Dr. Bussard Passes Away, Polywell Fusion Continues

Vinz writes "Dr Bussard, the man behind the Bussard Collector and inventor of the Polywell fusion device, passed away last Sunday in the morning. He leaves behind him a legacy of EM fusion devices, and a team determined to continue his efforts. The news of funding extension for the construction of his WB-7 fusion devices made it to slashdot months ago (as well as his talk at google). They may be a serious candidate in the run to bring commercial fusion, and may work at lower scales than other projects. Let's hope the project continues in good shape despite his departure."

Comments

Electron losses

[BlueParrot]

The catch to these devises appears to be that if you have a strong enough electrostatic field to contain the ions then you will also lose A LOT of high energy electrons (Rider 1995), thus reducing the confinement efficiency. As Rider notes, capturing the escaping electrons to recover their energy may make the scheme feasible for D-T fusion ( there are other issues as well however).

Personally I think stellarators are more promising. For those who don't know stellarators are a bit like Tokamaks, except rather than relying on an electric current in the plasma to create the necessary twist to the magnetic field for confinement, they twist the confinement vessel itself ( a bit like a moebius strip ), making them a lot more stable than Tokamaks, and allowing them to operate continuously (You can't induce a DC current in the plasma so Tokamaks necessarily operate in pulses ). Main problem seems to be that since stellerators have a lot less symmetry than Tokamaks the calculations become more difficult, but if computing power continues to rise this will probably be solveable.

As a bonus stellarators look damn cool ; )
http://www.efda.org/pictures_html/stellarator_schema_and_live.jpg
http://www.psl.wisc.edu/hsx.jpg

Re:Electron losses

[BCGlorfindel]

The catch to these devises appears to be that if you have a strong enough electrostatic field to contain the ions then you will also lose A LOT of high energy electrons (Rider 1995)

And Bussard insisted that Rider's math model was flat out wrong. Recent experiments by Yoshikawa and MIT have both demonstrated that Rider's model is, in fact, wrong.
The Polywell design has tremendous merit to it and the experiments that Bussard managed at the end of his life were successful in measuring fusion scaling factors and electron loss factors. From those experimental results Bussard's team rushed together what was expected to be their last device in WB-6. On analyzing the data it generated, it achieved record breaking fusion rates. Now that the navy has re-funded his team to finish WB-7, expect to see some big announcements in a year or so.

For more on Polywell theory and background go here.

Re:Electron losses

[BlueParrot]

the idea in the polywell isn't to contain the ions, but to contain the elcetrons, which will attrack the ions.

From Maxwell's equations div B = 0, so magnetic field lines cannot suddenly stop, and thus magnetic fields alone cannot confine charged particles in a plasma which has the same topology as a sphere ( a charged particle that travels along a magnetic field line will escape the confinement ). Consequentially you WILL have electrons leaking out of the magnetic mirrors, and this effect will increase as the potential well height increases.

Tokamaks and Stellarators don't have this problem because they are topologically equivalent to a torus, and thus their magnetic field lines can completely enclose the plasma, while simultaneously not penetrating the plasma facing components.

There are further problems with the polywell design. As an example, even at optimal energy levels the reactants will fail to fusion in many of the collisions, and thus the ions will thermalise much quicker than they fuse. Bussard claimed he could avoid thermalisation of the ions, but this is simply not possible in the polywell design since it would require a spontaneous process to transfer energy between the ions in such a way that their overall entropy decreases. While the polywell is not a closed system, and thus not subject to the second law of thermodynamics, there is no meaningful energy input other than the initial potential energy of the ions, and thus for thermalisation to be avoided there would have to be a large entropy flow out of the plasma, and thus it would quickly cool to levels bellow that required for meaningful fusion. In short, you will rapidly get thermalisation of the ions, which in turn leads to X-ray losses from the electrons. If you did heat the plasma, by say injecting microwaves or neutral particle beams, it would still not avoid the problem of thermalisation unless you managed to selectively accelerate the low energy ions, while simultaneously slowing the fast ones ( and of course, if this energy exceeds the fusion power, as it will have to do in order to overcome the speed of thermalisation, then you won't get net energy out of the device ).

While we are at it, no, you are not going to produce a Boron plasma with any significant number density without getting electrons in it, just calculate the electrostatic force you would get on an electron outside the device from 1 mole of boron nuclei and you quickly see that this is absolutely impossible. Even if the proton/electron ratio is just 5/4, Q = N_a, so you are talking roughly 6*10^23 times the proton charge ( or 60 million Coulomb ).

You then have to take into consideration other problems, like sputtering of plasma facing compounds, giving impurities that cool the plasma ( and since all potential plasma facing compounds have Z numbers of 6 or above, this will further increase X-ray losses ). There is no proposed way to design a divertor, so the device could most likely not operate for extended periods of time.

Basically I don't see this getting a confinement time even close to that of a Tokamak or Stellarator. The number density will be dramatically less ( since it is limited by the height of the potential well ), and it just doesn't seem likely you will get even close to the lawson criterion. Granted, you don't need to achieve ignition in order to extract a lot of energy, but you won't get a high value of the nTtau triple product without raising T to very high energies, which impacts the amount of energy you can gain.

Re:Electron losses

[KonoWatakushi]

Instead of producing lengthy expositions about the flaws of technologies that you don't understand, why don't you try learning about them instead? From your post, it is clear that you neither understand, nor have you read any of Dr. Bussard's papers on the subject. Given the topic of this story, you could at least have enough respect to do so, before spreading FUD about ideas.

First of all, no one is claiming that the divergence of a magnetic field is non-zero. The fact is, the "wiffle ball" trapping of electrons in a Polywell is more than adequate for the task. Electrons escaping through the cusps do not equate to losses, as they usually follow the field lines right back into the machine.

In any case, it is highly disingenuous to claim that a Tokamak has no difficulty confining a plasma. While the topology of a Tokamak (or a dipole as in the LDX) may be a better configuration for containing charged particles, this ignores the fact that the ions have a much greater mass. After a number of collisions, it is inevitable that they will smash into a wall. The only solution to this problem is to make the machine bigger, but it is still far from ideal.

Your calculations concerning a Boron plasma are complete nonsense; as described in his recent paper, only a slight deviation (1E-6) from neutrality is necessary to make a well nearly as deep as the drive energy.

Overall, there are at least as many, if not more challenges, in producing a commercially viable Tokamak. I won't discount either approach yet, but the Polywell certainly looks a lot more promising. A quasi-spherical potential well simply seems like a much better place for a sustainable fusion reaction than a divergence-less B field. Wether or not it works out, it certainly deserves more attention and less unfounded condemnation.

Oh ho, someone died

[Anonymous Coward]

Cue the inevitable silly poster who chastises us all for making jokes about someone's death, and the +5 Insightful reply explaining how laughter is a way of coping. Don't forget the meta-comments describing the very phenomenon ;D!

Re:Aw, man...

[rickola]

I posted the following to the TrekBBS: "Dr. Robert W. Bussard has died. Star Trek tech fans will know the name, though perhaps not the man himself, from the matter scooping mechanism he proposed and which we finally named in his honor on The Next Generation's U.S.S. Enterprise. By all means check Google and Wikipedia for further information about his accomplishments and hopes for new energy and propulsion systems. I knew him for a time in the early 1980s, as I had written an article on interstellar travel for Science Digest magazine that included his concepts as well as those of Drs. Robert Forward and Robert Enzmann (the three "Bobs" of far future flight). As part of the scientific help Bob Bussard gave me for that article, he sat in my living room in Irvine, California, scribbling calculations for waste heat radiators on a new version of his ramjet ship, and it was a wonder to watch and listen. It was an honor to translate his doodles and numbers into finished art. He was generous with his time and knowledge, and while I hadn't talked to him for a few years, I will miss him." It seemed fairly obvious that folks like "Franz Joseph" Schnaubelt, who drew up the deck plans for the original series U.S.S. Enterprise, knew something of Bussard's ideas for the ramscoop, since the spinny red caps were labeled as "matter/energy sinks" or something very close to that (my copy of the plans is away in a box). I'm not certain that Matt Jefferies heard about the scoop at the time he sketched the ship out, but he might have. When it came time to do Star Trek: The Next Generation, we in the techy side of the art department decided to give a name to the hardware, and so it shall stick. Rick Sternbach

Re:Final arrangements for his body.

[Tetsujin]

I hope they cremate him in a fusion reactor. I'm sure it's hot enough. Of course it is. Everybody knows there's no such thing as cold fusion...

Godspeed, Doc.

[Anonymous Coward]

.."inventor of the Polywell fusion device, passed away last Sunday in the morning. He leaves behind him 2 long flaming tyre tracks and a mysterious note about Libyans"

Re:A remark captured my attention

[nutshell42]

The other thing that caught my attention was Bussard's comment that they should go straight to full scale. He may or may not be right. Most people who have been around the block more than once would be sceptical though. When you are trying something new, there is almost always a gotcha or two.

It's one of the things that had the alarm bells ringing about the Polywell because it's something you'd expect from someone who wants to sell you the Brooklyn Bridge.

But I think in his case he just saw the writing on the wall. He knew he wouldn't see a full-scale reactor if it was done step-by-step, he was just too old for that.

I really hope someone with the required expertise will take an honest look at the Polywell. The concept sounds good and the central question seems to be whether the plasma will move into thermal equilibrium or not. And the paper every critic cites is one master thesis written by the student of one of Bussard's rivals for Navy funding. Hmmmm...

Now, the fact that your opponent's not trustworthy doesn't mean that you are, but I think that considering all the money that goes into ITER a few million for looking at different approaches (mostly this and lasers/inertial confinement =) are a good investment.

Makes perfect sense to me.

[Ungrounded Lightning]

The other thing that caught my attention was Bussard's comment that they should go straight to full scale. ... Most people ... would be sceptical... When you are trying something new, there is almost always a gotcha or two.

In this case he believed he had the scaling laws down. With power proportional to the seventh power of the radius and energy gain proportional to the fifth power, you were only talking about building a device maybe 10 times the radius of the lab device. That's TINY as fusion experiments go, and also compared to fission plants. And the thing is basically a slightly gassy vacuum tube with some magnets in it, i.e. mostly empty space, very little material.

If there are any gotchas you'd have to scale it up about that much to find them. So why go halfway and then build a full-size one when, if it turns out there AREN'T any gotchas you've got an operating power plant on the next step?

His plan was to do two more small prototypes, to get some more solid data than his three-neutron final run and compare two geometries for the final deaign, then go for the gotchas-or-gold. If it works, it gets you to production right away and you didn't spend a dime on yet another intermediate prototype. If it doesn't, you're not out all that much more than if you built some intermediate size that was maybe big enough to find the gotchas.

Suppose there AREN'T any gotchas. Then we get to working fusion power years sooner. Ditto if there are gotchas that only show up at the scale between the intermediate prototype and the full-size design. In either case the time spent on the middle-size below-break-even prototype was wasted.

Baby steps are for people who get their money from researching and will be looking for a new job once things are actually working. Big steps are for people who want to get to the finish line.