Carlson and Nebel

Discuss how polywell fusion works; share theoretical questions and answers.

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TallDave
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Carlson and Nebel

Post by TallDave »

Comments from the MSNBC article, presented for your consideration.

Carlson:
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.
Well, it's nice he agrees they should be allowed to finish and publish their results.

Nebel responds:
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.
Most of these I'm familiar with; I've cited that last paper several times. #3 I've always just accepted as true, but I should probably dig out those articles and get more detail.

The behavior at higher ion densities than WB-7 could achieve are what concern me most; the unknown unknowns are bound to crop up. But only experimental data is going to give us answeres on those issues one way or the other.

choff
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Post by choff »

I noticed Dr. Nebel mentioned the possibility of injecting fuel pellets into the Polywell, that makes me wonder if a heavy inert catalyst could be tried as well, crushing the fuel together between two molecules of a much heavier element. On that note, maybe that was happening in the fusors back in the 60's, a few iron molecules from the machining, causing the high fusion counts by crushing the fuel between them when they repeated collided in the core.
CHoff

93143
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Post by 93143 »

I doubt it. You never know with quantum mechanics, but...

...in gas kinetics, we do all our calculations assuming two-body collisions. Three-body collisions are supposed to be so rare that they can be neglected. This becomes less true for charged particles (Coulomb collisions) but what you're talking about requires at least a four-body collision - in a specific orientation, no less...

choff
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Post by choff »

If the collisions happen at r=0 and everything is directed inward, I know its unlikely but??? Also the heavy material would get reused without taking part in fusion, and if the machine pulsed???
CHoff

kcdodd
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Post by kcdodd »

I think the introduction of heavy elements would just knock all the light ones out of the reaction area. The velocity of a iron atom would not change much compared to hydrogen, or even boron. And considering it would be at several times the energy I would think the lighter elements would gain so much energy in a single collision they would leave the potential well.
Carter

dch24
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Post by dch24 »

Thanks, TallDave for posting the discussion. Art Carlson and Dr. Nebel have more to say:
Art Carlson wrote:
Prometheus wrote:Meanwhile with a few $ billion's spent the Tokomaks Are still 10-15 orders of magnitude away [from the necessary conditions for sustained positive output].
This is a bizarre statement considering that the tokamak JET has already produced plasmas where the fusion power produced was greater than the heating power.

Art Carlson, Munich, Germany (Sent Monday, June 16, 2008 9:54 AM)
Art Carlson wrote:
TallDave wrote:
Alex wrote:That was about 15 orders of magnitude short of break-even.
True, but no [one] expects break-even from a machine of this size and budget. The power scaling law is roughly radius ^ 7 (r^3 for the volume of ions x B^4 for the power increase from density created by the magnetic field, which scales roughly with the radius), so a machine about 1.5M in diameter would in theory be able to produce something around 100MW of net power.
Scaling is a tricky business. If you want to buy 14 orders of magnitude with R^7 scaling by increasing the volume a factor of (100)^3 and the field a factor of (100)^4, the ion gyroradius will shrink relative to the machine by a factor of (100)^5 = 10^10. Considering it is critical to the concept that the ions be practically unmagnetized, I'd say you have a problem (even if you think you the improvement you need is much more modest.)

Art Carlson, Munich, Germany (Sent Monday, June 16, 2008 10:08 AM)
rnebel wrote:The scaling laws quoted by TallDave and Dr. Carlson are the power output scaling laws. The B**4*R**3 scaling is just the "constant Beta" scaling which applies to every magnetic confinement device (that I know of) and is theoretically founded in something as simple as force balance. It works for Tokamaks, Reverse Field Pinches, Spheromaks, etc. This one I’m not worried about.

The one you have to worry about is the input power scaling, because that one is related to the plasma losses (or transport). This one answers the question of "How much power do I need to supply to the device to maintain constant Beta". Theoretical modeling of transport has a much poorer track record than plasma equilibrium has. These scaling laws are where the major risks for the larger device reside. The major saving grace is that for the Polywell is that the projected average densities are ~ 2 orders of magnitude higher than they are in Tokamaks so the energy confinement times don’t have to be all that good. (It’s the product of the density and the confinement time that’s important.)

Our contention is that since our projections for a power producing device only require a machine like the one TallDave described, we might as well build the next one in that size range and accept the risk. The machines just aren’t all that expensive. Also, there are a multitude of things that can be done to improve confinement (such as pulse discharge cleaning, pellet injection, etc.) that have been successful in the magnetic confinement program that can be instituted if our projections fall short. This approach will minimize the development time and lead to a lower costs for the overall program.

rnebel (Sent Monday, June 16, 2008 4:07 PM)
seedload wrote:Mr. Carlson has gone from saying it won't work to saying that it won't scale. Next step, autographed polywell posters on his bedroom walls.

seedload (Sent Monday, June 16, 2008 4:11 PM)

dch24
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Post by dch24 »

Catalysts would be a great topic for a separate thread.

drmike
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Post by drmike »

Thanks for the excerpts, I don't have time to visit the other site much.

MSimon
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Post by MSimon »

Engineering is the art of making what you want from what you can get at a profit.

TallDave
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Post by TallDave »

Thanks for adding the additional comments.

You know, I stupidly kept waiting for more comments to appear on the first page and never noticed that "Next" link till today. Argh.

The "magnetized ion" problem is one I hadn't heard of before. It sounds like Carlson means the magnetic field must not impinge on the ions, and Nebel responded by noting the B=1 condition will push the magnetic field back to the edge (as discussed here in other threads), where only a small tail of the ions' velocity distribution could reach.

Jeff Peachman
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Post by Jeff Peachman »

Obviously Carlson knows more than the average person about what he's saying or else I see no reason why Nebel would take even a moment of his time to debunk him.

Does Nebel know of Carlson? (Is Carlson in the Tokamak community?)

Haha, I wouldn't be suprised. Sucker =p
- Jeff Peachman

rnebel
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Post by rnebel »

Carlson is a Tokamak researcher from Garching (Germany). I don't know him personally. I have had some off-line exchanges with him and he has some interest, even if that interest is primarily in shooting down Polywells. I've never quite understood why this is, but a lot of the plasma people not only think that Polywells won't work, but they apparently don't want them to work. Polywells have been vetted by some pretty good independent plasma physicists (like Nick Krall) and if there was an obvious showstopper I think it would have been found. Maybe Carlson will come around. I think we have probably convinced him that we are serious people and not 100% Looney Tunes (maybe just 50%). He has been very civil in our e-mails, which isn't always the case with Tokamak people. I'm simply going to try to answer any questions that he might have. I think the best thing with these guys is to be honest and up front both about what you do know and what you don't know.

TallDave
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Post by TallDave »

Yeah, I've had a little bit of a running battle with him on the Polywell wikipedia entry. He has his biases (as do we) but he's always been pretty reasonable and civil.

Possibly interesting aside: he had a dust-up with Lerner at Wikipedia over focus fusion. It ended with a committee banning Lerner from further editing. Reviewing the discussion, my sympathies were all with Carlson there.

Carlson also argued against the Rostoker/Monckton CBFR in this Science Mag discussion.

http://www.sciencemag.org/cgi/content/f ... /5375/307a
Last edited by TallDave on Fri Jun 20, 2008 8:24 pm, edited 1 time in total.

seedload
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Post by seedload »

Carlson's latest
B~R scaling: It looks like Bussard really was thinking about a fixed current density and a fixed ratio of conductor size to machine size. The statements I found were too brief to judge whether they are correct, but it could well be that the engineering constraints work that way, at least up to some maximum field on the order of 10-20 T. That notwithstanding, I am still worried about the physics of the scaling. There are statements from Bussard that the ions must be unmagnetized, and a calculation by Krall about how big the field can be before the ions get knocked off center. On the other hand, maybe that is not really so important. (Dr. Nebel has suggested that the convergence of the ions is not as important as previously assumed.) Can someone supply some numbers? Above all, what is the (maximum) field strength envisaged for a polywell power reactor? From that we can calculate the ion Larmor radius, the electron larmor radius, and the Debye length.

Zero-field cusps: Apparently my assumptions about the coil geometry were those used in the first machines. Bussard eventually discovered the problem himself. If I understand correctly, the current designs have coils which do not touch each other. I'm still chewing on the implications of this. For example, do the point cusps at the corners start to trun into line cusps that wind around the machine? Bussard himself seemed to think that it is essential that the cusps be points.
Cracking?

rcain
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Post by rcain »

I've been keeping up with the Carlson Vs Nebel discourse also and I suggest that it really must be welcomed:

If, as has been suggested, Carlson is 'cleverer than the average bear' and if he in any way represents the opinions of the wider (Tokamak) fusion community 'against' the Polywell approach, then surely his conversion from utter skepticism, by whatever fraction can only have a beneficial effect on converting that 'wider anti-Polywell camp to the same (more positive) conclusions.

I say, may the debate continue and broaden still further - but not for too long, obviously; peer reviewed, experimental results are what we are all really looking forward to.

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