Ion Loss

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

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Solo
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Ion Loss

Post by Solo »

Is there such a thing as ion loss in the polywell? I guess neutralization shouldn't be possible because of the high energies all around, but what about loosing energy some way, maybe brems. or inelastic collisions or something. I can't imagine that an ion is always going to keep its same speed up, eventually they are going to start loosing energy, as a group. The ion oscillations are going to damp and the whole plasma is going to condense toward the center of the well. Then what?

The ions will eventually be a so low a potential they won't fuse. Sure, you can add a stream of fresh ions at high potential, but that will only help for so long. So what happens when your ion population is not energetic enough? I haven't heard anything addressing this, to my knowledge. Maybe neutralization will take place and carry away the low energy ions, but I doubt it. Since the plasma is not maxwellian, the whole plasma will go down in energy together, so there won't be any 'skimming off' of the lowest energy ions, because they will all be about the same energy.

I suppose that inputing a POPS-like RF drive of some kind might fix that, add energy back to the ions. Maybe that's what's got MSimon so excited about that vein of research.

BTW, I imagine the same situation would apply to electrons if it weren't for the fact that they are rapidly lost to the grid. Perhaps the same fix could work for them, if we ever get the losses that low.

drmike
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Post by drmike »

The ions will circulate just like the electrons (but maybe in the opposite direction!) Because they are massive, they could go out to the walls far more easily and be lost there. If they have lost their energy, then they are good targets for the high energy ions. And I think RF will easily be able to pump them back up to high energy in the right direction easily enough.

Loss to the walls will be acceptable if there is very little energy loss with it. Preventing loss to the grids from really high energy particles may also be an interesting, but limited, problem.

If it works, then it really is just engineering details to keep it going. That's not simple. But it is doable.

MSimon
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Post by MSimon »

drmike wrote: If it works, then it really is just engineering details to keep it going. That's not simple. But it is doable.
That is what attracts me so much to this as an engineering problem.

The plant is simplicity itself. Yet making it work is a very difficult cross discipline problem.

Once we get past "could it work with the right design?" We then have to get the right design.

Heat loads from ions, direct conversion, and figuring out exactly what is happening in the reactor will be the big hurdles. The heat heat loads and the reactor arrangement for direct conversion will be the toughest. The "what is happening in the reactor" question will probably be the the part of the job with the highest effort. However, that is mostly a matter of grinding it out.

Solo
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Post by Solo »

So I take it that low-energy ions clogging up the core and neutralizing the potential well are not a percieved problem. Good.

Here's a related thought: if the ions are going to pseudo-thermalize to a non-maxwellian state, all the ions will have the same energy, correct? As in, they will come out with a varied amount from collisions in the center, but they will 'anneal' at the edges. Now, the trick is, Bussard only used deuterium. All the atoms have the same mass. But if we are using H and B, then we have ions that are charged +1 and +5 respectively. As these ions travel out of the well, the potential does five times more work on the boron (Potential energy = charge * field * distance). So the boron atoms will form their annealing sphere at a smaller radius than they hydrogen. This might create some problems. The hydrogen atoms will have significant KE both coming and going through the boron layer. This could disrupt both the hydrogen and boron annealing processes. It could also cause fusion collisions a significant distance from the core. And probably other effects I can't think of right now.

Thoughts?

drmike
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Post by drmike »

That is what makes the problem so interesting! There will probably be several zones that all have particular behavior. Designing the whole system to deal with that behavior is just part of the job. And there might be ways to take advantage of various properties of the different ions in different regions - the heavy massive B ions moving slowly can help trap electrons in the outer regions and help reduce losses.

Details matter, so we'll have to take a lot of measurements to make sure we understand enough detail to make things work well.

93143
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Post by 93143 »

That isn't the problem. The ions all start out in the ECR region at the same distance from the core, which means that the low-speed edge region for both particle potential maps is the same, at least in the ideal case.

However, the energy of the borons at (or near) the core is five times that of the protons, and they have about two-thirds of the speed.

This will affect scattering. If it enhances collisional scattering enough, edge annealing could work for D-D but not p-11B. In addition, since the boron has a higher temperature at the core, it could transfer energy to the protons and actually wind up not coming back up as far, on average - with a result similar to what you suggest. Of course, then the slow borons partway up the well would tend to get rear-ended by protons...

Now that I look at the numbers, I don't think scattering will be all that bad. Hard to say, really...

MSimon
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Post by MSimon »

93143 wrote: Hard to say, really...
If we are going to get these devices fielded quickly we are going to need to to do a lot of experiments.

One of the problems is that we have no really good way of measuring low density small volume plasmas. Langmuir probes are about it.

Another problem is that we really need a continuous operation machine (seconds to minutes) to do a lot of the testing.

Our level of ignorance is similar to where fission was when Meitner, Hahn and Strassmann figured out what was going on in 1939. There was a long way from that discovery to power to the grid in Calder Hall (50 MWe - 1956).

They needed to work out industrial scale processes for refining graphite. They had to find the neutron cross sections of a great number of materials. They had to find out exactly how many neutrons were released per fission. Develop instrumentation. Enrich the uranium. Extract plutonium (for plutonium reactor experiments) etc.

One advantage we have over the ITER guys is that they are fighting plasma "instabilities". We are treating them as self organizing principles.

More: We do have a lot of advantages. The state of electronics is much better. We have cheap computer resources. The industrial base just needs expansion. Not invention and expansion. Turbopumps are off the shelf. B11 is produced in high purity for semiconductor production. High purity heavy water (Deuterium) can be obtained by the gallon if not truck load. You can buy truck loads of LN2 delivered to any place in America for about $4 a gallon. There are all kinds of exotic welding technologies available including laser and electron beam. Particle detectors of all kinds are off the shelf. etc.

The big hole in the development is just the round ball containing the plasma.

Roger
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Post by Roger »

MSimon wrote:One advantage we have over the ITER guys is that they are fighting plasma "instabilities". We are treating them as self organizing principles.
The Huge advantage, that at the end of the day may show why Polywell was bound to work. And the torus was just bound not to.
I like the p-B11 resonance peak at 50 KV acceleration. In2 years we'll know.

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