Slashdot Mirror
New 'Stellarator' Design for Fusion Reactors
eldavojohn writes "The holy grail of fusion reactors has always seemed 'just a few years off' for many decades. But a recent design enhancement termed a 'Stellarator' may change all that. The point at which a fusion reactor crashes is when particles begin escaping due to disruptions in the plasma. A NYU team has discovered that coiling specific wires to form a magnetic field may contain the plasma. This may be a a viable way to create a plasma body with axial symmetry, and a far better chance of remaining stable. Like other forms of containment this does require energy itself, but could bring us closer to a stable fusion reactor. It may not be cold fusion or 'table top' fusion but it certainly is a step forward. The paper is up for peer review in the Proceedings of the National Academy of Sciences."
In fact, the stellarator design is almost as old as the Tokamak design. The first one was built in 1951.
Somebody over at physorg got a little too excited about a fairly low-impact paper from NYU. If you read the abstract, you'll see that the paper just deals with the design of the coils for a stellarator.
Most likely, this is for the National Compact Stellarator Experiment (NCSX) being built at nearby Princeton, which will be the first stellarator designed with a computer optimized plasma geometry. I think it will also be the largest stellarator to date, with 12 MW of heating capacity. In contrast, the JET Tokamak has 37 MW and the ITER Tokamak will have 110 MW of heating. Unlike ITER, NCSX will not be capable of break-even operation.
Stellarators often get mentioned in fusion power discussions because they provide a more stable containment design, whereas a Tokamak needs one extra set of electromagnets to deal with the fact that the magnetic field is weaker at the outside of a torus of magnets than at the inside. Although a stellarator is therefore a little simpler in that regard, the geometry and plasma modelling is much more complex, and this in turn creates problems for designing the coils and the exhaust diverter. Because of this, most of the funding and research effort has gone to the Tokamaks.
A little more info here: http://en.wikipedia.org/wiki/Stellarator
Anybody care to bet on whether this shows up on CNN's tech page in a day or two as some major "recent design enhancement?"
Well, I'm not a plasma physicist, so I'm not intimately familiar with all the details, but one thing that jumps out at me right away is the distinction between energy and power.
Energy is the ability to do work. Power is the rate at which work is done or energy is extracted.
The plasma contains a great amount of thermal energy with a tendency to do work (by difussing to the reactor walls), so you have to set up a barrier to accomplishing that work. This is analogous to a dam holding back water. The water, due to it's elevation, has a lot of potential energy, but no power is required to hold it back. Power is extracted as it's let through the turbines.
It's a little more complicated for a plasma. A charged particle moving through a varying magnetic field (like that surrounding the reactor) does work and thereby loses energy. As a result, there is a tendency, although less definite than with a dam and water, for the hydrogen ions to only move around in the reactor along lines of constant magnetic field strength.
Once a magnetic field is established, it ideally takes no energy to maintain, except as charged particles move through it. So power only has to be supplied to the electromagnets to account for their inefficiency (0 under ideal conditions in a superconducting Tokamak) or as work is done on the field by charged particles escaping. Since most of the energy from the reactions is carried away by neutrons, which have no electric charge and therefore don't affect the field, the containment power is sufficiently smaller than the reaction power that this is theoretically feasible as a power plant.
Actually, the biggest power demand in a Tokamak as I understand is for heating the plasma to a temperature where fusion will take place. The hotter it gets, the faster fusion occurs, eventually reaching a breakeven point energy is released by fusion faster than it is carried away by escaping neutrons and gamma rays. Then the plasma can sustain itself. We haven't gotten there yet.
Sorry, the dam analogy isn't great and talking about charged particles in a magnetic field is a little abstract. Hope this helps.
"Energy" in the context of containing a plasma is actually work. They have the same units, so they're like exchangeable currencies (i.e. some energy will buy you work, and some negative work will buy you energy)
The energy that a plasma intrinsically has (like kinetic energy) is just that; energy.
Here's a related (but certainly not airtight) analogy: A brick can have some gravitational potential energy relative to the earth's surface. If you're standing on the ground, that brick will have some nominal gravitational potential energy. If you lift that brick 1 meter, you'll do some amount of work. If you're hanging over the edge of a helicopter at a couple hundred meters, that brick has substantially higher gravitational potential energy. However, if you lift the brick a distance of 1 meter, you'll still do the same amount of work.
So, what's going on here is that a plasma can indeed have a lot of energy (relative to the earth's environment). However, the "energy" we're putting in is actually work to contain that plasma.
Sig free's the way to be.