Fusion Moving Forward

FUSION QUEST GOES FORWARD

The Emc2 team has been ramping up its tests over the past few months, with the aim of using WB-7 to verify Bussard's WB-6 results. Today, Nebel said he's confident that the answers will be forthcoming, one way or the other.

"We're fully operational and we're getting data," Nebel said. "The machine runs like a top. You can just sit there and take data all afternoon."

So was Bussard correct? Will it be worth putting hundreds of millions of dollars into a larger-scale demonstration project, to show that Bussard's Polywell concept could be a viable route to fusion power?

Nebel said it's way too early to talk about the answers to those questions. For one thing, it's up to the project's funders to assess the data. Toward that end, an independent panel of experts will be coming to Santa Fe this summer to review the WB-7 experiment, Nebel said.

The [other] really interesting thing about this news item is the interactive nature of the online world.

A skeptic who knows enough about fusion to speak intelligently left the following comment below the story:

It's fun to daydream, isn't it? And it's easy, too, as long as you don't know too much.

There's more reasons than you can shake a stick at that this won't work. For starters, you can forget about aneutronic fusion. It's not just the temperature, Bremstrahlung is almost to certain radiate more energy than you produce by fusion no matter how good your confinement is. Even if you somehow manage to get a decent power balance, for a given plasma pressure and fusion power, a p-B11 reactor would have to be about 1000 times bigger (and more expensive) than a corresponding D-T reactor.

The next thing to worry about is the electrons. The magnetic configuration has not only lines of radial field from the center to the edge, which is bad enough judging from the experience with mirror machines, it also has lines of *zero* field along which the electrons will gush out. The idea of recycling electrons lost through the cusps won't work because they will come out almost parallel to the field but hit the return cusp with a large perpendicular velocity component they picked up going around the bend.

And the ions? The device is conceived to utilize a bi-modal velocity distribution, which will be destroyed very quickly by the two-stream instability. The anisotropy of the velocity distribution is also know to be a big problem, again from experience in the mirror program.

We haven't even started to talk about energy loss to the grids, the consequences of tiny field misalignments, charge-exchange ion losses, energy coupling between electrons and ions, and whether the potential distribution envisioned is even possible at a non-trivial ion density.

Since they managed to sweet talk somebody into giving them money, let them finish and publish their results, but let's not stop looking for ways to save energy and trying to develop other, less sexy but more reliable energy sources.

Art Carlson, Munich, Germany

But the even cooler thing is Dr. Nebel who is running the experiment was able to post a reply! Talk about getting the inside story in near real-time!

Just a few comments for Mr. Carlson

1. The theory says that you can beat Bremstrahlung, but it's a challenge. The key is to keep the Boron concentration low compared the proton concentration so Z isn’t too bad. You pay for it in power density, but there is an optimum which works. You also gain because the electron energies are low in the high density regions.

2. The size arguments apply for machines where confinement is limited by cross-field diffusion like Tokamaks. They don't apply for electrostatic machines.

3. The Polywell doesn't have any lines of zero field. Take a look at the original papers on the configuration. See :
Bussard R.W., FusionTechnology, Vol. 19, 273, (1991) .
or
Krall N.A., Fusion Technology. Vol. 22, 42 (1992).

Furthermore, one expects adiabatic behavior along the field lines external to the device. Thus, what goes out comes back in. Phase space scattering is small because the density is small external to the device.

4. The machine does not use a bi-modal velocity distribution. We have looked at two-stream in detail, and it is not an issue for this machine. The most definitive treatise on the ions is : L. Chacon, G. H. Miley, D. C. Barnes, D. A. Knoll, Phys. Plasmas 7, 4547 (2000) which concluded partially relaxed ion distributions work just fine. Furthermore, the Polywell doesn’t even require ion convergence to work (unlike most other electrostatic devices). It helps, but it isn’t a requirement.

5. The system doesn’t have grids. It has magnetically insulated coil cases to provide the electrostatic acceleration. That’s what keeps the losses tolerable.

6. The electrostatic potential well is an issue. Maintaining it depends on the detailed particle balance. The “knobs” that affect it are the electron confinement time, the ion confinement time, and the electron injection current. There are methods of controlling all of these knobs.

rnebel

We'll know more soon.