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You recently got the chance to ask a group of MIT researchers questions about fusion power, and they've now finished writing some incredibly detailed answers. They discuss the things we've learned about fusion in the past decade, how long it's likely to take for fusion to power your home, the biggest problems fusion researchers are working to solve, and why it's important to continue funding fusion projects. They also delve into the specifics of tokamak operation, like dealing with disruption events and the limitations on reactor size, and provide some insight into fusion as a career. Hit the link below for a wealth of information about fusion. 1. What have we learned?
by jank1887

Fusion is one of those technologies that is always '50 years away,’ even 50 years ago, maybe even 50 years from now. So, looking at what's actually happened recently: What do we actually know now that we didn't know 10-15 years ago that gives support to the notion that we're making progress? Or, what are the 'big' things we know now? Similarly, what are the things we still don't know that we could reasonably expect to find answers for in the next 10-15 years?

MIT Researchers: As researchers in this field, we have heard the expression "Fusion is 50 years away and always will be" more times than we would like to admit. The implication of this statement is that no real progress has been made in the field, which is simply not true! We have made a great deal of progress, even in the last 10–15 years (which have been very lean times for funding). We’ll try to summarize some of the new findings, in no particular order:

1) Internal Transport Barriers/Reversed Shear operation –
We have actually discovered a way to improve upon the performance that we get in H- mode plasmas. These improvements come in the form for so called internal transport barriers. In the past 10–15 years we have begun to understand how to modify the current flowing in tokamak plasmas so that we create effectively what is a barrier in the middle of the plasma. Like the edge barrier in H-mode plasmas, this barrier restricts particles and energy from escaping the plasma and enhancing the overall performance. 2) I-mode –
In just the last 5 years, a new operational regime has been discovered on the Alcator C-Mod tokamak at MIT. This is termed the I-mode, or “Improved L-mode” regime. When the tokamak is operated in this manner it exhibits excellent energy confinement properties, keeping the plasma hot. At the same time the plasma does an excellent job of expelling impurities which dilute the fusion fuel and reduce the number of fusion reactions which can occur. It is particularly important to us as it was first observed on Alcator C-Mod, and is now under active development at many other tokamaks around the world.

3) Development of Predictive Models –
Great advances have been made in the development of predictive computer models, such as gyrokinetic and magnetohydrodynamic (MHD) formulations. Years of experiments have revealed that plasma turbulence is often primarily responsible for the loss of particles and energy from fusion reactors. In the past 10–15 years we have developed advanced models which are thought to contain sufficient physics to simulate plasma turbulence and predict the performance of future fusion devices. At this time we are in the process of validating these models, i.e. comparing them directly with experiment to ensure they are correct, but we are approaching the ability to reliably predict the performance of fusion plasmas without the need for a fusion reactor. This can motivate engineering design and operational choices for future fusion devices.

4) Self-acceleration of the plasma (intrinsic rotation) –
Over the past decade, it has been discovered (on Alcator C-Mod and elsewhere) that plasmas can spontaneously rotate, at speeds of tens of kilometers per second. (Imagine the donut-shaped plasma spinning on its axis.) This turns out to have beneficial effects for stabilizing turbulence at the edge, as the spinning plasma causes the turbulent eddies to break up before they can carry hot plasma out of the core. This is an exciting area of research that could have big implications for the performance of a tokamak reactor.

5) Disruption mitigation –
One of the main problems with a tokamak is the ‘disruption’, when the plasma energy is suddenly lost, stopping any fusion that is occurring and requiring a restart of the reactor. (See the question below for a lot of detail about this!) In extreme cases, these disruptions can cause damage to the wall of the tokamak – which would require repairs before the machine can be restarted. Over the past decade, we have developed techniques to mitigate these disruptions, causing the plasma to come to a rapid shutdown that does not negatively affect the wall condition. Work is underway to scale these techniques up to a reactor-size device (ITER).

6) ELM control/avoidance –
Another longstanding problem with tokamaks is periodic ‘bursts’ of energy from the edge called Edge-Localized Modes (ELMs). In today’s devices, ELMs are not a problem, but in ITER and future reactors, they could carry enough energy to damage the wall in the divertor region (where most of the energy comes out). There has been rapid progress lately (past 15 years) in ways to control these ELMs by making them more rapid and smaller, such as using resonant magnetic perturbation (RMP) coils to distort the shape of the confining magnetic field, or ‘pellet pacing’ (firing small pellets of deuterium fuel into the machine 50–60 times per second, which triggers an ELM), or vertical ‘kicks’ in which the control system suddenly jogs the plasma position a few centimeters vertically, also triggering an ELM. Between these techniques and the recently discovered I-mode (which doesn’t have ELMs), this is a problem that is well on the way to being solved.

7) High-Z walls –
This is a particular point of pride for Alcator C-Mod. Running a tokamak with walls made of refractory metals has many advantages because of the extreme capacity of these materials to absorb heat loads, but there are disadvantages as well, such as how radiative these high atomic number elements are if they get into the plasma as impurities, or how metallic materials distort when they melt, rather than ablating like carbon-fiber composites. Alcator C-Mod (which has a molybdenum wall) and other tokamaks have recently shown that it is possible to reliably run a tokamak with high-Z refractory metal walls, which will almost certainly be a feature of future reactors.

2. Power Loss Scenario in Alcator C-Mod?
by eldavojohn

Not to raise any fears -- rather out of genuine curiosity -- what happens when the magnetic fields that hold the 90,000,000 degrees Celsius plasma in place fail or loser power on the Alcator C-Mod? I understand it's probably in prototype mode, but what sort of safety advantages or disadvantages do Alcator C-Mod designs offer over conventional, large-scale designs? Does the plasma come into contact with the toroidal superconducting coil? Then what?

Geoff Olynyk answers: Actually, that’s exactly what my research is on! The event you describe is called a "disruption." Holding a hot plasma stationary using magnetic fields without it ever touching material surfaces is very difficult – Richard Feynman once compared it to trying to "hold Jello with rubber bands." For any number of reasons, like a magnetic coil losing power, the control system not being able to juggle the plasma position quickly enough, or the plasma hitting a stability limit (pressure or density goes too high), it’s possible for the plasma to hit the wall. The most important thing to know, though, is that when this occurs (and it does, frequently, in today’s experiments – although it’ll have to be a very rare occurrence in a real power reactor so it produces uninterrupted electricity), it is no risk to the environment or to safety.

To understand what happens, you have to realize that the plasma is very, very light. In the Alcator C-Mod tokamak, it has a mass of only about 0.001 grams – about one- fiftieth as much as the smallest drop of water you can get from an eyedropper. (This is with a plasma volume of about a cubic meter – a fusion plasma is actually a pretty good vacuum!) So even though it’s very hot, it doesn’t actually have a lot of thermal stored energy to flow into the wall if confinement is suddenly lost. There is actually more energy stored in the current flowing in the plasma (in C-Mod, about a million amperes), which also gets deposited on the wall. In C-Mod, thermal stored energy is about 50– 150 kJ and magnetic stored energy is almost 1000 kJ. The problem is that as we go to larger machines (like ITER, or a reactor), the amount of stored energy in the plasma scales like the cube of the size, and the wall area only scales like the square of the size. So the energy deposited per square meter of wall area gets worse (larger) as we go up in machine size.

The plasma doesn’t hit the superconducting coils - it hits (really, deposits its energy on) the “first wall” of the chamber closest to the plasma. So, we do two things to make sure that the walls can survive these disruption events. The first is making them out of materials that can take a blast of heat, like tungsten, or else materials that ablate away rather than melting, like carbon fiber composites. The second is to develop “disruption mitigation” systems which can cause the plasma to radiate all its energy evenly over the entire wall surface, spreading the heat out and lessening the chance of causing localized melting. But I want to stress again - disruptions are an operational problem, meaning they might cause a power plant to be offline for a while, but they’re not a safety problem. There is no chance of a runaway reaction or meltdown in a fusion reactor.

3. Ubiquitous Fusion Power
by monsted

When will fusion power my house (or vehicle)?

MIT Researchers: This is obviously an impossible question to answer, but we can give some thoughts about when it might happen, and why. First, the current official plan is that ITER will demonstrate net fusion gain (Q = 10, that is, ten times more fusion power out than heating power put in) in about 2028 or 2029. (Construction will be done by about 2022 but there’s a six-year shakedown process of steadily increasing the power and learning how to run the machine before the full-power fusion shots.) At that point, designs can begin for a “DEMO”, which is the fusion community’s term for a demonstration power plant. That would come online around 2040 (and would putt watts on the grid, although probably at an economic loss at first), and would be followed by (profitable, economic) commercial plants around 2050.

This seems like a long time, and it is, but it’s important to understand that this is not the only possible path. You might say that we’re not a certain number of years away from a working fusion power plant, but rather about $80-billion away (in worldwide funding). We’ll get into this more in response to one of the other questions, but there are other experiments that could be done in parallel with ITER that would certainly speed up the goal of a demonstration power plant, if there were the money for it. Here is a graph based on a 1976 ERDA (predecessor to today’s DOE) fusion development plan, showing their four paths to a reactor, as well as a business-as-usual funding case that would never lead to a reactor, and in black is the actual funding amounts. (All values are adjusted to 2012 dollars.)

In the U.S. at least, fusion funding hasn’t been anywhere close to what would be required for a “crash program” to get to a reactor. If it were, it would probably be possible to have a demonstration reactor in about twenty years. (This is not actually that long - given that it takes almost a decade to build a large fission reactor or hydroelectric dam!)

Fusion has a reputation of “always being thirty years away” (or fifty, or twenty). We want to address that head-on here: aside from a few over-optimistic predictions made in the very early days of magnetic fusion research (the 1950s), this reputation is undeserved. The reason it has taken so long to get to breakeven (ITER) is because since the end of the 1970s, funding for fusion research has been continually slashed, up to today, when the U.S. is proposing shuttering one of three remaining tokamak experiments, the Alcator C-Mod device at MIT that we all work on. Despite this, progress has been continuous. But if we had the money, we would be getting there quicker.

4. What are the economic numbers for a successful, commercial reactor?
by kestasjk
I know that the economics of larger reactor = more economical are well known with tokamaks. Does this mean you have a good idea of the minimum cost / generating capacity of the first commercial reactors? If so, what do those numbers look like?
7. Lower Limit on Tokamak Design
by gyepi
Are there any good guesstimates on how small a tokamak-based fusion reactor (which produces more energy than it consumes) can become? Theoretical limitations on the size of the reactor would have obvious implications for pragmatic issues.

MIT Researchers: Questions 4 and 7 are similar and we answer them together here.

The current thinking is that a tokamak fusion reactor will be about 1 gigawatt electrical, and about 2–3.5 gigawatts thermal (depending on how high-temperature the blanket is and thus how thermally efficient it can be). This is about the size of a current fission reactor or large coal-fired power plant.

Fusion researchers are working on smaller designs, though! At MIT, some students are working on a concept for a 350–500 MW (thermal) class fusion reactor, which would be cheaper to field and thus more likely to be built by private industry with limited access to capital. This is still early work, though, and the economic analysis is not done yet.

Cost estimates for a new technology like fusion cannot be terribly reliable, but several studies suggest that, with suitable developments in science and technology, the costs could be competitive with other methods of electricity generation. We recommend you read the ARIES-AT study (google it), which goes through all the factors that go into the cost of electricity (COE) for a fusion reactor, and compares their concept to other electricity generation options (fission, fossil fuels, etc.) A key advantage of fusion is in what economists call "external costs." These are costs borne by society as a whole and not by the generating industry. Environmental pollution, nuclear proliferation, and military operations to protect oil supplies are all examples of external costs for energy.

5. What Problems are Holding Back Successful Reactions?
by Bucc5062

Can you explain to a non-scientist what the biggest stumbling blocks are for an effective fusion reaction? Is it truly a matter of throwing money down an energy hole, or are there verifiable, measurable benchmarks that lead us from one step to the next? I.e. we’ve achieved X, now we need Y; when we get Y, we get Z and then achieve fusion. Is it the technology holding us back, the politics, or the science?

MIT Researchers: We know exactly what we need to do. Not everything has a solution yet – that’s why it’s still a research project! – but we generally know what the big challenges are to get to a working magnetic fusion reactor. Here is a non-exhaustive list:

• 1 – Non-inductive current drive. We can’t rely on inductors to drive the plasma current since they are inherently pulsed (not steady-state). We think that lower hybrid current drive might be the solution, and are actively researching this on Alcator C-Mod.
• 2 – Confining a 'burning plasma.' This is the big question that ITER will resolve – can we really confine a plasma that is dominantly self-heated – that is, most of its energy comes from fusion reactions rather than external heating. Will new instabilities appear? Or can we confine the plasma as we expect we can.
• 3 – Confining a steady-state burning plasma while avoiding off-normal events. We have to do both of the previous points at the same time! And we can’t have disruptions too often or else the power plant won’t have a high enough duty factor. The goal is to have disruptions (which require a shutdown) occur less than once per year.
• 4 – Validated predictive capability for fusion-grade plasmas. We have made great progress in this field already (see our answer to an earlier question), but it’s not at the point yet that, say, fluid mechanics codes are, where Boeing can design an entire plane in the computer before ever building a scale model. We need our models of fusion plasma behavior to be accurate and reliable enough to design first-of-a-kind machines that we are 100% sure will work the way we think they will.
• 5 – Diagnosing a burning plasma. It’s really hard to tell any of the properties of the plasma even today, when we use pure deuterium fuel (instead of ‘live’ deuterium–tritium fuel), and our plasmas are colder than they would be in a reactor! You can’t, for example, stick a thermometer in to tell the temperature! We have to use subtle effects like bouncing a laser beam off the electrons and telling the temperature from the Doppler shift of the laser from the moving electrons (a technique known as Thomson scattering). Making these diagnostics work in the reactor environment, with higher plasma temperatures and a ferocious flux of neutrons coming out, is a great challenge.
• 6 – Better understanding of plasma–wall interactions. The plasma is confined by magnetic fields, and ideally doesn’t touch the wall at all, except in a very small area called the divertor. This means that the material challenges in the divertor are severe – we have to figure out a way to operate the plasma so that it’s hot in the center, but cold near the divertor, so that it doesn’t erode the wall too fast. This will be a limiting factor on how long you can run a fusion power plant for before you have to shut it down in order to do maintenance. Ideally, we’d want this to be every 2 years or so, like fission power plants today.
• 7 – Materials for plasma-facing components. We need to develop new materials that can withstand the high temperatures of the wall of a fusion reactor while resisting neutron damage and not becoming too activated by the neutrons that will pass through them. (There is some progress on this front with ferritic steels and silicon carbide.)
• 8 – Magnets that meet the plasma physics requirements and allow reactor maintainability at reasonable costs. (Some of us are working on demountable superconducting coil concepts that may eventually be the solution to this!)
• 9 – Design and materials for tritium fuel cycle and power extraction. Fusion reactors will breed their own tritium fuel from deuterium – this process has to be experimentally tested on a large scale (which will obviously require a burning plasma tokamak).
• 10 – Reliability, availability, maintainability, and inspectability (RAMI) of the reactor designs. We have to show that our concepts for reactors really are as good as we think they can be.

The point is that it’s not a money pit. There are unsolved challenges, but we know what they are, and with adequate support, these challenges will be overcome. This is why we are urging everyone to go to fusionfuture.org and write Congress asking them to keep supporting U.S. fusion research! (It’s very easy – there’s a link at the right on the website.)

6. NIMBY
by GeneralTurgidson

How do you explain the safety/benefits of fusion to a generation of people terrified of nuclear anything?

MIT Researchers:This is where fusion really shines. The two big problems (at least, perceived problems) of fission reactors are the risk of a meltdown, and what you do with the high-level radioactive waste. Fusion has neither of these issues!

Regarding the first, the reason why a worst-case accident in a fission reactor can be so devastating is because there is a lot of fuel in the reactor at any one time. There are well known accidents at Chernobyl (where the reaction ‘ran away’, making more power than the reactor was designed to handle) and Fukushima, where the fission chain reaction was safely shut down, but the cores melted down when the tsunami knocked out the cooling systems, due to ‘decay heat’ which is produced by the used fuel even after shutdown.

In a fusion reactor, it’s a completely different story. There will be less than a gram of fuel in a reactor at any one time—fresh deuterium–tritium fuel is continually added as it is burned—and so a runaway reaction is simply not possible. Decay heat isn’t a problem in a pure fusion system, again because there just isn’t any fuel sitting there undergoing nuclear reactions once the reactor is shut down. In general, this is one area where it’s a benefit that a fusion reaction is so hard to sustain! We have to try really hard to keep the plasma hot enough to undergo fusion in the first place, so if we just turn off the heating and fuelling systems, the fusion reaction will shut down very quickly.

As for the second benefit of fusion (waste), the reaction is completely different from that in a fission reactor. In fission, uranium (or other heavy elements like plutonium) split into pieces, producing hundreds of different isotopes, some of which are radioactive, with half-lives ranging from fractions of a second to millions of years. In fusion, the reaction is simple, deuterium + tritium helium + neutron. So there is no “waste” from the unburned fuel – any tritium that isn’t burned gets pumped out of the chamber and recirculated back in.

This is not to say that there will be no radioactive waste from a fusion plant. The reactor vessel itself will become activated because of the flux of neutrons passing through it, and will have to be treated accordingly when the plant is decommissioned (after, say, a 50-year operational period). But it’s important to note that this kind of radioactive waste is of a much lower level – it won’t have to be stored for very long before it will be “cool” enough to simply bury in the ground safely. And there is active research going on into new materials for fusion reactors that are more resistant to activation by neutrons, such as ferritic steel and silicon carbide.

Finally, fusion has great advantages for nuclear non-proliferation. Creating enough fission power plants to avoid climate change would mean that the plutonium moving around the world would be enough to create about 100,000 nuclear weapons. For fusion, it is much more difficult to use a reactor to make fuel for weapons. This is also something that we think a nuclear-skeptical public will appreciate about fusion power.

All of us are strong supporters of fission power, and we agree that at times, the nuclear power industry has not received a fair shake when compared to other sources of energy. But we think that the advantages of fusion power speak for themselves, and the public will be able to understand the risks and will support the construction of these plants. Obviously, having media that are able to explain things clearly and fairly are a necessity.

8. What do the numbers really look like?
by Erich

ITER is a hugely expensive project, and won't produce a commercially viable power generation system. In a lot of areas where research is done on things which don't work yet -- rockets, bridges, transmission systems, etc -- there's a general idea of how things might be able to "scale up" to meet the goals. Is tokamak fusion really in sight of being a commercially viable source of energy? If we need unobtanium to make a commercially viable reactor, wouldn't it make sense to wait until the materials are viable before making even larger tokamaks? Or is it still worth learning from these new, bigger, more expensive reactors?

MIT Researchers: You are exactly correct in your statement; ITER is an expensive project which will not produce electricity upon completion. However, ITER’s main purpose is not to put watts on the grid, but to demonstrate the scientific feasibility of fusion by creating a Q=10 plasma (10 times as much energy out as we put in). We do have a good idea of how to proceed with devices following ITER, namely DEMO, a full demonstration fusion power plant which will use the steam cycle to generate electricity from the fusion reactor. The basic layout of a reactor can be found here: http://www.fusionfuture.org/what-is-alcator- c-mod/c-mod-for-energy/

Although there is still plenty of research which remains, fusion is in sight of being a commercially viable energy source. We believe that we now understand the physics well enough to create the appropriate plasma conditions (this will be demonstrated on ITER) and we are working on the engineering challenges that lay between us and a commercial fusion reactor.

It is obviously impossible to predict when fusion will put power on the grid since the estimate can change drastically based on demand and overall funding levels. You are however, correct in noting that some of the biggest challenges involve the discovery/ development of materials which can resist the unique and harsh conditions associated with fusion reactors, namely, high heat and neutron fluxes. Due to its importance to the success of future devices, this is a very active and important area of research.

The international fusion community is attempting to address these issues in the following manner: Given the scope of the ITER project and the time required to build and test it, we are planning on constructing a materials testing facility named, IFMIF which stands for International Fusion Materials Irradiation Facility. This facility should be operated at the same time as ITER and will be addressing the materials issues associated with an eventual fusion power plant while ITER is demonstrated the scientific feasibility of a fusion reactor.

Given the time-scales for reactor construction, we think it would be unwise to wait for this materials testing to be complete before starting new machine construction. Addressing the remaining problems in parallel will most likely result in the quickest path to fusion energy.

9. Careers in Fusion?
by benjfowler

As practicing researchers, can you tell us about the health of the pipeline of young researchers coming into the field? Is there a glut of trained physicists at this stage, or is there still a need for trained specialists to enter the field, especially with ITER and follow-on machines coming online in the next couple of decades?

Nathan Howard answers: At this point in time fusion is actually a pretty healthy field in terms of young researchers and with emergence of the next generation devices such as ITER, there should be an influx of researchers stepping up to meet the need for trained specialists on these next gen devices. Currently in Europe and Asia, emphasis on fusion research is ramping up to support the research needs. These newly trained researchers are going to be the scientists working on ITER in 10-15 years.

Unfortunately, the US fusion program is in danger of going the opposite direction of the Asian and European programs. The current proposals made by the US are threatening the health of fusion in the US. The President’s 2013 budget proposal calls for drastic cuts to the domestic fusion budget to pay for increased funding for the ITER budget. However, if these cuts continue, there will not be a field for the young researchers to enter and the US fusion program is in danger of dissolving before ITER comes online.

This does not mean that a need for trained specialist will not remain, it just means that the young researchers in Europe and Asia will be filling these positions. Dr. Stewart Prager, the head of Princeton Plasma Physics Lab said it best, “We have a clear choice before us: The United States can either design and build fusion energy plants or we can buy them from Asia or Europe.”

As a young researcher myself, I am particularly affected by the choices that the US is currently making. Myself and other graduate students have been urging others who support fusion research to contact congress and tell them to continue to fund domestic fusion research. We put together a website, www.fusionfuture.org, which provides more information and people the ability to quickly and easily contact their congressmen to tell them to support research. Please support US fusion research and check out the site.

12. Patents?
by Anonymous Coward

Will patents get in the way of your research?

MIT Researchers: In general, we find that the tokamak labs of the world are extremely cooperative; patents have never been a problem. It does seem likely that the technologies supporting power plants will be highly patentable, but the sort of scientific knowledge we’re accumulating at present really isn’t. At some point, we expect to move from a collaborative to a competitive phase – but we’re not there yet.

11. What level of investment would get fusion going?
by Tragek
Do you think a program the size of the Apollo program could kickstart fusion to general availability? Or would a smaller program suffice?
14. What could you do with unlimited resources?
by petes_PoV
Given $1 trillion, the pick of the best brains in the world to work willingly on the project, a large enough location away from any and all governmental regulation and every facility you could ever need - when would fusion be commercially viable?

MIT Researchers: Questions 11 and 14 are similar and we have answered them together.

Any kind of question asking about a hypothetical massive increase in funding is tricky to answer. We probably couldn’t even spend a trillion dollars if we wanted to – just because it would take a long time to get enough people trained in plasma physics and fusion energy.

We can say this: an increase in funding would allow for different paths to be tried in parallel, like stellarators, tokamaks (ITER), spherical tokamaks, etc. Plus, we could build a facility in the United States to study the problem of plasma–wall interactions, which is a very important topic that has not been adequately studied up to this point (see our answer above about what steps are needed to get to a reactor).

We think that we’re roughly $80-billion away from a reactor. At current levels of funding (worldwide), that’s about 40 years. Even given access to huge amounts of money, it’s unlikely that a working reactor could be built in less than a decade – there are just too many facilities to build between current devices and a full-scale reactor in order to ensure success. But we could certainly do it faster than 40 years!

We want to note that “crash” programs like Apollo or the Manhattan Project succeeded because they took risks – they started work on building their systems before they had done all the homework. That is inherently risky, but these risks are mitigated by pursuing alternatives in parallel. Something similar could be done in fusion, given the money.

15. Your favorite books?
by eldavojohn

I'm not a physicist (software guy), but I've taken a few physics classes. At an early age I found a tattered copy of George Gamow's One Two Three . . . Infinity, which, although incorrect in some parts (I guess that's why they revised it and that's why 'speculations' was in the title), was perfectly written for my then-fifth-grade mind. It set me on a path toward science, and a few weeks ago I saw the same 1960s Viking Press edition and flipped through it, noticing what was slightly off and remembering it. I've since grown to love other obvious books by authors like Hawking, Penrose, Hofstadter, etc. So, quite simply, what are your favorite books for all minds, young and old? Also, can you annotate which are written for the layman's entry into the given field and which are written to encompass the field for the researcher? I find that some books start off with the jargon so strong and the references and footnotes so thick that you start to have to re-read every paragraph, as they're clearly condensing entire historic papers into lengthy sentences. Any fiction books worthy of influencing your work and desires?

Ian Hutchinson: My all time favorite novel is Godric by Fredrick Buechner. It's a wonderful first-person portrait of the prior life of a medieval hermit. My favorite physics teaching text is the Feynman Lectures on Physics, which comes from a remarkable effort by the most widely acclaimed american physicist of the 20th century to explain really advanced physics to undergraduates.

I really don't enjoy the genre of books that combine science popularization with metaphysical speculation. They are of course quite popular, but most are philosophically naive in a way that I find annoying.

Anne White: I like detective/adventure stories. I also enjoy reading plays, poetry and short stories – some authors I read over and over are Wolfgang Borchert, Julio Cortazar, Ray Bradbury and Samuel Beckett.

Recently, I've enjoyed reading The End of the Affair by Graham Greene, People of the Book by Geraldine Brooks, Jane Eyre by Charlotte Bronte and Her Fearful Symmetry by Audrey Niffenegger.

Influential books/stories that I remember reading when I was young : The Pearl (John Steinbeck), Catch-22 (Joseph Heller), Flatland, and Ender's Game.

Dennis Whyte:

• For science non-fiction books, it’s a tie: The Selfish Gene by Richard Dawkins, and Wonderful Life by Stephen Jay Gould.
• Novel (in general subject area of science): The Baroque Cycle by Neal Stephenson
• Speculative fiction: Starship Troopers by Robert Heinlein

Geoff Olynyk: The Making of the Atomic Bomb by Richard Rhodes is the best non-fiction book I’ve ever read. It’s a bit long, but is a fascinating, well-written exploration of the project to develop the atom bomb (both in the U.S. and elsewhere).

This is not a science book, but The Rebel Sell by Joseph Heath and Andrew Potter (sold in the United States as Nation of Rebels) changed my life. I was into counterculture, "culture jamming," anti-advertising, that kind of stuff, and this book made me seriously reconsider all of it. I now understand that trying to be unique is futile in a world of seven billion, and I should just try to be a good person and do good for the world (hence working on fusion!) Potter’s follow-up The Authenticity Hoax, explores the search for authenticity in more detail, but it’s not nearly as good of a book as Rebel Sell.

Nathan Howard: I first became interested in physics by reading about astrophysics. I was specifically interested in black holes and so one of the first books I read (after some of the popular books by Hawking which are written for general audiences, e.g. A Brief History of Time) was a book by Kip S. Thorne called Black Holes and Time Warps. I really enjoyed this book. It did not require much technical background, just some basic mathematics, and it gave good explanations of black holes, relativity, and gravitational waves.

16. Why is fusion more useful than exploiting thorium?
by gestalt_n_pepper

I understand that in the long term, we would want fusion. But we face increasing energy problems over the next 50 years and severe energy problems before 2100. Wouldn't it make sense to allocate research and development resources to something that we know works?

MIT Researchers: First of all, fusion will be putting watts on the grid before 2100. It’s not going to be tomorrow, but it’s not going to be a hundred years, either.

We know how to build thorium fission reactors. It's been done. They have none of the major attractions of a fusion reactor in terms of safety, fuel resources, reduced waste, or non-proliferation. Worldwide thorium fuel resources are about the same as those of uranium. Thorium reactors might become part of the commercial fission reactor mix in the future, but they don't offer transformative possibilities for nuclear power the way fusion does.

That said, we think that the that the scale of the energy/climate problem demands that we (meaning: government and private industry where appropriate) pursue multiple lines of development into new energy sources. Obviously nobody wants to waste taxpayer money, so all proposals have to be evaluated for chance of success – but today, it’s limited by funding more than by a lack of good ideas. This shouldn’t be the case.

The key thing we want to get across is that it shouldn’t be a contest between “fund fusion” or “fund thorium research”. Fusion is extremely important for humankind and should be funded – if thorium fission also has promise, it should be funded too.

17. How is fusion power harnessed?
by circletimessquare

The talk is always about reaching break-even with fusion. What about capturing the power? Are we generating heat that will drive steam turbines? What schemes exist for capture and harnessing the power generated by fusion?

MIT Researchers: In a magnetic fusion reactor, each deuterium-tritium fusion produces a 3.5 MeV (mega- electronvolt) alpha particle (helium nucleus) which deposits its energy in the plasma (this self-heating is how you can have an ‘ignited’ plasma which doesn’t require much or any external heating), and a 14.1 MeV neutron, which deposits its energy in a thick lithium blanket surrounding the toroidal reaction chamber. But in the end, all of it comes out as heat!

For a conservative fusion reactor design, this heat would be removed by a primary cooling loop (high-pressure steam or some sort of liquid metal) which would give the heat to a secondary steam loop (Rankine cycle) in a heat exchanger (steam generator). The steam would then turn a turbine, producing electricity, just like in a fission or coal power plant.

Of course, with a thermal process like a steam cycle, one is always limited by the Carnot efficiency, which increases as the temperature of the high-temperature reservoir goes up. So there are also designs to use a very high-temperature (800–1000 C) gas cooling loop and a Brayton cycle.

But the short answer is: the alpha power is captured by the plasma, and the neutron power is captured by the blanket. It all comes out as heat, which is used to heat a working fluid, which turns a turbine, producing electricity. This is not expected to be a technological problem – the challenge is in getting a confined thermonuclear plasma to produce the fusion energy in the first place!

19. Fusion Milestone Prizes?
by Baldrson

In 1992, with the assistance of fusion technologists such as Robert W. Bussard, I developed legislative language for a series of 12 milestones, each of which would be awarded a $(1992)100M prize for the achievement of objectives toward the attainment of practical fusion energy. This legislation also provided a grace period during which scientists and technologists that had been working on the US fusion program would be provided full salaries, without obligation, during which time they could seek support for their ideas to achieve these milestones. This legislation presaged a number of other prizes including the X-Prize and BAFAR / CATS prize. In 1995, Robert W. Bussard submitted this legislation to all relevant Congressional committees, copying all US plasma physics laboratories. Needless to say, the legislation wasn't passed. Do you think the time is right?

MIT Researchers: We think that the current approach, in which government-funded labs are not in direct competition, but have to justify their funding to the agency (in our case, the DOE), is the best option for the moment. Perhaps the X-PRIZE approach might work for the alternative concepts? (see our answer below regarding Polywell/Dense Plasma Focus/ IEC etc.)

20. ITER
by MpVpRb

Is the ITER project good science? Or is it a politically-motivated, pork-laden boondoggle?

MIT Researchers: ITER is absolutely good science. Governments representing over half the population of the world are backing the project because it is the logical next step – a prototype reactor that will produce ten times more fusion energy than heating power put in, for a few minutes at a time. It is also pushing forward the development of fusion reactor technology (materials, control systems, remote handling systems, etc.). The U.S. fusion community endorsed ITER as the best option for a next-step experiment at the Snowmass II conference in 2002 (see proceedings here).

All of that said, the cost of ITER has risen substantially from the original estimates, and because overall magnetic fusion funding has remained nearly flat in the United States, the U.S. contribution to ITER is threatening to swallow up the entire domestic program. This is starting with the planned closure of Alcator C-Mod in September 2012, but unless more money is allocated to fusion research, all three U.S. tokamak facilities are at risk in the next few years.

Graduate students at Alcator C-Mod have put together a web page explaining the problem: http://www.fusionfuture.org/faq/the-fusion-budget-problem/ and we urge you to go to this website and click the link to contact your member of Congress and urge them to fully fund a strong domestic program and the U.S. contribution to ITER!

21. NIF
by Grond

Is the NIF approach even plausibly capable of generating electricity in a useful way? Or is it purely a research platform / smokescreen for nuclear weapons research?

MIT Researchers: The primary mission of the National Ignition Facility (NIF) is "stockpile stewardship." That is, to ensure that U.S. nuclear weapons continue to be a credible deterrent. This is why NIF is funded by the National Nuclear Security Agency (the agency in charge of the nation’s nuclear stockpile), not the DOE Fusion Energy Sciences program. Thus, the weapons research mission of NIF is not a smokescreen, but is actually the publicly acknowledged primary objective for the facility.

Some researchers at NIF believe that their inertial fusion approach can be used for an energy source as well. We don’t want to speculate here on the plausibility of the LIFE (Laser Inertial Fusion Energy) concept. There is a National Academy of Science review of the prospects of inertial fusion energy under way right now; the final report is expected to be published before the end of this year.

18. Dense Plasma Focus
by mbradmoody
Do you see any merit in the "dense plasma focus" approach to commercial fusion power production, specifically the work of the Lawrenceville Plasma Physics group?
22. Focus Fusion / aneutronic fusion?
by mwk88
Focus Fusion Society is posting research on their project to do aneutronic (e.g. Proton Boron (pB11)) fusion. The concept sounds great, and as an engineer, I find several parts of their design, such as direct extraction of electric power, to be elegant. Is this credible research or pie-in-the-sky? I have not seen much mention of them in mainstream fusion research.
23. Polywell Fusion
by mknewman
What do you think of the efforts at EMC2 Fusion and Polywell Fusion? They seem to be making real, measurable, and open results, but the mainstream physics community seems to ignore this progress.
24. What’s wrong with IECs / Fusor?
by claytongulick
Why aren't IEC reactors based on Farnsworth's designs taken more seriously? From what I understand, IECs have been more effective at producing fusion, and they are cheap to build. People even build them in the garage. From everything I've read, no one really takes the "fusor" seriously in the fusion science realm, and it's considered a dead line of inquiry. I've never understood why.

MIT Researchers: These four questions (18, 22–24) are answered together here.

None of us are experts on inertial electrostatic confinement, magnetized target fusion / dense plasma focus, or Polywells, and so we don’t want to say too much about the specifics of those designs. We can say the following:

1. The amount of money that is being spent, especially in the United States, on fusion is far lower than the field deserves, given its track record and potential. This sounds self-serving, but we think it’s justifiable based on the facts. The graph we posted above shows how the fusion budget is far lower today than it was thirty years ago, even as we continue to make steady progress toward a reactor and the seriousness of the coupled energy/climate problem becomes more obvious.

The alternate confinement concepts program has also seen cuts. (“Alternative” in DOE Fusion Energy Sciences parlance means, basically, anything that isn’t tokamaks, stellarators, or laser [inertial] fusion.) The Levitated Dipole Experiment, an innovative magnetic-confinement arrangement based on planetary magnetic fields, was cancelled just as they were about to add significant auxiliary heating for the first time. And these small-scale alternative confinement projects are not very expensive! Some of these alternative concepts may very well be promising and deserve taxpayer money to be developed.

2. But on the other hand, these groups need to show that they deserve funding. It’s not enough to just tease these promising results and be secretive about the methods or technologies. Public funding can only come when the details are published in the open literature, and subjected to the scrutiny of peer review and the wider community reading the papers. The (hot) fusion community is still living with the aftermath of the cold fusion scandal from a quarter century ago - so it’s very important for the proponents of these alternate concepts to push the researchers to publish their results in peer-reviewed journals. Whatever negatives the tokamak might have, one thing you can’t say about it is that the research has been too secretive, and this has allowed the funding agencies to make the judgement that the tokamak is currently the most promising route to a fusion reactor, which is why this line of research gets the most money.

Special thanks to Dr. Martin Greenwald, Prof. Ian Hutchinson, Asst. Prof. Anne White, Prof. Dennis Whyte, Nathan Howard, and Geoff Olynyk for taking the time to answer our questions.

You recently got the chance to ask a group of MIT researchers questions about fusion power, and they've now finished writing some incredibly detailed answers. They discuss the things we've learned about fusion in the past decade, how long it's likely to take for fusion to power your home, the biggest problems fusion researchers are working to solve, and why it's important to continue funding fusion projects. They also delve into the specifics of tokamak operation, like dealing with disruption events and the limitations on reactor size, and provide some insight into fusion as a career. Hit the link below for a wealth of information about fusion. 1. What have we learned?
by jank1887

Fusion is one of those technologies that is always '50 years away,’ even 50 years ago, maybe even 50 years from now. So, looking at what's actually happened recently: What do we actually know now that we didn't know 10-15 years ago that gives support to the notion that we're making progress? Or, what are the 'big' things we know now? Similarly, what are the things we still don't know that we could reasonably expect to find answers for in the next 10-15 years?

MIT Researchers: As researchers in this field, we have heard the expression "Fusion is 50 years away and always will be" more times than we would like to admit. The implication of this statement is that no real progress has been made in the field, which is simply not true! We have made a great deal of progress, even in the last 10–15 years (which have been very lean times for funding). We’ll try to summarize some of the new findings, in no particular order:

1) Internal Transport Barriers/Reversed Shear operation –
We have actually discovered a way to improve upon the performance that we get in H- mode plasmas. These improvements come in the form for so called internal transport barriers. In the past 10–15 years we have begun to understand how to modify the current flowing in tokamak plasmas so that we create effectively what is a barrier in the middle of the plasma. Like the edge barrier in H-mode plasmas, this barrier restricts particles and energy from escaping the plasma and enhancing the overall performance. 2) I-mode –
In just the last 5 years, a new operational regime has been discovered on the Alcator C-Mod tokamak at MIT. This is termed the I-mode, or “Improved L-mode” regime. When the tokamak is operated in this manner it exhibits excellent energy confinement properties, keeping the plasma hot. At the same time the plasma does an excellent job of expelling impurities which dilute the fusion fuel and reduce the number of fusion reactions which can occur. It is particularly important to us as it was first observed on Alcator C-Mod, and is now under active development at many other tokamaks around the world.

3) Development of Predictive Models –
Great advances have been made in the development of predictive computer models, such as gyrokinetic and magnetohydrodynamic (MHD) formulations. Years of experiments have revealed that plasma turbulence is often primarily responsible for the loss of particles and energy from fusion reactors. In the past 10–15 years we have developed advanced models which are thought to contain sufficient physics to simulate plasma turbulence and predict the performance of future fusion devices. At this time we are in the process of validating these models, i.e. comparing them directly with experiment to ensure they are correct, but we are approaching the ability to reliably predict the performance of fusion plasmas without the need for a fusion reactor. This can motivate engineering design and operational choices for future fusion devices.

4) Self-acceleration of the plasma (intrinsic rotation) –
Over the past decade, it has been discovered (on Alcator C-Mod and elsewhere) that plasmas can spontaneously rotate, at speeds of tens of kilometers per second. (Imagine the donut-shaped plasma spinning on its axis.) This turns out to have beneficial effects for stabilizing turbulence at the edge, as the spinning plasma causes the turbulent eddies to break up before they can carry hot plasma out of the core. This is an exciting area of research that could have big implications for the performance of a tokamak reactor.

5) Disruption mitigation –
One of the main problems with a tokamak is the ‘disruption’, when the plasma energy is suddenly lost, stopping any fusion that is occurring and requiring a restart of the reactor. (See the question below for a lot of detail about this!) In extreme cases, these disruptions can cause damage to the wall of the tokamak – which would require repairs before the machine can be restarted. Over the past decade, we have developed techniques to mitigate these disruptions, causing the plasma to come to a rapid shutdown that does not negatively affect the wall condition. Work is underway to scale these techniques up to a reactor-size device (ITER).

6) ELM control/avoidance –
Another longstanding problem with tokamaks is periodic ‘bursts’ of energy from the edge called Edge-Localized Modes (ELMs). In today’s devices, ELMs are not a problem, but in ITER and future reactors, they could carry enough energy to damage the wall in the divertor region (where most of the energy comes out). There has been rapid progress lately (past 15 years) in ways to control these ELMs by making them more rapid and smaller, such as using resonant magnetic perturbation (RMP) coils to distort the shape of the confining magnetic field, or ‘pellet pacing’ (firing small pellets of deuterium fuel into the machine 50–60 times per second, which triggers an ELM), or vertical ‘kicks’ in which the control system suddenly jogs the plasma position a few centimeters vertically, also triggering an ELM. Between these techniques and the recently discovered I-mode (which doesn’t have ELMs), this is a problem that is well on the way to being solved.

7) High-Z walls –
This is a particular point of pride for Alcator C-Mod. Running a tokamak with walls made of refractory metals has many advantages because of the extreme capacity of these materials to absorb heat loads, but there are disadvantages as well, such as how radiative these high atomic number elements are if they get into the plasma as impurities, or how metallic materials distort when they melt, rather than ablating like carbon-fiber composites. Alcator C-Mod (which has a molybdenum wall) and other tokamaks have recently shown that it is possible to reliably run a tokamak with high-Z refractory metal walls, which will almost certainly be a feature of future reactors.

2. Power Loss Scenario in Alcator C-Mod?
by eldavojohn

Not to raise any fears -- rather out of genuine curiosity -- what happens when the magnetic fields that hold the 90,000,000 degrees Celsius plasma in place fail or loser power on the Alcator C-Mod? I understand it's probably in prototype mode, but what sort of safety advantages or disadvantages do Alcator C-Mod designs offer over conventional, large-scale designs? Does the plasma come into contact with the toroidal superconducting coil? Then what?

Geoff Olynyk answers: Actually, that’s exactly what my research is on! The event you describe is called a "disruption." Holding a hot plasma stationary using magnetic fields without it ever touching material surfaces is very difficult – Richard Feynman once compared it to trying to "hold Jello with rubber bands." For any number of reasons, like a magnetic coil losing power, the control system not being able to juggle the plasma position quickly enough, or the plasma hitting a stability limit (pressure or density goes too high), it’s possible for the plasma to hit the wall. The most important thing to know, though, is that when this occurs (and it does, frequently, in today’s experiments – although it’ll have to be a very rare occurrence in a real power reactor so it produces uninterrupted electricity), it is no risk to the environment or to safety.

To understand what happens, you have to realize that the plasma is very, very light. In the Alcator C-Mod tokamak, it has a mass of only about 0.001 grams – about one- fiftieth as much as the smallest drop of water you can get from an eyedropper. (This is with a plasma volume of about a cubic meter – a fusion plasma is actually a pretty good vacuum!) So even though it’s very hot, it doesn’t actually have a lot of thermal stored energy to flow into the wall if confinement is suddenly lost. There is actually more energy stored in the current flowing in the plasma (in C-Mod, about a million amperes), which also gets deposited on the wall. In C-Mod, thermal stored energy is about 50– 150 kJ and magnetic stored energy is almost 1000 kJ. The problem is that as we go to larger machines (like ITER, or a reactor), the amount of stored energy in the plasma scales like the cube of the size, and the wall area only scales like the square of the size. So the energy deposited per square meter of wall area gets worse (larger) as we go up in machine size.

The plasma doesn’t hit the superconducting coils - it hits (really, deposits its energy on) the “first wall” of the chamber closest to the plasma. So, we do two things to make sure that the walls can survive these disruption events. The first is making them out of materials that can take a blast of heat, like tungsten, or else materials that ablate away rather than melting, like carbon fiber composites. The second is to develop “disruption mitigation” systems which can cause the plasma to radiate all its energy evenly over the entire wall surface, spreading the heat out and lessening the chance of causing localized melting. But I want to stress again - disruptions are an operational problem, meaning they might cause a power plant to be offline for a while, but they’re not a safety problem. There is no chance of a runaway reaction or meltdown in a fusion reactor.

3. Ubiquitous Fusion Power
by monsted

When will fusion power my house (or vehicle)?

MIT Researchers: This is obviously an impossible question to answer, but we can give some thoughts about when it might happen, and why. First, the current official plan is that ITER will demonstrate net fusion gain (Q = 10, that is, ten times more fusion power out than heating power put in) in about 2028 or 2029. (Construction will be done by about 2022 but there’s a six-year shakedown process of steadily increasing the power and learning how to run the machine before the full-power fusion shots.) At that point, designs can begin for a “DEMO”, which is the fusion community’s term for a demonstration power plant. That would come online around 2040 (and would putt watts on the grid, although probably at an economic loss at first), and would be followed by (profitable, economic) commercial plants around 2050.

This seems like a long time, and it is, but it’s important to understand that this is not the only possible path. You might say that we’re not a certain number of years away from a working fusion power plant, but rather about $80-billion away (in worldwide funding). We’ll get into this more in response to one of the other questions, but there are other experiments that could be done in parallel with ITER that would certainly speed up the goal of a demonstration power plant, if there were the money for it. Here is a graph based on a 1976 ERDA (predecessor to today’s DOE) fusion development plan, showing their four paths to a reactor, as well as a business-as-usual funding case that would never lead to a reactor, and in black is the actual funding amounts. (All values are adjusted to 2012 dollars.)

In the U.S. at least, fusion funding hasn’t been anywhere close to what would be required for a “crash program” to get to a reactor. If it were, it would probably be possible to have a demonstration reactor in about twenty years. (This is not actually that long - given that it takes almost a decade to build a large fission reactor or hydroelectric dam!)

Fusion has a reputation of “always being thirty years away” (or fifty, or twenty). We want to address that head-on here: aside from a few over-optimistic predictions made in the very early days of magnetic fusion research (the 1950s), this reputation is undeserved. The reason it has taken so long to get to breakeven (ITER) is because since the end of the 1970s, funding for fusion research has been continually slashed, up to today, when the U.S. is proposing shuttering one of three remaining tokamak experiments, the Alcator C-Mod device at MIT that we all work on. Despite this, progress has been continuous. But if we had the money, we would be getting there quicker.

4. What are the economic numbers for a successful, commercial reactor?
by kestasjk
I know that the economics of larger reactor = more economical are well known with tokamaks. Does this mean you have a good idea of the minimum cost / generating capacity of the first commercial reactors? If so, what do those numbers look like?
7. Lower Limit on Tokamak Design
by gyepi
Are there any good guesstimates on how small a tokamak-based fusion reactor (which produces more energy than it consumes) can become? Theoretical limitations on the size of the reactor would have obvious implications for pragmatic issues.

MIT Researchers: Questions 4 and 7 are similar and we answer them together here.

The current thinking is that a tokamak fusion reactor will be about 1 gigawatt electrical, and about 2–3.5 gigawatts thermal (depending on how high-temperature the blanket is and thus how thermally efficient it can be). This is about the size of a current fission reactor or large coal-fired power plant.

Fusion researchers are working on smaller designs, though! At MIT, some students are working on a concept for a 350–500 MW (thermal) class fusion reactor, which would be cheaper to field and thus more likely to be built by private industry with limited access to capital. This is still early work, though, and the economic analysis is not done yet.

Cost estimates for a new technology like fusion cannot be terribly reliable, but several studies suggest that, with suitable developments in science and technology, the costs could be competitive with other methods of electricity generation. We recommend you read the ARIES-AT study (google it), which goes through all the factors that go into the cost of electricity (COE) for a fusion reactor, and compares their concept to other electricity generation options (fission, fossil fuels, etc.) A key advantage of fusion is in what economists call "external costs." These are costs borne by society as a whole and not by the generating industry. Environmental pollution, nuclear proliferation, and military operations to protect oil supplies are all examples of external costs for energy.

5. What Problems are Holding Back Successful Reactions?
by Bucc5062

Can you explain to a non-scientist what the biggest stumbling blocks are for an effective fusion reaction? Is it truly a matter of throwing money down an energy hole, or are there verifiable, measurable benchmarks that lead us from one step to the next? I.e. we’ve achieved X, now we need Y; when we get Y, we get Z and then achieve fusion. Is it the technology holding us back, the politics, or the science?

MIT Researchers: We know exactly what we need to do. Not everything has a solution yet – that’s why it’s still a research project! – but we generally know what the big challenges are to get to a working magnetic fusion reactor. Here is a non-exhaustive list:

• 1 – Non-inductive current drive. We can’t rely on inductors to drive the plasma current since they are inherently pulsed (not steady-state). We think that lower hybrid current drive might be the solution, and are actively researching this on Alcator C-Mod.
• 2 – Confining a 'burning plasma.' This is the big question that ITER will resolve – can we really confine a plasma that is dominantly self-heated – that is, most of its energy comes from fusion reactions rather than external heating. Will new instabilities appear? Or can we confine the plasma as we expect we can.
• 3 – Confining a steady-state burning plasma while avoiding off-normal events. We have to do both of the previous points at the same time! And we can’t have disruptions too often or else the power plant won’t have a high enough duty factor. The goal is to have disruptions (which require a shutdown) occur less than once per year.
• 4 – Validated predictive capability for fusion-grade plasmas. We have made great progress in this field already (see our answer to an earlier question), but it’s not at the point yet that, say, fluid mechanics codes are, where Boeing can design an entire plane in the computer before ever building a scale model. We need our models of fusion plasma behavior to be accurate and reliable enough to design first-of-a-kind machines that we are 100% sure will work the way we think they will.
• 5 – Diagnosing a burning plasma. It’s really hard to tell any of the properties of the plasma even today, when we use pure deuterium fuel (instead of ‘live’ deuterium–tritium fuel), and our plasmas are colder than they would be in a reactor! You can’t, for example, stick a thermometer in to tell the temperature! We have to use subtle effects like bouncing a laser beam off the electrons and telling the temperature from the Doppler shift of the laser from the moving electrons (a technique known as Thomson scattering). Making these diagnostics work in the reactor environment, with higher plasma temperatures and a ferocious flux of neutrons coming out, is a great challenge.
• 6 – Better understanding of plasma–wall interactions. The plasma is confined by magnetic fields, and ideally doesn’t touch the wall at all, except in a very small area called the divertor. This means that the material challenges in the divertor are severe – we have to figure out a way to operate the plasma so that it’s hot in the center, but cold near the divertor, so that it doesn’t erode the wall too fast. This will be a limiting factor on how long you can run a fusion power plant for before you have to shut it down in order to do maintenance. Ideally, we’d want this to be every 2 years or so, like fission power plants today.
• 7 – Materials for plasma-facing components. We need to develop new materials that can withstand the high temperatures of the wall of a fusion reactor while resisting neutron damage and not becoming too activated by the neutrons that will pass through them. (There is some progress on this front with ferritic steels and silicon carbide.)
• 8 – Magnets that meet the plasma physics requirements and allow reactor maintainability at reasonable costs. (Some of us are working on demountable superconducting coil concepts that may eventually be the solution to this!)
• 9 – Design and materials for tritium fuel cycle and power extraction. Fusion reactors will breed their own tritium fuel from deuterium – this process has to be experimentally tested on a large scale (which will obviously require a burning plasma tokamak).
• 10 – Reliability, availability, maintainability, and inspectability (RAMI) of the reactor designs. We have to show that our concepts for reactors really are as good as we think they can be.

The point is that it’s not a money pit. There are unsolved challenges, but we know what they are, and with adequate support, these challenges will be overcome. This is why we are urging everyone to go to fusionfuture.org and write Congress asking them to keep supporting U.S. fusion research! (It’s very easy – there’s a link at the right on the website.)

6. NIMBY
by GeneralTurgidson

How do you explain the safety/benefits of fusion to a generation of people terrified of nuclear anything?

MIT Researchers:This is where fusion really shines. The two big problems (at least, perceived problems) of fission reactors are the risk of a meltdown, and what you do with the high-level radioactive waste. Fusion has neither of these issues!

Regarding the first, the reason why a worst-case accident in a fission reactor can be so devastating is because there is a lot of fuel in the reactor at any one time. There are well known accidents at Chernobyl (where the reaction ‘ran away’, making more power than the reactor was designed to handle) and Fukushima, where the fission chain reaction was safely shut down, but the cores melted down when the tsunami knocked out the cooling systems, due to ‘decay heat’ which is produced by the used fuel even after shutdown.

In a fusion reactor, it’s a completely different story. There will be less than a gram of fuel in a reactor at any one time—fresh deuterium–tritium fuel is continually added as it is burned—and so a runaway reaction is simply not possible. Decay heat isn’t a problem in a pure fusion system, again because there just isn’t any fuel sitting there undergoing nuclear reactions once the reactor is shut down. In general, this is one area where it’s a benefit that a fusion reaction is so hard to sustain! We have to try really hard to keep the plasma hot enough to undergo fusion in the first place, so if we just turn off the heating and fuelling systems, the fusion reaction will shut down very quickly.

As for the second benefit of fusion (waste), the reaction is completely different from that in a fission reactor. In fission, uranium (or other heavy elements like plutonium) split into pieces, producing hundreds of different isotopes, some of which are radioactive, with half-lives ranging from fractions of a second to millions of years. In fusion, the reaction is simple, deuterium + tritium helium + neutron. So there is no “waste” from the unburned fuel – any tritium that isn’t burned gets pumped out of the chamber and recirculated back in.

This is not to say that there will be no radioactive waste from a fusion plant. The reactor vessel itself will become activated because of the flux of neutrons passing through it, and will have to be treated accordingly when the plant is decommissioned (after, say, a 50-year operational period). But it’s important to note that this kind of radioactive waste is of a much lower level – it won’t have to be stored for very long before it will be “cool” enough to simply bury in the ground safely. And there is active research going on into new materials for fusion reactors that are more resistant to activation by neutrons, such as ferritic steel and silicon carbide.

Finally, fusion has great advantages for nuclear non-proliferation. Creating enough fission power plants to avoid climate change would mean that the plutonium moving around the world would be enough to create about 100,000 nuclear weapons. For fusion, it is much more difficult to use a reactor to make fuel for weapons. This is also something that we think a nuclear-skeptical public will appreciate about fusion power.

All of us are strong supporters of fission power, and we agree that at times, the nuclear power industry has not received a fair shake when compared to other sources of energy. But we think that the advantages of fusion power speak for themselves, and the public will be able to understand the risks and will support the construction of these plants. Obviously, having media that are able to explain things clearly and fairly are a necessity.

8. What do the numbers really look like?
by Erich

ITER is a hugely expensive project, and won't produce a commercially viable power generation system. In a lot of areas where research is done on things which don't work yet -- rockets, bridges, transmission systems, etc -- there's a general idea of how things might be able to "scale up" to meet the goals. Is tokamak fusion really in sight of being a commercially viable source of energy? If we need unobtanium to make a commercially viable reactor, wouldn't it make sense to wait until the materials are viable before making even larger tokamaks? Or is it still worth learning from these new, bigger, more expensive reactors?

MIT Researchers: You are exactly correct in your statement; ITER is an expensive project which will not produce electricity upon completion. However, ITER’s main purpose is not to put watts on the grid, but to demonstrate the scientific feasibility of fusion by creating a Q=10 plasma (10 times as much energy out as we put in). We do have a good idea of how to proceed with devices following ITER, namely DEMO, a full demonstration fusion power plant which will use the steam cycle to generate electricity from the fusion reactor. The basic layout of a reactor can be found here: http://www.fusionfuture.org/what-is-alcator- c-mod/c-mod-for-energy/

Although there is still plenty of research which remains, fusion is in sight of being a commercially viable energy source. We believe that we now understand the physics well enough to create the appropriate plasma conditions (this will be demonstrated on ITER) and we are working on the engineering challenges that lay between us and a commercial fusion reactor.

It is obviously impossible to predict when fusion will put power on the grid since the estimate can change drastically based on demand and overall funding levels. You are however, correct in noting that some of the biggest challenges involve the discovery/ development of materials which can resist the unique and harsh conditions associated with fusion reactors, namely, high heat and neutron fluxes. Due to its importance to the success of future devices, this is a very active and important area of research.

The international fusion community is attempting to address these issues in the following manner: Given the scope of the ITER project and the time required to build and test it, we are planning on constructing a materials testing facility named, IFMIF which stands for International Fusion Materials Irradiation Facility. This facility should be operated at the same time as ITER and will be addressing the materials issues associated with an eventual fusion power plant while ITER is demonstrated the scientific feasibility of a fusion reactor.

Given the time-scales for reactor construction, we think it would be unwise to wait for this materials testing to be complete before starting new machine construction. Addressing the remaining problems in parallel will most likely result in the quickest path to fusion energy.

9. Careers in Fusion?
by benjfowler

As practicing researchers, can you tell us about the health of the pipeline of young researchers coming into the field? Is there a glut of trained physicists at this stage, or is there still a need for trained specialists to enter the field, especially with ITER and follow-on machines coming online in the next couple of decades?

Nathan Howard answers: At this point in time fusion is actually a pretty healthy field in terms of young researchers and with emergence of the next generation devices such as ITER, there should be an influx of researchers stepping up to meet the need for trained specialists on these next gen devices. Currently in Europe and Asia, emphasis on fusion research is ramping up to support the research needs. These newly trained researchers are going to be the scientists working on ITER in 10-15 years.

Unfortunately, the US fusion program is in danger of going the opposite direction of the Asian and European programs. The current proposals made by the US are threatening the health of fusion in the US. The President’s 2013 budget proposal calls for drastic cuts to the domestic fusion budget to pay for increased funding for the ITER budget. However, if these cuts continue, there will not be a field for the young researchers to enter and the US fusion program is in danger of dissolving before ITER comes online.

This does not mean that a need for trained specialist will not remain, it just means that the young researchers in Europe and Asia will be filling these positions. Dr. Stewart Prager, the head of Princeton Plasma Physics Lab said it best, “We have a clear choice before us: The United States can either design and build fusion energy plants or we can buy them from Asia or Europe.”

As a young researcher myself, I am particularly affected by the choices that the US is currently making. Myself and other graduate students have been urging others who support fusion research to contact congress and tell them to continue to fund domestic fusion research. We put together a website, www.fusionfuture.org, which provides more information and people the ability to quickly and easily contact their congressmen to tell them to support research. Please support US fusion research and check out the site.

12. Patents?
by Anonymous Coward

Will patents get in the way of your research?

MIT Researchers: In general, we find that the tokamak labs of the world are extremely cooperative; patents have never been a problem. It does seem likely that the technologies supporting power plants will be highly patentable, but the sort of scientific knowledge we’re accumulating at present really isn’t. At some point, we expect to move from a collaborative to a competitive phase – but we’re not there yet.

11. What level of investment would get fusion going?
by Tragek
Do you think a program the size of the Apollo program could kickstart fusion to general availability? Or would a smaller program suffice?
14. What could you do with unlimited resources?
by petes_PoV
Given $1 trillion, the pick of the best brains in the world to work willingly on the project, a large enough location away from any and all governmental regulation and every facility you could ever need - when would fusion be commercially viable?

MIT Researchers: Questions 11 and 14 are similar and we have answered them together.

Any kind of question asking about a hypothetical massive increase in funding is tricky to answer. We probably couldn’t even spend a trillion dollars if we wanted to – just because it would take a long time to get enough people trained in plasma physics and fusion energy.

We can say this: an increase in funding would allow for different paths to be tried in parallel, like stellarators, tokamaks (ITER), spherical tokamaks, etc. Plus, we could build a facility in the United States to study the problem of plasma–wall interactions, which is a very important topic that has not been adequately studied up to this point (see our answer above about what steps are needed to get to a reactor).

We think that we’re roughly $80-billion away from a reactor. At current levels of funding (worldwide), that’s about 40 years. Even given access to huge amounts of money, it’s unlikely that a working reactor could be built in less than a decade – there are just too many facilities to build between current devices and a full-scale reactor in order to ensure success. But we could certainly do it faster than 40 years!

We want to note that “crash” programs like Apollo or the Manhattan Project succeeded because they took risks – they started work on building their systems before they had done all the homework. That is inherently risky, but these risks are mitigated by pursuing alternatives in parallel. Something similar could be done in fusion, given the money.

15. Your favorite books?
by eldavojohn

I'm not a physicist (software guy), but I've taken a few physics classes. At an early age I found a tattered copy of George Gamow's One Two Three . . . Infinity, which, although incorrect in some parts (I guess that's why they revised it and that's why 'speculations' was in the title), was perfectly written for my then-fifth-grade mind. It set me on a path toward science, and a few weeks ago I saw the same 1960s Viking Press edition and flipped through it, noticing what was slightly off and remembering it. I've since grown to love other obvious books by authors like Hawking, Penrose, Hofstadter, etc. So, quite simply, what are your favorite books for all minds, young and old? Also, can you annotate which are written for the layman's entry into the given field and which are written to encompass the field for the researcher? I find that some books start off with the jargon so strong and the references and footnotes so thick that you start to have to re-read every paragraph, as they're clearly condensing entire historic papers into lengthy sentences. Any fiction books worthy of influencing your work and desires?

Ian Hutchinson: My all time favorite novel is Godric by Fredrick Buechner. It's a wonderful first-person portrait of the prior life of a medieval hermit. My favorite physics teaching text is the Feynman Lectures on Physics, which comes from a remarkable effort by the most widely acclaimed american physicist of the 20th century to explain really advanced physics to undergraduates.

I really don't enjoy the genre of books that combine science popularization with metaphysical speculation. They are of course quite popular, but most are philosophically naive in a way that I find annoying.

Anne White: I like detective/adventure stories. I also enjoy reading plays, poetry and short stories – some authors I read over and over are Wolfgang Borchert, Julio Cortazar, Ray Bradbury and Samuel Beckett.

Recently, I've enjoyed reading The End of the Affair by Graham Greene, People of the Book by Geraldine Brooks, Jane Eyre by Charlotte Bronte and Her Fearful Symmetry by Audrey Niffenegger.

Influential books/stories that I remember reading when I was young : The Pearl (John Steinbeck), Catch-22 (Joseph Heller), Flatland, and Ender's Game.

Dennis Whyte:

• For science non-fiction books, it’s a tie: The Selfish Gene by Richard Dawkins, and Wonderful Life by Stephen Jay Gould.
• Novel (in general subject area of science): The Baroque Cycle by Neal Stephenson
• Speculative fiction: Starship Troopers by Robert Heinlein

Geoff Olynyk: The Making of the Atomic Bomb by Richard Rhodes is the best non-fiction book I’ve ever read. It’s a bit long, but is a fascinating, well-written exploration of the project to develop the atom bomb (both in the U.S. and elsewhere).

This is not a science book, but The Rebel Sell by Joseph Heath and Andrew Potter (sold in the United States as Nation of Rebels) changed my life. I was into counterculture, "culture jamming," anti-advertising, that kind of stuff, and this book made me seriously reconsider all of it. I now understand that trying to be unique is futile in a world of seven billion, and I should just try to be a good person and do good for the world (hence working on fusion!) Potter’s follow-up The Authenticity Hoax, explores the search for authenticity in more detail, but it’s not nearly as good of a book as Rebel Sell.

Nathan Howard: I first became interested in physics by reading about astrophysics. I was specifically interested in black holes and so one of the first books I read (after some of the popular books by Hawking which are written for general audiences, e.g. A Brief History of Time) was a book by Kip S. Thorne called Black Holes and Time Warps. I really enjoyed this book. It did not require much technical background, just some basic mathematics, and it gave good explanations of black holes, relativity, and gravitational waves.

16. Why is fusion more useful than exploiting thorium?
by gestalt_n_pepper

I understand that in the long term, we would want fusion. But we face increasing energy problems over the next 50 years and severe energy problems before 2100. Wouldn't it make sense to allocate research and development resources to something that we know works?

MIT Researchers: First of all, fusion will be putting watts on the grid before 2100. It’s not going to be tomorrow, but it’s not going to be a hundred years, either.

We know how to build thorium fission reactors. It's been done. They have none of the major attractions of a fusion reactor in terms of safety, fuel resources, reduced waste, or non-proliferation. Worldwide thorium fuel resources are about the same as those of uranium. Thorium reactors might become part of the commercial fission reactor mix in the future, but they don't offer transformative possibilities for nuclear power the way fusion does.

That said, we think that the that the scale of the energy/climate problem demands that we (meaning: government and private industry where appropriate) pursue multiple lines of development into new energy sources. Obviously nobody wants to waste taxpayer money, so all proposals have to be evaluated for chance of success – but today, it’s limited by funding more than by a lack of good ideas. This shouldn’t be the case.

The key thing we want to get across is that it shouldn’t be a contest between “fund fusion” or “fund thorium research”. Fusion is extremely important for humankind and should be funded – if thorium fission also has promise, it should be funded too.

17. How is fusion power harnessed?
by circletimessquare

The talk is always about reaching break-even with fusion. What about capturing the power? Are we generating heat that will drive steam turbines? What schemes exist for capture and harnessing the power generated by fusion?

MIT Researchers: In a magnetic fusion reactor, each deuterium-tritium fusion produces a 3.5 MeV (mega- electronvolt) alpha particle (helium nucleus) which deposits its energy in the plasma (this self-heating is how you can have an ‘ignited’ plasma which doesn’t require much or any external heating), and a 14.1 MeV neutron, which deposits its energy in a thick lithium blanket surrounding the toroidal reaction chamber. But in the end, all of it comes out as heat!

For a conservative fusion reactor design, this heat would be removed by a primary cooling loop (high-pressure steam or some sort of liquid metal) which would give the heat to a secondary steam loop (Rankine cycle) in a heat exchanger (steam generator). The steam would then turn a turbine, producing electricity, just like in a fission or coal power plant.

Of course, with a thermal process like a steam cycle, one is always limited by the Carnot efficiency, which increases as the temperature of the high-temperature reservoir goes up. So there are also designs to use a very high-temperature (800–1000 C) gas cooling loop and a Brayton cycle.

But the short answer is: the alpha power is captured by the plasma, and the neutron power is captured by the blanket. It all comes out as heat, which is used to heat a working fluid, which turns a turbine, producing electricity. This is not expected to be a technological problem – the challenge is in getting a confined thermonuclear plasma to produce the fusion energy in the first place!

19. Fusion Milestone Prizes?
by Baldrson

In 1992, with the assistance of fusion technologists such as Robert W. Bussard, I developed legislative language for a series of 12 milestones, each of which would be awarded a $(1992)100M prize for the achievement of objectives toward the attainment of practical fusion energy. This legislation also provided a grace period during which scientists and technologists that had been working on the US fusion program would be provided full salaries, without obligation, during which time they could seek support for their ideas to achieve these milestones. This legislation presaged a number of other prizes including the X-Prize and BAFAR / CATS prize. In 1995, Robert W. Bussard submitted this legislation to all relevant Congressional committees, copying all US plasma physics laboratories. Needless to say, the legislation wasn't passed. Do you think the time is right?

MIT Researchers: We think that the current approach, in which government-funded labs are not in direct competition, but have to justify their funding to the agency (in our case, the DOE), is the best option for the moment. Perhaps the X-PRIZE approach might work for the alternative concepts? (see our answer below regarding Polywell/Dense Plasma Focus/ IEC etc.)

20. ITER
by MpVpRb

Is the ITER project good science? Or is it a politically-motivated, pork-laden boondoggle?

MIT Researchers: ITER is absolutely good science. Governments representing over half the population of the world are backing the project because it is the logical next step – a prototype reactor that will produce ten times more fusion energy than heating power put in, for a few minutes at a time. It is also pushing forward the development of fusion reactor technology (materials, control systems, remote handling systems, etc.). The U.S. fusion community endorsed ITER as the best option for a next-step experiment at the Snowmass II conference in 2002 (see proceedings here).

All of that said, the cost of ITER has risen substantially from the original estimates, and because overall magnetic fusion funding has remained nearly flat in the United States, the U.S. contribution to ITER is threatening to swallow up the entire domestic program. This is starting with the planned closure of Alcator C-Mod in September 2012, but unless more money is allocated to fusion research, all three U.S. tokamak facilities are at risk in the next few years.

Graduate students at Alcator C-Mod have put together a web page explaining the problem: http://www.fusionfuture.org/faq/the-fusion-budget-problem/ and we urge you to go to this website and click the link to contact your member of Congress and urge them to fully fund a strong domestic program and the U.S. contribution to ITER!

21. NIF
by Grond

Is the NIF approach even plausibly capable of generating electricity in a useful way? Or is it purely a research platform / smokescreen for nuclear weapons research?

MIT Researchers: The primary mission of the National Ignition Facility (NIF) is "stockpile stewardship." That is, to ensure that U.S. nuclear weapons continue to be a credible deterrent. This is why NIF is funded by the National Nuclear Security Agency (the agency in charge of the nation’s nuclear stockpile), not the DOE Fusion Energy Sciences program. Thus, the weapons research mission of NIF is not a smokescreen, but is actually the publicly acknowledged primary objective for the facility.

Some researchers at NIF believe that their inertial fusion approach can be used for an energy source as well. We don’t want to speculate here on the plausibility of the LIFE (Laser Inertial Fusion Energy) concept. There is a National Academy of Science review of the prospects of inertial fusion energy under way right now; the final report is expected to be published before the end of this year.

18. Dense Plasma Focus
by mbradmoody
Do you see any merit in the "dense plasma focus" approach to commercial fusion power production, specifically the work of the Lawrenceville Plasma Physics group?
22. Focus Fusion / aneutronic fusion?
by mwk88
Focus Fusion Society is posting research on their project to do aneutronic (e.g. Proton Boron (pB11)) fusion. The concept sounds great, and as an engineer, I find several parts of their design, such as direct extraction of electric power, to be elegant. Is this credible research or pie-in-the-sky? I have not seen much mention of them in mainstream fusion research.
23. Polywell Fusion
by mknewman
What do you think of the efforts at EMC2 Fusion and Polywell Fusion? They seem to be making real, measurable, and open results, but the mainstream physics community seems to ignore this progress.
24. What’s wrong with IECs / Fusor?
by claytongulick
Why aren't IEC reactors based on Farnsworth's designs taken more seriously? From what I understand, IECs have been more effective at producing fusion, and they are cheap to build. People even build them in the garage. From everything I've read, no one really takes the "fusor" seriously in the fusion science realm, and it's considered a dead line of inquiry. I've never understood why.

MIT Researchers: These four questions (18, 22–24) are answered together here.

None of us are experts on inertial electrostatic confinement, magnetized target fusion / dense plasma focus, or Polywells, and so we don’t want to say too much about the specifics of those designs. We can say the following:

1. The amount of money that is being spent, especially in the United States, on fusion is far lower than the field deserves, given its track record and potential. This sounds self-serving, but we think it’s justifiable based on the facts. The graph we posted above shows how the fusion budget is far lower today than it was thirty years ago, even as we continue to make steady progress toward a reactor and the seriousness of the coupled energy/climate problem becomes more obvious.

The alternate confinement concepts program has also seen cuts. (“Alternative” in DOE Fusion Energy Sciences parlance means, basically, anything that isn’t tokamaks, stellarators, or laser [inertial] fusion.) The Levitated Dipole Experiment, an innovative magnetic-confinement arrangement based on planetary magnetic fields, was cancelled just as they were about to add significant auxiliary heating for the first time. And these small-scale alternative confinement projects are not very expensive! Some of these alternative concepts may very well be promising and deserve taxpayer money to be developed.

2. But on the other hand, these groups need to show that they deserve funding. It’s not enough to just tease these promising results and be secretive about the methods or technologies. Public funding can only come when the details are published in the open literature, and subjected to the scrutiny of peer review and the wider community reading the papers. The (hot) fusion community is still living with the aftermath of the cold fusion scandal from a quarter century ago - so it’s very important for the proponents of these alternate concepts to push the researchers to publish their results in peer-reviewed journals. Whatever negatives the tokamak might have, one thing you can’t say about it is that the research has been too secretive, and this has allowed the funding agencies to make the judgement that the tokamak is currently the most promising route to a fusion reactor, which is why this line of research gets the most money.

Special thanks to Dr. Martin Greenwald, Prof. Ian Hutchinson, Asst. Prof. Anne White, Prof. Dennis Whyte, Nathan Howard, and Geoff Olynyk for taking the time to answer our questions.

You recently got the chance to ask a group of MIT researchers questions about fusion power, and they've now finished writing some incredibly detailed answers. They discuss the things we've learned about fusion in the past decade, how long it's likely to take for fusion to power your home, the biggest problems fusion researchers are working to solve, and why it's important to continue funding fusion projects. They also delve into the specifics of tokamak operation, like dealing with disruption events and the limitations on reactor size, and provide some insight into fusion as a career. Hit the link below for a wealth of information about fusion. 1. What have we learned?
by jank1887

Fusion is one of those technologies that is always '50 years away,’ even 50 years ago, maybe even 50 years from now. So, looking at what's actually happened recently: What do we actually know now that we didn't know 10-15 years ago that gives support to the notion that we're making progress? Or, what are the 'big' things we know now? Similarly, what are the things we still don't know that we could reasonably expect to find answers for in the next 10-15 years?

MIT Researchers: As researchers in this field, we have heard the expression "Fusion is 50 years away and always will be" more times than we would like to admit. The implication of this statement is that no real progress has been made in the field, which is simply not true! We have made a great deal of progress, even in the last 10–15 years (which have been very lean times for funding). We’ll try to summarize some of the new findings, in no particular order:

1) Internal Transport Barriers/Reversed Shear operation –
We have actually discovered a way to improve upon the performance that we get in H- mode plasmas. These improvements come in the form for so called internal transport barriers. In the past 10–15 years we have begun to understand how to modify the current flowing in tokamak plasmas so that we create effectively what is a barrier in the middle of the plasma. Like the edge barrier in H-mode plasmas, this barrier restricts particles and energy from escaping the plasma and enhancing the overall performance. 2) I-mode –
In just the last 5 years, a new operational regime has been discovered on the Alcator C-Mod tokamak at MIT. This is termed the I-mode, or “Improved L-mode” regime. When the tokamak is operated in this manner it exhibits excellent energy confinement properties, keeping the plasma hot. At the same time the plasma does an excellent job of expelling impurities which dilute the fusion fuel and reduce the number of fusion reactions which can occur. It is particularly important to us as it was first observed on Alcator C-Mod, and is now under active development at many other tokamaks around the world.

3) Development of Predictive Models –
Great advances have been made in the development of predictive computer models, such as gyrokinetic and magnetohydrodynamic (MHD) formulations. Years of experiments have revealed that plasma turbulence is often primarily responsible for the loss of particles and energy from fusion reactors. In the past 10–15 years we have developed advanced models which are thought to contain sufficient physics to simulate plasma turbulence and predict the performance of future fusion devices. At this time we are in the process of validating these models, i.e. comparing them directly with experiment to ensure they are correct, but we are approaching the ability to reliably predict the performance of fusion plasmas without the need for a fusion reactor. This can motivate engineering design and operational choices for future fusion devices.

4) Self-acceleration of the plasma (intrinsic rotation) –
Over the past decade, it has been discovered (on Alcator C-Mod and elsewhere) that plasmas can spontaneously rotate, at speeds of tens of kilometers per second. (Imagine the donut-shaped plasma spinning on its axis.) This turns out to have beneficial effects for stabilizing turbulence at the edge, as the spinning plasma causes the turbulent eddies to break up before they can carry hot plasma out of the core. This is an exciting area of research that could have big implications for the performance of a tokamak reactor.

5) Disruption mitigation –
One of the main problems with a tokamak is the ‘disruption’, when the plasma energy is suddenly lost, stopping any fusion that is occurring and requiring a restart of the reactor. (See the question below for a lot of detail about this!) In extreme cases, these disruptions can cause damage to the wall of the tokamak – which would require repairs before the machine can be restarted. Over the past decade, we have developed techniques to mitigate these disruptions, causing the plasma to come to a rapid shutdown that does not negatively affect the wall condition. Work is underway to scale these techniques up to a reactor-size device (ITER).

6) ELM control/avoidance –
Another longstanding problem with tokamaks is periodic ‘bursts’ of energy from the edge called Edge-Localized Modes (ELMs). In today’s devices, ELMs are not a problem, but in ITER and future reactors, they could carry enough energy to damage the wall in the divertor region (where most of the energy comes out). There has been rapid progress lately (past 15 years) in ways to control these ELMs by making them more rapid and smaller, such as using resonant magnetic perturbation (RMP) coils to distort the shape of the confining magnetic field, or ‘pellet pacing’ (firing small pellets of deuterium fuel into the machine 50–60 times per second, which triggers an ELM), or vertical ‘kicks’ in which the control system suddenly jogs the plasma position a few centimeters vertically, also triggering an ELM. Between these techniques and the recently discovered I-mode (which doesn’t have ELMs), this is a problem that is well on the way to being solved.

7) High-Z walls –
This is a particular point of pride for Alcator C-Mod. Running a tokamak with walls made of refractory metals has many advantages because of the extreme capacity of these materials to absorb heat loads, but there are disadvantages as well, such as how radiative these high atomic number elements are if they get into the plasma as impurities, or how metallic materials distort when they melt, rather than ablating like carbon-fiber composites. Alcator C-Mod (which has a molybdenum wall) and other tokamaks have recently shown that it is possible to reliably run a tokamak with high-Z refractory metal walls, which will almost certainly be a feature of future reactors.

2. Power Loss Scenario in Alcator C-Mod?
by eldavojohn

Not to raise any fears -- rather out of genuine curiosity -- what happens when the magnetic fields that hold the 90,000,000 degrees Celsius plasma in place fail or loser power on the Alcator C-Mod? I understand it's probably in prototype mode, but what sort of safety advantages or disadvantages do Alcator C-Mod designs offer over conventional, large-scale designs? Does the plasma come into contact with the toroidal superconducting coil? Then what?

Geoff Olynyk answers: Actually, that’s exactly what my research is on! The event you describe is called a "disruption." Holding a hot plasma stationary using magnetic fields without it ever touching material surfaces is very difficult – Richard Feynman once compared it to trying to "hold Jello with rubber bands." For any number of reasons, like a magnetic coil losing power, the control system not being able to juggle the plasma position quickly enough, or the plasma hitting a stability limit (pressure or density goes too high), it’s possible for the plasma to hit the wall. The most important thing to know, though, is that when this occurs (and it does, frequently, in today’s experiments – although it’ll have to be a very rare occurrence in a real power reactor so it produces uninterrupted electricity), it is no risk to the environment or to safety.

To understand what happens, you have to realize that the plasma is very, very light. In the Alcator C-Mod tokamak, it has a mass of only about 0.001 grams – about one- fiftieth as much as the smallest drop of water you can get from an eyedropper. (This is with a plasma volume of about a cubic meter – a fusion plasma is actually a pretty good vacuum!) So even though it’s very hot, it doesn’t actually have a lot of thermal stored energy to flow into the wall if confinement is suddenly lost. There is actually more energy stored in the current flowing in the plasma (in C-Mod, about a million amperes), which also gets deposited on the wall. In C-Mod, thermal stored energy is about 50– 150 kJ and magnetic stored energy is almost 1000 kJ. The problem is that as we go to larger machines (like ITER, or a reactor), the amount of stored energy in the plasma scales like the cube of the size, and the wall area only scales like the square of the size. So the energy deposited per square meter of wall area gets worse (larger) as we go up in machine size.

The plasma doesn’t hit the superconducting coils - it hits (really, deposits its energy on) the “first wall” of the chamber closest to the plasma. So, we do two things to make sure that the walls can survive these disruption events. The first is making them out of materials that can take a blast of heat, like tungsten, or else materials that ablate away rather than melting, like carbon fiber composites. The second is to develop “disruption mitigation” systems which can cause the plasma to radiate all its energy evenly over the entire wall surface, spreading the heat out and lessening the chance of causing localized melting. But I want to stress again - disruptions are an operational problem, meaning they might cause a power plant to be offline for a while, but they’re not a safety problem. There is no chance of a runaway reaction or meltdown in a fusion reactor.

3. Ubiquitous Fusion Power
by monsted

When will fusion power my house (or vehicle)?

MIT Researchers: This is obviously an impossible question to answer, but we can give some thoughts about when it might happen, and why. First, the current official plan is that ITER will demonstrate net fusion gain (Q = 10, that is, ten times more fusion power out than heating power put in) in about 2028 or 2029. (Construction will be done by about 2022 but there’s a six-year shakedown process of steadily increasing the power and learning how to run the machine before the full-power fusion shots.) At that point, designs can begin for a “DEMO”, which is the fusion community’s term for a demonstration power plant. That would come online around 2040 (and would putt watts on the grid, although probably at an economic loss at first), and would be followed by (profitable, economic) commercial plants around 2050.

This seems like a long time, and it is, but it’s important to understand that this is not the only possible path. You might say that we’re not a certain number of years away from a working fusion power plant, but rather about $80-billion away (in worldwide funding). We’ll get into this more in response to one of the other questions, but there are other experiments that could be done in parallel with ITER that would certainly speed up the goal of a demonstration power plant, if there were the money for it. Here is a graph based on a 1976 ERDA (predecessor to today’s DOE) fusion development plan, showing their four paths to a reactor, as well as a business-as-usual funding case that would never lead to a reactor, and in black is the actual funding amounts. (All values are adjusted to 2012 dollars.)

In the U.S. at least, fusion funding hasn’t been anywhere close to what would be required for a “crash program” to get to a reactor. If it were, it would probably be possible to have a demonstration reactor in about twenty years. (This is not actually that long - given that it takes almost a decade to build a large fission reactor or hydroelectric dam!)

Fusion has a reputation of “always being thirty years away” (or fifty, or twenty). We want to address that head-on here: aside from a few over-optimistic predictions made in the very early days of magnetic fusion research (the 1950s), this reputation is undeserved. The reason it has taken so long to get to breakeven (ITER) is because since the end of the 1970s, funding for fusion research has been continually slashed, up to today, when the U.S. is proposing shuttering one of three remaining tokamak experiments, the Alcator C-Mod device at MIT that we all work on. Despite this, progress has been continuous. But if we had the money, we would be getting there quicker.

4. What are the economic numbers for a successful, commercial reactor?
by kestasjk
I know that the economics of larger reactor = more economical are well known with tokamaks. Does this mean you have a good idea of the minimum cost / generating capacity of the first commercial reactors? If so, what do those numbers look like?
7. Lower Limit on Tokamak Design
by gyepi
Are there any good guesstimates on how small a tokamak-based fusion reactor (which produces more energy than it consumes) can become? Theoretical limitations on the size of the reactor would have obvious implications for pragmatic issues.

MIT Researchers: Questions 4 and 7 are similar and we answer them together here.

The current thinking is that a tokamak fusion reactor will be about 1 gigawatt electrical, and about 2–3.5 gigawatts thermal (depending on how high-temperature the blanket is and thus how thermally efficient it can be). This is about the size of a current fission reactor or large coal-fired power plant.

Fusion researchers are working on smaller designs, though! At MIT, some students are working on a concept for a 350–500 MW (thermal) class fusion reactor, which would be cheaper to field and thus more likely to be built by private industry with limited access to capital. This is still early work, though, and the economic analysis is not done yet.

Cost estimates for a new technology like fusion cannot be terribly reliable, but several studies suggest that, with suitable developments in science and technology, the costs could be competitive with other methods of electricity generation. We recommend you read the ARIES-AT study (google it), which goes through all the factors that go into the cost of electricity (COE) for a fusion reactor, and compares their concept to other electricity generation options (fission, fossil fuels, etc.) A key advantage of fusion is in what economists call "external costs." These are costs borne by society as a whole and not by the generating industry. Environmental pollution, nuclear proliferation, and military operations to protect oil supplies are all examples of external costs for energy.

5. What Problems are Holding Back Successful Reactions?
by Bucc5062

Can you explain to a non-scientist what the biggest stumbling blocks are for an effective fusion reaction? Is it truly a matter of throwing money down an energy hole, or are there verifiable, measurable benchmarks that lead us from one step to the next? I.e. we’ve achieved X, now we need Y; when we get Y, we get Z and then achieve fusion. Is it the technology holding us back, the politics, or the science?

MIT Researchers: We know exactly what we need to do. Not everything has a solution yet – that’s why it’s still a research project! – but we generally know what the big challenges are to get to a working magnetic fusion reactor. Here is a non-exhaustive list:

• 1 – Non-inductive current drive. We can’t rely on inductors to drive the plasma current since they are inherently pulsed (not steady-state). We think that lower hybrid current drive might be the solution, and are actively researching this on Alcator C-Mod.
• 2 – Confining a 'burning plasma.' This is the big question that ITER will resolve – can we really confine a plasma that is dominantly self-heated – that is, most of its energy comes from fusion reactions rather than external heating. Will new instabilities appear? Or can we confine the plasma as we expect we can.
• 3 – Confining a steady-state burning plasma while avoiding off-normal events. We have to do both of the previous points at the same time! And we can’t have disruptions too often or else the power plant won’t have a high enough duty factor. The goal is to have disruptions (which require a shutdown) occur less than once per year.
• 4 – Validated predictive capability for fusion-grade plasmas. We have made great progress in this field already (see our answer to an earlier question), but it’s not at the point yet that, say, fluid mechanics codes are, where Boeing can design an entire plane in the computer before ever building a scale model. We need our models of fusion plasma behavior to be accurate and reliable enough to design first-of-a-kind machines that we are 100% sure will work the way we think they will.
• 5 – Diagnosing a burning plasma. It’s really hard to tell any of the properties of the plasma even today, when we use pure deuterium fuel (instead of ‘live’ deuterium–tritium fuel), and our plasmas are colder than they would be in a reactor! You can’t, for example, stick a thermometer in to tell the temperature! We have to use subtle effects like bouncing a laser beam off the electrons and telling the temperature from the Doppler shift of the laser from the moving electrons (a technique known as Thomson scattering). Making these diagnostics work in the reactor environment, with higher plasma temperatures and a ferocious flux of neutrons coming out, is a great challenge.
• 6 – Better understanding of plasma–wall interactions. The plasma is confined by magnetic fields, and ideally doesn’t touch the wall at all, except in a very small area called the divertor. This means that the material challenges in the divertor are severe – we have to figure out a way to operate the plasma so that it’s hot in the center, but cold near the divertor, so that it doesn’t erode the wall too fast. This will be a limiting factor on how long you can run a fusion power plant for before you have to shut it down in order to do maintenance. Ideally, we’d want this to be every 2 years or so, like fission power plants today.
• 7 – Materials for plasma-facing components. We need to develop new materials that can withstand the high temperatures of the wall of a fusion reactor while resisting neutron damage and not becoming too activated by the neutrons that will pass through them. (There is some progress on this front with ferritic steels and silicon carbide.)
• 8 – Magnets that meet the plasma physics requirements and allow reactor maintainability at reasonable costs. (Some of us are working on demountable superconducting coil concepts that may eventually be the solution to this!)
• 9 – Design and materials for tritium fuel cycle and power extraction. Fusion reactors will breed their own tritium fuel from deuterium – this process has to be experimentally tested on a large scale (which will obviously require a burning plasma tokamak).
• 10 – Reliability, availability, maintainability, and inspectability (RAMI) of the reactor designs. We have to show that our concepts for reactors really are as good as we think they can be.

The point is that it’s not a money pit. There are unsolved challenges, but we know what they are, and with adequate support, these challenges will be overcome. This is why we are urging everyone to go to fusionfuture.org and write Congress asking them to keep supporting U.S. fusion research! (It’s very easy – there’s a link at the right on the website.)

6. NIMBY
by GeneralTurgidson

How do you explain the safety/benefits of fusion to a generation of people terrified of nuclear anything?

MIT Researchers:This is where fusion really shines. The two big problems (at least, perceived problems) of fission reactors are the risk of a meltdown, and what you do with the high-level radioactive waste. Fusion has neither of these issues!

Regarding the first, the reason why a worst-case accident in a fission reactor can be so devastating is because there is a lot of fuel in the reactor at any one time. There are well known accidents at Chernobyl (where the reaction ‘ran away’, making more power than the reactor was designed to handle) and Fukushima, where the fission chain reaction was safely shut down, but the cores melted down when the tsunami knocked out the cooling systems, due to ‘decay heat’ which is produced by the used fuel even after shutdown.

In a fusion reactor, it’s a completely different story. There will be less than a gram of fuel in a reactor at any one time—fresh deuterium–tritium fuel is continually added as it is burned—and so a runaway reaction is simply not possible. Decay heat isn’t a problem in a pure fusion system, again because there just isn’t any fuel sitting there undergoing nuclear reactions once the reactor is shut down. In general, this is one area where it’s a benefit that a fusion reaction is so hard to sustain! We have to try really hard to keep the plasma hot enough to undergo fusion in the first place, so if we just turn off the heating and fuelling systems, the fusion reaction will shut down very quickly.

As for the second benefit of fusion (waste), the reaction is completely different from that in a fission reactor. In fission, uranium (or other heavy elements like plutonium) split into pieces, producing hundreds of different isotopes, some of which are radioactive, with half-lives ranging from fractions of a second to millions of years. In fusion, the reaction is simple, deuterium + tritium helium + neutron. So there is no “waste” from the unburned fuel – any tritium that isn’t burned gets pumped out of the chamber and recirculated back in.

This is not to say that there will be no radioactive waste from a fusion plant. The reactor vessel itself will become activated because of the flux of neutrons passing through it, and will have to be treated accordingly when the plant is decommissioned (after, say, a 50-year operational period). But it’s important to note that this kind of radioactive waste is of a much lower level – it won’t have to be stored for very long before it will be “cool” enough to simply bury in the ground safely. And there is active research going on into new materials for fusion reactors that are more resistant to activation by neutrons, such as ferritic steel and silicon carbide.

Finally, fusion has great advantages for nuclear non-proliferation. Creating enough fission power plants to avoid climate change would mean that the plutonium moving around the world would be enough to create about 100,000 nuclear weapons. For fusion, it is much more difficult to use a reactor to make fuel for weapons. This is also something that we think a nuclear-skeptical public will appreciate about fusion power.

All of us are strong supporters of fission power, and we agree that at times, the nuclear power industry has not received a fair shake when compared to other sources of energy. But we think that the advantages of fusion power speak for themselves, and the public will be able to understand the risks and will support the construction of these plants. Obviously, having media that are able to explain things clearly and fairly are a necessity.

8. What do the numbers really look like?
by Erich

ITER is a hugely expensive project, and won't produce a commercially viable power generation system. In a lot of areas where research is done on things which don't work yet -- rockets, bridges, transmission systems, etc -- there's a general idea of how things might be able to "scale up" to meet the goals. Is tokamak fusion really in sight of being a commercially viable source of energy? If we need unobtanium to make a commercially viable reactor, wouldn't it make sense to wait until the materials are viable before making even larger tokamaks? Or is it still worth learning from these new, bigger, more expensive reactors?

MIT Researchers: You are exactly correct in your statement; ITER is an expensive project which will not produce electricity upon completion. However, ITER’s main purpose is not to put watts on the grid, but to demonstrate the scientific feasibility of fusion by creating a Q=10 plasma (10 times as much energy out as we put in). We do have a good idea of how to proceed with devices following ITER, namely DEMO, a full demonstration fusion power plant which will use the steam cycle to generate electricity from the fusion reactor. The basic layout of a reactor can be found here: http://www.fusionfuture.org/what-is-alcator- c-mod/c-mod-for-energy/

Although there is still plenty of research which remains, fusion is in sight of being a commercially viable energy source. We believe that we now understand the physics well enough to create the appropriate plasma conditions (this will be demonstrated on ITER) and we are working on the engineering challenges that lay between us and a commercial fusion reactor.

It is obviously impossible to predict when fusion will put power on the grid since the estimate can change drastically based on demand and overall funding levels. You are however, correct in noting that some of the biggest challenges involve the discovery/ development of materials which can resist the unique and harsh conditions associated with fusion reactors, namely, high heat and neutron fluxes. Due to its importance to the success of future devices, this is a very active and important area of research.

The international fusion community is attempting to address these issues in the following manner: Given the scope of the ITER project and the time required to build and test it, we are planning on constructing a materials testing facility named, IFMIF which stands for International Fusion Materials Irradiation Facility. This facility should be operated at the same time as ITER and will be addressing the materials issues associated with an eventual fusion power plant while ITER is demonstrated the scientific feasibility of a fusion reactor.

Given the time-scales for reactor construction, we think it would be unwise to wait for this materials testing to be complete before starting new machine construction. Addressing the remaining problems in parallel will most likely result in the quickest path to fusion energy.

9. Careers in Fusion?
by benjfowler

As practicing researchers, can you tell us about the health of the pipeline of young researchers coming into the field? Is there a glut of trained physicists at this stage, or is there still a need for trained specialists to enter the field, especially with ITER and follow-on machines coming online in the next couple of decades?

Nathan Howard answers: At this point in time fusion is actually a pretty healthy field in terms of young researchers and with emergence of the next generation devices such as ITER, there should be an influx of researchers stepping up to meet the need for trained specialists on these next gen devices. Currently in Europe and Asia, emphasis on fusion research is ramping up to support the research needs. These newly trained researchers are going to be the scientists working on ITER in 10-15 years.

Unfortunately, the US fusion program is in danger of going the opposite direction of the Asian and European programs. The current proposals made by the US are threatening the health of fusion in the US. The President’s 2013 budget proposal calls for drastic cuts to the domestic fusion budget to pay for increased funding for the ITER budget. However, if these cuts continue, there will not be a field for the young researchers to enter and the US fusion program is in danger of dissolving before ITER comes online.

This does not mean that a need for trained specialist will not remain, it just means that the young researchers in Europe and Asia will be filling these positions. Dr. Stewart Prager, the head of Princeton Plasma Physics Lab said it best, “We have a clear choice before us: The United States can either design and build fusion energy plants or we can buy them from Asia or Europe.”

As a young researcher myself, I am particularly affected by the choices that the US is currently making. Myself and other graduate students have been urging others who support fusion research to contact congress and tell them to continue to fund domestic fusion research. We put together a website, www.fusionfuture.org, which provides more information and people the ability to quickly and easily contact their congressmen to tell them to support research. Please support US fusion research and check out the site.

12. Patents?
by Anonymous Coward

Will patents get in the way of your research?

MIT Researchers: In general, we find that the tokamak labs of the world are extremely cooperative; patents have never been a problem. It does seem likely that the technologies supporting power plants will be highly patentable, but the sort of scientific knowledge we’re accumulating at present really isn’t. At some point, we expect to move from a collaborative to a competitive phase – but we’re not there yet.

11. What level of investment would get fusion going?
by Tragek
Do you think a program the size of the Apollo program could kickstart fusion to general availability? Or would a smaller program suffice?
14. What could you do with unlimited resources?
by petes_PoV
Given $1 trillion, the pick of the best brains in the world to work willingly on the project, a large enough location away from any and all governmental regulation and every facility you could ever need - when would fusion be commercially viable?

MIT Researchers: Questions 11 and 14 are similar and we have answered them together.

Any kind of question asking about a hypothetical massive increase in funding is tricky to answer. We probably couldn’t even spend a trillion dollars if we wanted to – just because it would take a long time to get enough people trained in plasma physics and fusion energy.

We can say this: an increase in funding would allow for different paths to be tried in parallel, like stellarators, tokamaks (ITER), spherical tokamaks, etc. Plus, we could build a facility in the United States to study the problem of plasma–wall interactions, which is a very important topic that has not been adequately studied up to this point (see our answer above about what steps are needed to get to a reactor).

We think that we’re roughly $80-billion away from a reactor. At current levels of funding (worldwide), that’s about 40 years. Even given access to huge amounts of money, it’s unlikely that a working reactor could be built in less than a decade – there are just too many facilities to build between current devices and a full-scale reactor in order to ensure success. But we could certainly do it faster than 40 years!

We want to note that “crash” programs like Apollo or the Manhattan Project succeeded because they took risks – they started work on building their systems before they had done all the homework. That is inherently risky, but these risks are mitigated by pursuing alternatives in parallel. Something similar could be done in fusion, given the money.

15. Your favorite books?
by eldavojohn

I'm not a physicist (software guy), but I've taken a few physics classes. At an early age I found a tattered copy of George Gamow's One Two Three . . . Infinity, which, although incorrect in some parts (I guess that's why they revised it and that's why 'speculations' was in the title), was perfectly written for my then-fifth-grade mind. It set me on a path toward science, and a few weeks ago I saw the same 1960s Viking Press edition and flipped through it, noticing what was slightly off and remembering it. I've since grown to love other obvious books by authors like Hawking, Penrose, Hofstadter, etc. So, quite simply, what are your favorite books for all minds, young and old? Also, can you annotate which are written for the layman's entry into the given field and which are written to encompass the field for the researcher? I find that some books start off with the jargon so strong and the references and footnotes so thick that you start to have to re-read every paragraph, as they're clearly condensing entire historic papers into lengthy sentences. Any fiction books worthy of influencing your work and desires?

Ian Hutchinson: My all time favorite novel is Godric by Fredrick Buechner. It's a wonderful first-person portrait of the prior life of a medieval hermit. My favorite physics teaching text is the Feynman Lectures on Physics, which comes from a remarkable effort by the most widely acclaimed american physicist of the 20th century to explain really advanced physics to undergraduates.

I really don't enjoy the genre of books that combine science popularization with metaphysical speculation. They are of course quite popular, but most are philosophically naive in a way that I find annoying.

Anne White: I like detective/adventure stories. I also enjoy reading plays, poetry and short stories – some authors I read over and over are Wolfgang Borchert, Julio Cortazar, Ray Bradbury and Samuel Beckett.

Recently, I've enjoyed reading The End of the Affair by Graham Greene, People of the Book by Geraldine Brooks, Jane Eyre by Charlotte Bronte and Her Fearful Symmetry by Audrey Niffenegger.

Influential books/stories that I remember reading when I was young : The Pearl (John Steinbeck), Catch-22 (Joseph Heller), Flatland, and Ender's Game.

Dennis Whyte:

• For science non-fiction books, it’s a tie: The Selfish Gene by Richard Dawkins, and Wonderful Life by Stephen Jay Gould.
• Novel (in general subject area of science): The Baroque Cycle by Neal Stephenson
• Speculative fiction: Starship Troopers by Robert Heinlein

Geoff Olynyk: The Making of the Atomic Bomb by Richard Rhodes is the best non-fiction book I’ve ever read. It’s a bit long, but is a fascinating, well-written exploration of the project to develop the atom bomb (both in the U.S. and elsewhere).

This is not a science book, but The Rebel Sell by Joseph Heath and Andrew Potter (sold in the United States as Nation of Rebels) changed my life. I was into counterculture, "culture jamming," anti-advertising, that kind of stuff, and this book made me seriously reconsider all of it. I now understand that trying to be unique is futile in a world of seven billion, and I should just try to be a good person and do good for the world (hence working on fusion!) Potter’s follow-up The Authenticity Hoax, explores the search for authenticity in more detail, but it’s not nearly as good of a book as Rebel Sell.

Nathan Howard: I first became interested in physics by reading about astrophysics. I was specifically interested in black holes and so one of the first books I read (after some of the popular books by Hawking which are written for general audiences, e.g. A Brief History of Time) was a book by Kip S. Thorne called Black Holes and Time Warps. I really enjoyed this book. It did not require much technical background, just some basic mathematics, and it gave good explanations of black holes, relativity, and gravitational waves.

16. Why is fusion more useful than exploiting thorium?
by gestalt_n_pepper

I understand that in the long term, we would want fusion. But we face increasing energy problems over the next 50 years and severe energy problems before 2100. Wouldn't it make sense to allocate research and development resources to something that we know works?

MIT Researchers: First of all, fusion will be putting watts on the grid before 2100. It’s not going to be tomorrow, but it’s not going to be a hundred years, either.

We know how to build thorium fission reactors. It's been done. They have none of the major attractions of a fusion reactor in terms of safety, fuel resources, reduced waste, or non-proliferation. Worldwide thorium fuel resources are about the same as those of uranium. Thorium reactors might become part of the commercial fission reactor mix in the future, but they don't offer transformative possibilities for nuclear power the way fusion does.

That said, we think that the that the scale of the energy/climate problem demands that we (meaning: government and private industry where appropriate) pursue multiple lines of development into new energy sources. Obviously nobody wants to waste taxpayer money, so all proposals have to be evaluated for chance of success – but today, it’s limited by funding more than by a lack of good ideas. This shouldn’t be the case.

The key thing we want to get across is that it shouldn’t be a contest between “fund fusion” or “fund thorium research”. Fusion is extremely important for humankind and should be funded – if thorium fission also has promise, it should be funded too.

17. How is fusion power harnessed?
by circletimessquare

The talk is always about reaching break-even with fusion. What about capturing the power? Are we generating heat that will drive steam turbines? What schemes exist for capture and harnessing the power generated by fusion?

MIT Researchers: In a magnetic fusion reactor, each deuterium-tritium fusion produces a 3.5 MeV (mega- electronvolt) alpha particle (helium nucleus) which deposits its energy in the plasma (this self-heating is how you can have an ‘ignited’ plasma which doesn’t require much or any external heating), and a 14.1 MeV neutron, which deposits its energy in a thick lithium blanket surrounding the toroidal reaction chamber. But in the end, all of it comes out as heat!

For a conservative fusion reactor design, this heat would be removed by a primary cooling loop (high-pressure steam or some sort of liquid metal) which would give the heat to a secondary steam loop (Rankine cycle) in a heat exchanger (steam generator). The steam would then turn a turbine, producing electricity, just like in a fission or coal power plant.

Of course, with a thermal process like a steam cycle, one is always limited by the Carnot efficiency, which increases as the temperature of the high-temperature reservoir goes up. So there are also designs to use a very high-temperature (800–1000 C) gas cooling loop and a Brayton cycle.

But the short answer is: the alpha power is captured by the plasma, and the neutron power is captured by the blanket. It all comes out as heat, which is used to heat a working fluid, which turns a turbine, producing electricity. This is not expected to be a technological problem – the challenge is in getting a confined thermonuclear plasma to produce the fusion energy in the first place!

19. Fusion Milestone Prizes?
by Baldrson

In 1992, with the assistance of fusion technologists such as Robert W. Bussard, I developed legislative language for a series of 12 milestones, each of which would be awarded a $(1992)100M prize for the achievement of objectives toward the attainment of practical fusion energy. This legislation also provided a grace period during which scientists and technologists that had been working on the US fusion program would be provided full salaries, without obligation, during which time they could seek support for their ideas to achieve these milestones. This legislation presaged a number of other prizes including the X-Prize and BAFAR / CATS prize. In 1995, Robert W. Bussard submitted this legislation to all relevant Congressional committees, copying all US plasma physics laboratories. Needless to say, the legislation wasn't passed. Do you think the time is right?

MIT Researchers: We think that the current approach, in which government-funded labs are not in direct competition, but have to justify their funding to the agency (in our case, the DOE), is the best option for the moment. Perhaps the X-PRIZE approach might work for the alternative concepts? (see our answer below regarding Polywell/Dense Plasma Focus/ IEC etc.)

20. ITER
by MpVpRb

Is the ITER project good science? Or is it a politically-motivated, pork-laden boondoggle?

MIT Researchers: ITER is absolutely good science. Governments representing over half the population of the world are backing the project because it is the logical next step – a prototype reactor that will produce ten times more fusion energy than heating power put in, for a few minutes at a time. It is also pushing forward the development of fusion reactor technology (materials, control systems, remote handling systems, etc.). The U.S. fusion community endorsed ITER as the best option for a next-step experiment at the Snowmass II conference in 2002 (see proceedings here).

All of that said, the cost of ITER has risen substantially from the original estimates, and because overall magnetic fusion funding has remained nearly flat in the United States, the U.S. contribution to ITER is threatening to swallow up the entire domestic program. This is starting with the planned closure of Alcator C-Mod in September 2012, but unless more money is allocated to fusion research, all three U.S. tokamak facilities are at risk in the next few years.

Graduate students at Alcator C-Mod have put together a web page explaining the problem: http://www.fusionfuture.org/faq/the-fusion-budget-problem/ and we urge you to go to this website and click the link to contact your member of Congress and urge them to fully fund a strong domestic program and the U.S. contribution to ITER!

21. NIF
by Grond

Is the NIF approach even plausibly capable of generating electricity in a useful way? Or is it purely a research platform / smokescreen for nuclear weapons research?

MIT Researchers: The primary mission of the National Ignition Facility (NIF) is "stockpile stewardship." That is, to ensure that U.S. nuclear weapons continue to be a credible deterrent. This is why NIF is funded by the National Nuclear Security Agency (the agency in charge of the nation’s nuclear stockpile), not the DOE Fusion Energy Sciences program. Thus, the weapons research mission of NIF is not a smokescreen, but is actually the publicly acknowledged primary objective for the facility.

Some researchers at NIF believe that their inertial fusion approach can be used for an energy source as well. We don’t want to speculate here on the plausibility of the LIFE (Laser Inertial Fusion Energy) concept. There is a National Academy of Science review of the prospects of inertial fusion energy under way right now; the final report is expected to be published before the end of this year.

18. Dense Plasma Focus
by mbradmoody
Do you see any merit in the "dense plasma focus" approach to commercial fusion power production, specifically the work of the Lawrenceville Plasma Physics group?
22. Focus Fusion / aneutronic fusion?
by mwk88
Focus Fusion Society is posting research on their project to do aneutronic (e.g. Proton Boron (pB11)) fusion. The concept sounds great, and as an engineer, I find several parts of their design, such as direct extraction of electric power, to be elegant. Is this credible research or pie-in-the-sky? I have not seen much mention of them in mainstream fusion research.
23. Polywell Fusion
by mknewman
What do you think of the efforts at EMC2 Fusion and Polywell Fusion? They seem to be making real, measurable, and open results, but the mainstream physics community seems to ignore this progress.
24. What’s wrong with IECs / Fusor?
by claytongulick
Why aren't IEC reactors based on Farnsworth's designs taken more seriously? From what I understand, IECs have been more effective at producing fusion, and they are cheap to build. People even build them in the garage. From everything I've read, no one really takes the "fusor" seriously in the fusion science realm, and it's considered a dead line of inquiry. I've never understood why.

MIT Researchers: These four questions (18, 22–24) are answered together here.

None of us are experts on inertial electrostatic confinement, magnetized target fusion / dense plasma focus, or Polywells, and so we don’t want to say too much about the specifics of those designs. We can say the following:

1. The amount of money that is being spent, especially in the United States, on fusion is far lower than the field deserves, given its track record and potential. This sounds self-serving, but we think it’s justifiable based on the facts. The graph we posted above shows how the fusion budget is far lower today than it was thirty years ago, even as we continue to make steady progress toward a reactor and the seriousness of the coupled energy/climate problem becomes more obvious.

The alternate confinement concepts program has also seen cuts. (“Alternative” in DOE Fusion Energy Sciences parlance means, basically, anything that isn’t tokamaks, stellarators, or laser [inertial] fusion.) The Levitated Dipole Experiment, an innovative magnetic-confinement arrangement based on planetary magnetic fields, was cancelled just as they were about to add significant auxiliary heating for the first time. And these small-scale alternative confinement projects are not very expensive! Some of these alternative concepts may very well be promising and deserve taxpayer money to be developed.

2. But on the other hand, these groups need to show that they deserve funding. It’s not enough to just tease these promising results and be secretive about the methods or technologies. Public funding can only come when the details are published in the open literature, and subjected to the scrutiny of peer review and the wider community reading the papers. The (hot) fusion community is still living with the aftermath of the cold fusion scandal from a quarter century ago - so it’s very important for the proponents of these alternate concepts to push the researchers to publish their results in peer-reviewed journals. Whatever negatives the tokamak might have, one thing you can’t say about it is that the research has been too secretive, and this has allowed the funding agencies to make the judgement that the tokamak is currently the most promising route to a fusion reactor, which is why this line of research gets the most money.

Special thanks to Dr. Martin Greenwald, Prof. Ian Hutchinson, Asst. Prof. Anne White, Prof. Dennis Whyte, Nathan Howard, and Geoff Olynyk for taking the time to answer our questions.

You recently got the chance to ask a group of MIT researchers questions about fusion power, and they've now finished writing some incredibly detailed answers. They discuss the things we've learned about fusion in the past decade, how long it's likely to take for fusion to power your home, the biggest problems fusion researchers are working to solve, and why it's important to continue funding fusion projects. They also delve into the specifics of tokamak operation, like dealing with disruption events and the limitations on reactor size, and provide some insight into fusion as a career. Hit the link below for a wealth of information about fusion. 1. What have we learned?
by jank1887

Fusion is one of those technologies that is always '50 years away,’ even 50 years ago, maybe even 50 years from now. So, looking at what's actually happened recently: What do we actually know now that we didn't know 10-15 years ago that gives support to the notion that we're making progress? Or, what are the 'big' things we know now? Similarly, what are the things we still don't know that we could reasonably expect to find answers for in the next 10-15 years?

MIT Researchers: As researchers in this field, we have heard the expression "Fusion is 50 years away and always will be" more times than we would like to admit. The implication of this statement is that no real progress has been made in the field, which is simply not true! We have made a great deal of progress, even in the last 10–15 years (which have been very lean times for funding). We’ll try to summarize some of the new findings, in no particular order:

1) Internal Transport Barriers/Reversed Shear operation –
We have actually discovered a way to improve upon the performance that we get in H- mode plasmas. These improvements come in the form for so called internal transport barriers. In the past 10–15 years we have begun to understand how to modify the current flowing in tokamak plasmas so that we create effectively what is a barrier in the middle of the plasma. Like the edge barrier in H-mode plasmas, this barrier restricts particles and energy from escaping the plasma and enhancing the overall performance. 2) I-mode –
In just the last 5 years, a new operational regime has been discovered on the Alcator C-Mod tokamak at MIT. This is termed the I-mode, or “Improved L-mode” regime. When the tokamak is operated in this manner it exhibits excellent energy confinement properties, keeping the plasma hot. At the same time the plasma does an excellent job of expelling impurities which dilute the fusion fuel and reduce the number of fusion reactions which can occur. It is particularly important to us as it was first observed on Alcator C-Mod, and is now under active development at many other tokamaks around the world.

3) Development of Predictive Models –
Great advances have been made in the development of predictive computer models, such as gyrokinetic and magnetohydrodynamic (MHD) formulations. Years of experiments have revealed that plasma turbulence is often primarily responsible for the loss of particles and energy from fusion reactors. In the past 10–15 years we have developed advanced models which are thought to contain sufficient physics to simulate plasma turbulence and predict the performance of future fusion devices. At this time we are in the process of validating these models, i.e. comparing them directly with experiment to ensure they are correct, but we are approaching the ability to reliably predict the performance of fusion plasmas without the need for a fusion reactor. This can motivate engineering design and operational choices for future fusion devices.

4) Self-acceleration of the plasma (intrinsic rotation) –
Over the past decade, it has been discovered (on Alcator C-Mod and elsewhere) that plasmas can spontaneously rotate, at speeds of tens of kilometers per second. (Imagine the donut-shaped plasma spinning on its axis.) This turns out to have beneficial effects for stabilizing turbulence at the edge, as the spinning plasma causes the turbulent eddies to break up before they can carry hot plasma out of the core. This is an exciting area of research that could have big implications for the performance of a tokamak reactor.

5) Disruption mitigation –
One of the main problems with a tokamak is the ‘disruption’, when the plasma energy is suddenly lost, stopping any fusion that is occurring and requiring a restart of the reactor. (See the question below for a lot of detail about this!) In extreme cases, these disruptions can cause damage to the wall of the tokamak – which would require repairs before the machine can be restarted. Over the past decade, we have developed techniques to mitigate these disruptions, causing the plasma to come to a rapid shutdown that does not negatively affect the wall condition. Work is underway to scale these techniques up to a reactor-size device (ITER).

6) ELM control/avoidance –
Another longstanding problem with tokamaks is periodic ‘bursts’ of energy from the edge called Edge-Localized Modes (ELMs). In today’s devices, ELMs are not a problem, but in ITER and future reactors, they could carry enough energy to damage the wall in the divertor region (where most of the energy comes out). There has been rapid progress lately (past 15 years) in ways to control these ELMs by making them more rapid and smaller, such as using resonant magnetic perturbation (RMP) coils to distort the shape of the confining magnetic field, or ‘pellet pacing’ (firing small pellets of deuterium fuel into the machine 50–60 times per second, which triggers an ELM), or vertical ‘kicks’ in which the control system suddenly jogs the plasma position a few centimeters vertically, also triggering an ELM. Between these techniques and the recently discovered I-mode (which doesn’t have ELMs), this is a problem that is well on the way to being solved.

7) High-Z walls –
This is a particular point of pride for Alcator C-Mod. Running a tokamak with walls made of refractory metals has many advantages because of the extreme capacity of these materials to absorb heat loads, but there are disadvantages as well, such as how radiative these high atomic number elements are if they get into the plasma as impurities, or how metallic materials distort when they melt, rather than ablating like carbon-fiber composites. Alcator C-Mod (which has a molybdenum wall) and other tokamaks have recently shown that it is possible to reliably run a tokamak with high-Z refractory metal walls, which will almost certainly be a feature of future reactors.

2. Power Loss Scenario in Alcator C-Mod?
by eldavojohn

Not to raise any fears -- rather out of genuine curiosity -- what happens when the magnetic fields that hold the 90,000,000 degrees Celsius plasma in place fail or loser power on the Alcator C-Mod? I understand it's probably in prototype mode, but what sort of safety advantages or disadvantages do Alcator C-Mod designs offer over conventional, large-scale designs? Does the plasma come into contact with the toroidal superconducting coil? Then what?

Geoff Olynyk answers: Actually, that’s exactly what my research is on! The event you describe is called a "disruption." Holding a hot plasma stationary using magnetic fields without it ever touching material surfaces is very difficult – Richard Feynman once compared it to trying to "hold Jello with rubber bands." For any number of reasons, like a magnetic coil losing power, the control system not being able to juggle the plasma position quickly enough, or the plasma hitting a stability limit (pressure or density goes too high), it’s possible for the plasma to hit the wall. The most important thing to know, though, is that when this occurs (and it does, frequently, in today’s experiments – although it’ll have to be a very rare occurrence in a real power reactor so it produces uninterrupted electricity), it is no risk to the environment or to safety.

To understand what happens, you have to realize that the plasma is very, very light. In the Alcator C-Mod tokamak, it has a mass of only about 0.001 grams – about one- fiftieth as much as the smallest drop of water you can get from an eyedropper. (This is with a plasma volume of about a cubic meter – a fusion plasma is actually a pretty good vacuum!) So even though it’s very hot, it doesn’t actually have a lot of thermal stored energy to flow into the wall if confinement is suddenly lost. There is actually more energy stored in the current flowing in the plasma (in C-Mod, about a million amperes), which also gets deposited on the wall. In C-Mod, thermal stored energy is about 50– 150 kJ and magnetic stored energy is almost 1000 kJ. The problem is that as we go to larger machines (like ITER, or a reactor), the amount of stored energy in the plasma scales like the cube of the size, and the wall area only scales like the square of the size. So the energy deposited per square meter of wall area gets worse (larger) as we go up in machine size.

The plasma doesn’t hit the superconducting coils - it hits (really, deposits its energy on) the “first wall” of the chamber closest to the plasma. So, we do two things to make sure that the walls can survive these disruption events. The first is making them out of materials that can take a blast of heat, like tungsten, or else materials that ablate away rather than melting, like carbon fiber composites. The second is to develop “disruption mitigation” systems which can cause the plasma to radiate all its energy evenly over the entire wall surface, spreading the heat out and lessening the chance of causing localized melting. But I want to stress again - disruptions are an operational problem, meaning they might cause a power plant to be offline for a while, but they’re not a safety problem. There is no chance of a runaway reaction or meltdown in a fusion reactor.

3. Ubiquitous Fusion Power
by monsted

When will fusion power my house (or vehicle)?

MIT Researchers: This is obviously an impossible question to answer, but we can give some thoughts about when it might happen, and why. First, the current official plan is that ITER will demonstrate net fusion gain (Q = 10, that is, ten times more fusion power out than heating power put in) in about 2028 or 2029. (Construction will be done by about 2022 but there’s a six-year shakedown process of steadily increasing the power and learning how to run the machine before the full-power fusion shots.) At that point, designs can begin for a “DEMO”, which is the fusion community’s term for a demonstration power plant. That would come online around 2040 (and would putt watts on the grid, although probably at an economic loss at first), and would be followed by (profitable, economic) commercial plants around 2050.

This seems like a long time, and it is, but it’s important to understand that this is not the only possible path. You might say that we’re not a certain number of years away from a working fusion power plant, but rather about $80-billion away (in worldwide funding). We’ll get into this more in response to one of the other questions, but there are other experiments that could be done in parallel with ITER that would certainly speed up the goal of a demonstration power plant, if there were the money for it. Here is a graph based on a 1976 ERDA (predecessor to today’s DOE) fusion development plan, showing their four paths to a reactor, as well as a business-as-usual funding case that would never lead to a reactor, and in black is the actual funding amounts. (All values are adjusted to 2012 dollars.)

In the U.S. at least, fusion funding hasn’t been anywhere close to what would be required for a “crash program” to get to a reactor. If it were, it would probably be possible to have a demonstration reactor in about twenty years. (This is not actually that long - given that it takes almost a decade to build a large fission reactor or hydroelectric dam!)

Fusion has a reputation of “always being thirty years away” (or fifty, or twenty). We want to address that head-on here: aside from a few over-optimistic predictions made in the very early days of magnetic fusion research (the 1950s), this reputation is undeserved. The reason it has taken so long to get to breakeven (ITER) is because since the end of the 1970s, funding for fusion research has been continually slashed, up to today, when the U.S. is proposing shuttering one of three remaining tokamak experiments, the Alcator C-Mod device at MIT that we all work on. Despite this, progress has been continuous. But if we had the money, we would be getting there quicker.

4. What are the economic numbers for a successful, commercial reactor?
by kestasjk
I know that the economics of larger reactor = more economical are well known with tokamaks. Does this mean you have a good idea of the minimum cost / generating capacity of the first commercial reactors? If so, what do those numbers look like?
7. Lower Limit on Tokamak Design
by gyepi
Are there any good guesstimates on how small a tokamak-based fusion reactor (which produces more energy than it consumes) can become? Theoretical limitations on the size of the reactor would have obvious implications for pragmatic issues.

MIT Researchers: Questions 4 and 7 are similar and we answer them together here.

The current thinking is that a tokamak fusion reactor will be about 1 gigawatt electrical, and about 2–3.5 gigawatts thermal (depending on how high-temperature the blanket is and thus how thermally efficient it can be). This is about the size of a current fission reactor or large coal-fired power plant.

Fusion researchers are working on smaller designs, though! At MIT, some students are working on a concept for a 350–500 MW (thermal) class fusion reactor, which would be cheaper to field and thus more likely to be built by private industry with limited access to capital. This is still early work, though, and the economic analysis is not done yet.

Cost estimates for a new technology like fusion cannot be terribly reliable, but several studies suggest that, with suitable developments in science and technology, the costs could be competitive with other methods of electricity generation. We recommend you read the ARIES-AT study (google it), which goes through all the factors that go into the cost of electricity (COE) for a fusion reactor, and compares their concept to other electricity generation options (fission, fossil fuels, etc.) A key advantage of fusion is in what economists call "external costs." These are costs borne by society as a whole and not by the generating industry. Environmental pollution, nuclear proliferation, and military operations to protect oil supplies are all examples of external costs for energy.

5. What Problems are Holding Back Successful Reactions?
by Bucc5062

Can you explain to a non-scientist what the biggest stumbling blocks are for an effective fusion reaction? Is it truly a matter of throwing money down an energy hole, or are there verifiable, measurable benchmarks that lead us from one step to the next? I.e. we’ve achieved X, now we need Y; when we get Y, we get Z and then achieve fusion. Is it the technology holding us back, the politics, or the science?

MIT Researchers: We know exactly what we need to do. Not everything has a solution yet – that’s why it’s still a research project! – but we generally know what the big challenges are to get to a working magnetic fusion reactor. Here is a non-exhaustive list:

• 1 – Non-inductive current drive. We can’t rely on inductors to drive the plasma current since they are inherently pulsed (not steady-state). We think that lower hybrid current drive might be the solution, and are actively researching this on Alcator C-Mod.
• 2 – Confining a 'burning plasma.' This is the big question that ITER will resolve – can we really confine a plasma that is dominantly self-heated – that is, most of its energy comes from fusion reactions rather than external heating. Will new instabilities appear? Or can we confine the plasma as we expect we can.
• 3 – Confining a steady-state burning plasma while avoiding off-normal events. We have to do both of the previous points at the same time! And we can’t have disruptions too often or else the power plant won’t have a high enough duty factor. The goal is to have disruptions (which require a shutdown) occur less than once per year.
• 4 – Validated predictive capability for fusion-grade plasmas. We have made great progress in this field already (see our answer to an earlier question), but it’s not at the point yet that, say, fluid mechanics codes are, where Boeing can design an entire plane in the computer before ever building a scale model. We need our models of fusion plasma behavior to be accurate and reliable enough to design first-of-a-kind machines that we are 100% sure will work the way we think they will.
• 5 – Diagnosing a burning plasma. It’s really hard to tell any of the properties of the plasma even today, when we use pure deuterium fuel (instead of ‘live’ deuterium–tritium fuel), and our plasmas are colder than they would be in a reactor! You can’t, for example, stick a thermometer in to tell the temperature! We have to use subtle effects like bouncing a laser beam off the electrons and telling the temperature from the Doppler shift of the laser from the moving electrons (a technique known as Thomson scattering). Making these diagnostics work in the reactor environment, with higher plasma temperatures and a ferocious flux of neutrons coming out, is a great challenge.
• 6 – Better understanding of plasma–wall interactions. The plasma is confined by magnetic fields, and ideally doesn’t touch the wall at all, except in a very small area called the divertor. This means that the material challenges in the divertor are severe – we have to figure out a way to operate the plasma so that it’s hot in the center, but cold near the divertor, so that it doesn’t erode the wall too fast. This will be a limiting factor on how long you can run a fusion power plant for before you have to shut it down in order to do maintenance. Ideally, we’d want this to be every 2 years or so, like fission power plants today.
• 7 – Materials for plasma-facing components. We need to develop new materials that can withstand the high temperatures of the wall of a fusion reactor while resisting neutron damage and not becoming too activated by the neutrons that will pass through them. (There is some progress on this front with ferritic steels and silicon carbide.)
• 8 – Magnets that meet the plasma physics requirements and allow reactor maintainability at reasonable costs. (Some of us are working on demountable superconducting coil concepts that may eventually be the solution to this!)
• 9 – Design and materials for tritium fuel cycle and power extraction. Fusion reactors will breed their own tritium fuel from deuterium – this process has to be experimentally tested on a large scale (which will obviously require a burning plasma tokamak).
• 10 – Reliability, availability, maintainability, and inspectability (RAMI) of the reactor designs. We have to show that our concepts for reactors really are as good as we think they can be.

The point is that it’s not a money pit. There are unsolved challenges, but we know what they are, and with adequate support, these challenges will be overcome. This is why we are urging everyone to go to fusionfuture.org and write Congress asking them to keep supporting U.S. fusion research! (It’s very easy – there’s a link at the right on the website.)

6. NIMBY
by GeneralTurgidson

How do you explain the safety/benefits of fusion to a generation of people terrified of nuclear anything?

MIT Researchers:This is where fusion really shines. The two big problems (at least, perceived problems) of fission reactors are the risk of a meltdown, and what you do with the high-level radioactive waste. Fusion has neither of these issues!

Regarding the first, the reason why a worst-case accident in a fission reactor can be so devastating is because there is a lot of fuel in the reactor at any one time. There are well known accidents at Chernobyl (where the reaction ‘ran away’, making more power than the reactor was designed to handle) and Fukushima, where the fission chain reaction was safely shut down, but the cores melted down when the tsunami knocked out the cooling systems, due to ‘decay heat’ which is produced by the used fuel even after shutdown.

In a fusion reactor, it’s a completely different story. There will be less than a gram of fuel in a reactor at any one time—fresh deuterium–tritium fuel is continually added as it is burned—and so a runaway reaction is simply not possible. Decay heat isn’t a problem in a pure fusion system, again because there just isn’t any fuel sitting there undergoing nuclear reactions once the reactor is shut down. In general, this is one area where it’s a benefit that a fusion reaction is so hard to sustain! We have to try really hard to keep the plasma hot enough to undergo fusion in the first place, so if we just turn off the heating and fuelling systems, the fusion reaction will shut down very quickly.

As for the second benefit of fusion (waste), the reaction is completely different from that in a fission reactor. In fission, uranium (or other heavy elements like plutonium) split into pieces, producing hundreds of different isotopes, some of which are radioactive, with half-lives ranging from fractions of a second to millions of years. In fusion, the reaction is simple, deuterium + tritium helium + neutron. So there is no “waste” from the unburned fuel – any tritium that isn’t burned gets pumped out of the chamber and recirculated back in.

This is not to say that there will be no radioactive waste from a fusion plant. The reactor vessel itself will become activated because of the flux of neutrons passing through it, and will have to be treated accordingly when the plant is decommissioned (after, say, a 50-year operational period). But it’s important to note that this kind of radioactive waste is of a much lower level – it won’t have to be stored for very long before it will be “cool” enough to simply bury in the ground safely. And there is active research going on into new materials for fusion reactors that are more resistant to activation by neutrons, such as ferritic steel and silicon carbide.

Finally, fusion has great advantages for nuclear non-proliferation. Creating enough fission power plants to avoid climate change would mean that the plutonium moving around the world would be enough to create about 100,000 nuclear weapons. For fusion, it is much more difficult to use a reactor to make fuel for weapons. This is also something that we think a nuclear-skeptical public will appreciate about fusion power.

All of us are strong supporters of fission power, and we agree that at times, the nuclear power industry has not received a fair shake when compared to other sources of energy. But we think that the advantages of fusion power speak for themselves, and the public will be able to understand the risks and will support the construction of these plants. Obviously, having media that are able to explain things clearly and fairly are a necessity.

8. What do the numbers really look like?
by Erich

ITER is a hugely expensive project, and won't produce a commercially viable power generation system. In a lot of areas where research is done on things which don't work yet -- rockets, bridges, transmission systems, etc -- there's a general idea of how things might be able to "scale up" to meet the goals. Is tokamak fusion really in sight of being a commercially viable source of energy? If we need unobtanium to make a commercially viable reactor, wouldn't it make sense to wait until the materials are viable before making even larger tokamaks? Or is it still worth learning from these new, bigger, more expensive reactors?

MIT Researchers: You are exactly correct in your statement; ITER is an expensive project which will not produce electricity upon completion. However, ITER’s main purpose is not to put watts on the grid, but to demonstrate the scientific feasibility of fusion by creating a Q=10 plasma (10 times as much energy out as we put in). We do have a good idea of how to proceed with devices following ITER, namely DEMO, a full demonstration fusion power plant which will use the steam cycle to generate electricity from the fusion reactor. The basic layout of a reactor can be found here: http://www.fusionfuture.org/what-is-alcator- c-mod/c-mod-for-energy/

Although there is still plenty of research which remains, fusion is in sight of being a commercially viable energy source. We believe that we now understand the physics well enough to create the appropriate plasma conditions (this will be demonstrated on ITER) and we are working on the engineering challenges that lay between us and a commercial fusion reactor.

It is obviously impossible to predict when fusion will put power on the grid since the estimate can change drastically based on demand and overall funding levels. You are however, correct in noting that some of the biggest challenges involve the discovery/ development of materials which can resist the unique and harsh conditions associated with fusion reactors, namely, high heat and neutron fluxes. Due to its importance to the success of future devices, this is a very active and important area of research.

The international fusion community is attempting to address these issues in the following manner: Given the scope of the ITER project and the time required to build and test it, we are planning on constructing a materials testing facility named, IFMIF which stands for International Fusion Materials Irradiation Facility. This facility should be operated at the same time as ITER and will be addressing the materials issues associated with an eventual fusion power plant while ITER is demonstrated the scientific feasibility of a fusion reactor.

Given the time-scales for reactor construction, we think it would be unwise to wait for this materials testing to be complete before starting new machine construction. Addressing the remaining problems in parallel will most likely result in the quickest path to fusion energy.

9. Careers in Fusion?
by benjfowler

As practicing researchers, can you tell us about the health of the pipeline of young researchers coming into the field? Is there a glut of trained physicists at this stage, or is there still a need for trained specialists to enter the field, especially with ITER and follow-on machines coming online in the next couple of decades?

Nathan Howard answers: At this point in time fusion is actually a pretty healthy field in terms of young researchers and with emergence of the next generation devices such as ITER, there should be an influx of researchers stepping up to meet the need for trained specialists on these next gen devices. Currently in Europe and Asia, emphasis on fusion research is ramping up to support the research needs. These newly trained researchers are going to be the scientists working on ITER in 10-15 years.

Unfortunately, the US fusion program is in danger of going the opposite direction of the Asian and European programs. The current proposals made by the US are threatening the health of fusion in the US. The President’s 2013 budget proposal calls for drastic cuts to the domestic fusion budget to pay for increased funding for the ITER budget. However, if these cuts continue, there will not be a field for the young researchers to enter and the US fusion program is in danger of dissolving before ITER comes online.

This does not mean that a need for trained specialist will not remain, it just means that the young researchers in Europe and Asia will be filling these positions. Dr. Stewart Prager, the head of Princeton Plasma Physics Lab said it best, “We have a clear choice before us: The United States can either design and build fusion energy plants or we can buy them from Asia or Europe.”

As a young researcher myself, I am particularly affected by the choices that the US is currently making. Myself and other graduate students have been urging others who support fusion research to contact congress and tell them to continue to fund domestic fusion research. We put together a website, www.fusionfuture.org, which provides more information and people the ability to quickly and easily contact their congressmen to tell them to support research. Please support US fusion research and check out the site.

12. Patents?
by Anonymous Coward

Will patents get in the way of your research?

MIT Researchers: In general, we find that the tokamak labs of the world are extremely cooperative; patents have never been a problem. It does seem likely that the technologies supporting power plants will be highly patentable, but the sort of scientific knowledge we’re accumulating at present really isn’t. At some point, we expect to move from a collaborative to a competitive phase – but we’re not there yet.

11. What level of investment would get fusion going?
by Tragek
Do you think a program the size of the Apollo program could kickstart fusion to general availability? Or would a smaller program suffice?
14. What could you do with unlimited resources?
by petes_PoV
Given $1 trillion, the pick of the best brains in the world to work willingly on the project, a large enough location away from any and all governmental regulation and every facility you could ever need - when would fusion be commercially viable?

MIT Researchers: Questions 11 and 14 are similar and we have answered them together.

Any kind of question asking about a hypothetical massive increase in funding is tricky to answer. We probably couldn’t even spend a trillion dollars if we wanted to – just because it would take a long time to get enough people trained in plasma physics and fusion energy.

We can say this: an increase in funding would allow for different paths to be tried in parallel, like stellarators, tokamaks (ITER), spherical tokamaks, etc. Plus, we could build a facility in the United States to study the problem of plasma–wall interactions, which is a very important topic that has not been adequately studied up to this point (see our answer above about what steps are needed to get to a reactor).

We think that we’re roughly $80-billion away from a reactor. At current levels of funding (worldwide), that’s about 40 years. Even given access to huge amounts of money, it’s unlikely that a working reactor could be built in less than a decade – there are just too many facilities to build between current devices and a full-scale reactor in order to ensure success. But we could certainly do it faster than 40 years!

We want to note that “crash” programs like Apollo or the Manhattan Project succeeded because they took risks – they started work on building their systems before they had done all the homework. That is inherently risky, but these risks are mitigated by pursuing alternatives in parallel. Something similar could be done in fusion, given the money.

15. Your favorite books?
by eldavojohn

I'm not a physicist (software guy), but I've taken a few physics classes. At an early age I found a tattered copy of George Gamow's One Two Three . . . Infinity, which, although incorrect in some parts (I guess that's why they revised it and that's why 'speculations' was in the title), was perfectly written for my then-fifth-grade mind. It set me on a path toward science, and a few weeks ago I saw the same 1960s Viking Press edition and flipped through it, noticing what was slightly off and remembering it. I've since grown to love other obvious books by authors like Hawking, Penrose, Hofstadter, etc. So, quite simply, what are your favorite books for all minds, young and old? Also, can you annotate which are written for the layman's entry into the given field and which are written to encompass the field for the researcher? I find that some books start off with the jargon so strong and the references and footnotes so thick that you start to have to re-read every paragraph, as they're clearly condensing entire historic papers into lengthy sentences. Any fiction books worthy of influencing your work and desires?

Ian Hutchinson: My all time favorite novel is Godric by Fredrick Buechner. It's a wonderful first-person portrait of the prior life of a medieval hermit. My favorite physics teaching text is the Feynman Lectures on Physics, which comes from a remarkable effort by the most widely acclaimed american physicist of the 20th century to explain really advanced physics to undergraduates.

I really don't enjoy the genre of books that combine science popularization with metaphysical speculation. They are of course quite popular, but most are philosophically naive in a way that I find annoying.

Anne White: I like detective/adventure stories. I also enjoy reading plays, poetry and short stories – some authors I read over and over are Wolfgang Borchert, Julio Cortazar, Ray Bradbury and Samuel Beckett.

Recently, I've enjoyed reading The End of the Affair by Graham Greene, People of the Book by Geraldine Brooks, Jane Eyre by Charlotte Bronte and Her Fearful Symmetry by Audrey Niffenegger.

Influential books/stories that I remember reading when I was young : The Pearl (John Steinbeck), Catch-22 (Joseph Heller), Flatland, and Ender's Game.

Dennis Whyte:

• For science non-fiction books, it’s a tie: The Selfish Gene by Richard Dawkins, and Wonderful Life by Stephen Jay Gould.
• Novel (in general subject area of science): The Baroque Cycle by Neal Stephenson
• Speculative fiction: Starship Troopers by Robert Heinlein

Geoff Olynyk: The Making of the Atomic Bomb by Richard Rhodes is the best non-fiction book I’ve ever read. It’s a bit long, but is a fascinating, well-written exploration of the project to develop the atom bomb (both in the U.S. and elsewhere).

This is not a science book, but The Rebel Sell by Joseph Heath and Andrew Potter (sold in the United States as Nation of Rebels) changed my life. I was into counterculture, "culture jamming," anti-advertising, that kind of stuff, and this book made me seriously reconsider all of it. I now understand that trying to be unique is futile in a world of seven billion, and I should just try to be a good person and do good for the world (hence working on fusion!) Potter’s follow-up The Authenticity Hoax, explores the search for authenticity in more detail, but it’s not nearly as good of a book as Rebel Sell.

Nathan Howard: I first became interested in physics by reading about astrophysics. I was specifically interested in black holes and so one of the first books I read (after some of the popular books by Hawking which are written for general audiences, e.g. A Brief History of Time) was a book by Kip S. Thorne called Black Holes and Time Warps. I really enjoyed this book. It did not require much technical background, just some basic mathematics, and it gave good explanations of black holes, relativity, and gravitational waves.

16. Why is fusion more useful than exploiting thorium?
by gestalt_n_pepper

I understand that in the long term, we would want fusion. But we face increasing energy problems over the next 50 years and severe energy problems before 2100. Wouldn't it make sense to allocate research and development resources to something that we know works?

MIT Researchers: First of all, fusion will be putting watts on the grid before 2100. It’s not going to be tomorrow, but it’s not going to be a hundred years, either.

We know how to build thorium fission reactors. It's been done. They have none of the major attractions of a fusion reactor in terms of safety, fuel resources, reduced waste, or non-proliferation. Worldwide thorium fuel resources are about the same as those of uranium. Thorium reactors might become part of the commercial fission reactor mix in the future, but they don't offer transformative possibilities for nuclear power the way fusion does.

That said, we think that the that the scale of the energy/climate problem demands that we (meaning: government and private industry where appropriate) pursue multiple lines of development into new energy sources. Obviously nobody wants to waste taxpayer money, so all proposals have to be evaluated for chance of success – but today, it’s limited by funding more than by a lack of good ideas. This shouldn’t be the case.

The key thing we want to get across is that it shouldn’t be a contest between “fund fusion” or “fund thorium research”. Fusion is extremely important for humankind and should be funded – if thorium fission also has promise, it should be funded too.

17. How is fusion power harnessed?
by circletimessquare

The talk is always about reaching break-even with fusion. What about capturing the power? Are we generating heat that will drive steam turbines? What schemes exist for capture and harnessing the power generated by fusion?

MIT Researchers: In a magnetic fusion reactor, each deuterium-tritium fusion produces a 3.5 MeV (mega- electronvolt) alpha particle (helium nucleus) which deposits its energy in the plasma (this self-heating is how you can have an ‘ignited’ plasma which doesn’t require much or any external heating), and a 14.1 MeV neutron, which deposits its energy in a thick lithium blanket surrounding the toroidal reaction chamber. But in the end, all of it comes out as heat!

For a conservative fusion reactor design, this heat would be removed by a primary cooling loop (high-pressure steam or some sort of liquid metal) which would give the heat to a secondary steam loop (Rankine cycle) in a heat exchanger (steam generator). The steam would then turn a turbine, producing electricity, just like in a fission or coal power plant.

Of course, with a thermal process like a steam cycle, one is always limited by the Carnot efficiency, which increases as the temperature of the high-temperature reservoir goes up. So there are also designs to use a very high-temperature (800–1000 C) gas cooling loop and a Brayton cycle.

But the short answer is: the alpha power is captured by the plasma, and the neutron power is captured by the blanket. It all comes out as heat, which is used to heat a working fluid, which turns a turbine, producing electricity. This is not expected to be a technological problem – the challenge is in getting a confined thermonuclear plasma to produce the fusion energy in the first place!

19. Fusion Milestone Prizes?
by Baldrson

In 1992, with the assistance of fusion technologists such as Robert W. Bussard, I developed legislative language for a series of 12 milestones, each of which would be awarded a $(1992)100M prize for the achievement of objectives toward the attainment of practical fusion energy. This legislation also provided a grace period during which scientists and technologists that had been working on the US fusion program would be provided full salaries, without obligation, during which time they could seek support for their ideas to achieve these milestones. This legislation presaged a number of other prizes including the X-Prize and BAFAR / CATS prize. In 1995, Robert W. Bussard submitted this legislation to all relevant Congressional committees, copying all US plasma physics laboratories. Needless to say, the legislation wasn't passed. Do you think the time is right?

MIT Researchers: We think that the current approach, in which government-funded labs are not in direct competition, but have to justify their funding to the agency (in our case, the DOE), is the best option for the moment. Perhaps the X-PRIZE approach might work for the alternative concepts? (see our answer below regarding Polywell/Dense Plasma Focus/ IEC etc.)

20. ITER
by MpVpRb

Is the ITER project good science? Or is it a politically-motivated, pork-laden boondoggle?

MIT Researchers: ITER is absolutely good science. Governments representing over half the population of the world are backing the project because it is the logical next step – a prototype reactor that will produce ten times more fusion energy than heating power put in, for a few minutes at a time. It is also pushing forward the development of fusion reactor technology (materials, control systems, remote handling systems, etc.). The U.S. fusion community endorsed ITER as the best option for a next-step experiment at the Snowmass II conference in 2002 (see proceedings here).

All of that said, the cost of ITER has risen substantially from the original estimates, and because overall magnetic fusion funding has remained nearly flat in the United States, the U.S. contribution to ITER is threatening to swallow up the entire domestic program. This is starting with the planned closure of Alcator C-Mod in September 2012, but unless more money is allocated to fusion research, all three U.S. tokamak facilities are at risk in the next few years.

Graduate students at Alcator C-Mod have put together a web page explaining the problem: http://www.fusionfuture.org/faq/the-fusion-budget-problem/ and we urge you to go to this website and click the link to contact your member of Congress and urge them to fully fund a strong domestic program and the U.S. contribution to ITER!

21. NIF
by Grond

Is the NIF approach even plausibly capable of generating electricity in a useful way? Or is it purely a research platform / smokescreen for nuclear weapons research?

MIT Researchers: The primary mission of the National Ignition Facility (NIF) is "stockpile stewardship." That is, to ensure that U.S. nuclear weapons continue to be a credible deterrent. This is why NIF is funded by the National Nuclear Security Agency (the agency in charge of the nation’s nuclear stockpile), not the DOE Fusion Energy Sciences program. Thus, the weapons research mission of NIF is not a smokescreen, but is actually the publicly acknowledged primary objective for the facility.

Some researchers at NIF believe that their inertial fusion approach can be used for an energy source as well. We don’t want to speculate here on the plausibility of the LIFE (Laser Inertial Fusion Energy) concept. There is a National Academy of Science review of the prospects of inertial fusion energy under way right now; the final report is expected to be published before the end of this year.

18. Dense Plasma Focus
by mbradmoody
Do you see any merit in the "dense plasma focus" approach to commercial fusion power production, specifically the work of the Lawrenceville Plasma Physics group?
22. Focus Fusion / aneutronic fusion?
by mwk88
Focus Fusion Society is posting research on their project to do aneutronic (e.g. Proton Boron (pB11)) fusion. The concept sounds great, and as an engineer, I find several parts of their design, such as direct extraction of electric power, to be elegant. Is this credible research or pie-in-the-sky? I have not seen much mention of them in mainstream fusion research.
23. Polywell Fusion
by mknewman
What do you think of the efforts at EMC2 Fusion and Polywell Fusion? They seem to be making real, measurable, and open results, but the mainstream physics community seems to ignore this progress.
24. What’s wrong with IECs / Fusor?
by claytongulick
Why aren't IEC reactors based on Farnsworth's designs taken more seriously? From what I understand, IECs have been more effective at producing fusion, and they are cheap to build. People even build them in the garage. From everything I've read, no one really takes the "fusor" seriously in the fusion science realm, and it's considered a dead line of inquiry. I've never understood why.

MIT Researchers: These four questions (18, 22–24) are answered together here.

None of us are experts on inertial electrostatic confinement, magnetized target fusion / dense plasma focus, or Polywells, and so we don’t want to say too much about the specifics of those designs. We can say the following:

1. The amount of money that is being spent, especially in the United States, on fusion is far lower than the field deserves, given its track record and potential. This sounds self-serving, but we think it’s justifiable based on the facts. The graph we posted above shows how the fusion budget is far lower today than it was thirty years ago, even as we continue to make steady progress toward a reactor and the seriousness of the coupled energy/climate problem becomes more obvious.

The alternate confinement concepts program has also seen cuts. (“Alternative” in DOE Fusion Energy Sciences parlance means, basically, anything that isn’t tokamaks, stellarators, or laser [inertial] fusion.) The Levitated Dipole Experiment, an innovative magnetic-confinement arrangement based on planetary magnetic fields, was cancelled just as they were about to add significant auxiliary heating for the first time. And these small-scale alternative confinement projects are not very expensive! Some of these alternative concepts may very well be promising and deserve taxpayer money to be developed.

2. But on the other hand, these groups need to show that they deserve funding. It’s not enough to just tease these promising results and be secretive about the methods or technologies. Public funding can only come when the details are published in the open literature, and subjected to the scrutiny of peer review and the wider community reading the papers. The (hot) fusion community is still living with the aftermath of the cold fusion scandal from a quarter century ago - so it’s very important for the proponents of these alternate concepts to push the researchers to publish their results in peer-reviewed journals. Whatever negatives the tokamak might have, one thing you can’t say about it is that the research has been too secretive, and this has allowed the funding agencies to make the judgement that the tokamak is currently the most promising route to a fusion reactor, which is why this line of research gets the most money.

Special thanks to Dr. Martin Greenwald, Prof. Ian Hutchinson, Asst. Prof. Anne White, Prof. Dennis Whyte, Nathan Howard, and Geoff Olynyk for taking the time to answer our questions.

Nuclear fusion power is the process of fusing light nuclei together to release energy, and ultimately, to put electricity on the grid. Today, we have six researchers from MIT's Plasma Science and Fusion Center here to answer your questions about fusion power, tokamaks, and public support and funding in the U.S. for this research. The Obama Administration's budget request for fiscal year 2013 is paying for the U.S. share of ITER construction out of the domestic program, starting with the closure of the MIT fusion lab. The interviewees are ready to answer technical and policy questions, so don't be shy! And, as always, please break unrelated questions into separate posts. Read on for information about the researchers who will answer your questions. Dr. Martin Greenwald is a Senior Scientist and Associate Director of the MIT Plasma Science and Fusion Center. His experimental work focuses on turbulence and transport, density limits, and pellet fueling of magnetically confined plasmas. More recently, Dr. Greenwald has been heavily involved with data management, computation, simulation, networks, and remote collaborations for fusion research.

Professor Ian Hutchinson is interested in plasma control in tokamaks, as well as spatially resolved measurements of the radiated power coming from the plasma. He is the author of the standard fusion textbook Principles of Plasma Diagnostics. Prof. Hutchinson also works on particle-in-cell simulations of astrophysical and laboratory plasmas.

Assistant Professor Anne White researches turbulence phenomena on the Alcator C-Mod tokamak, developing new diagnostics to resolve the small fluctuations which cause energy and particles to leak out. She is the recent recipient of the U.S. Department of Energy Early Career Award.

Professor Dennis Whyte pursues research into plasma–material interactions; that is, the way the hot plasma in a magnetic fusion reactor interacts with the surrounding solid materials walls. His team is also developing novel diagnostics for fusion nuclear science, which is critical as fusion reactors start producing power (and neutrons) over long periods of time.

Nathan Howard and Geoff Olynyk are Ph.D students on the Alcator C-Mod project. Nathan, who is in the final year of his studies, studies turbulent transport phenomena experimentally and through simulation. Geoff, in his fourth year, is working on disruption mitigation, which is a way to quickly and safely shut a tokamak plasma down in a few thousandths of a second.

Nuclear fusion power is the process of fusing light nuclei together to release energy, and ultimately, to put electricity on the grid. Today, we have six researchers from MIT's Plasma Science and Fusion Center here to answer your questions about fusion power, tokamaks, and public support and funding in the U.S. for this research. The Obama Administration's budget request for fiscal year 2013 is paying for the U.S. share of ITER construction out of the domestic program, starting with the closure of the MIT fusion lab. The interviewees are ready to answer technical and policy questions, so don't be shy! And, as always, please break unrelated questions into separate posts. Read on for information about the researchers who will answer your questions. Dr. Martin Greenwald is a Senior Scientist and Associate Director of the MIT Plasma Science and Fusion Center. His experimental work focuses on turbulence and transport, density limits, and pellet fueling of magnetically confined plasmas. More recently, Dr. Greenwald has been heavily involved with data management, computation, simulation, networks, and remote collaborations for fusion research.

Professor Ian Hutchinson is interested in plasma control in tokamaks, as well as spatially resolved measurements of the radiated power coming from the plasma. He is the author of the standard fusion textbook Principles of Plasma Diagnostics. Prof. Hutchinson also works on particle-in-cell simulations of astrophysical and laboratory plasmas.

Assistant Professor Anne White researches turbulence phenomena on the Alcator C-Mod tokamak, developing new diagnostics to resolve the small fluctuations which cause energy and particles to leak out. She is the recent recipient of the U.S. Department of Energy Early Career Award.

Professor Dennis Whyte pursues research into plasma–material interactions; that is, the way the hot plasma in a magnetic fusion reactor interacts with the surrounding solid materials walls. His team is also developing novel diagnostics for fusion nuclear science, which is critical as fusion reactors start producing power (and neutrons) over long periods of time.

Nathan Howard and Geoff Olynyk are Ph.D students on the Alcator C-Mod project. Nathan, who is in the final year of his studies, studies turbulent transport phenomena experimentally and through simulation. Geoff, in his fourth year, is working on disruption mitigation, which is a way to quickly and safely shut a tokamak plasma down in a few thousandths of a second.