Helium exhaust. Sputtering contamination.

[chrismb]

Another question. In another thread I have been enlighted to the fact that a hollow charged sphere has no effect on a charged particle inside of it. But, is not the decellerating grids that are susposed to decelerate the alpha particles the same thing? Or does the centrally located structures (magrid and entraped fields) shield the opposite side of the sphere enough that this is trivial?


Dsn Tibbets

I believe the 'notion' of deceleration grids for the generation of direct electricity is just that, a notion. I am not aware of any practical experimental proof that such a mechanism can work.

Your question appears to me to be wholly valid.

[chrismb]

I am just an interested layman, but this has been discussed at length on the forum. The system will need to operate in a vacuum, which will need to be maintained. The pumps that maintain the vacuum will remove the helium byproduct.

Only a short length, to the technical detail that interests me, to the extent that the helium will be sucked out by the vacuum kit.

The issue is - how does the helium get to that evacuation port?

A helium, duly neutralised, will 'wander' (or bounce) around the chamber. Why would it know to go straight to the evacuation port?

There is a very good chance it will get into the 'shell' inside the magrids and there be ionised. It will then bounce around indefinitely, never neutralising, never fusing, never leaving the reaction space and thus 'polluting' it.

That is one of the quite clever aspects of the machine. As I understand it, the alpha particles will two distinct energy characteristics. There will be a potential gradient to move out of the machine, the alpha particles will need to 'do work' to climb the potential gradient, as they get to the near zero energy location they will be collected by an external electrode, actually two, one for each major species produced. Someone else has described this process like throwing a ball vertically, just at the top of its trajectory, as it starts to fall back to earth, sliding a shelf beneath the ball to arrest its return.

I've yet to see a practical demonstration of this. Before the tokamak was even built, it was already well-proven that energy could be recovered from fast neutrons, so energy recovery means was proven. Unlike the tokamak, this ion recovery process has not been demonstrated practically, as far as I am aware. Maybe it has been done, I just don't know of any. Has anyone information on practical power recovery from fast beams to link to?

You are right about the two-energies of 4He. One third will be at ~5.8MeV, two thirds at ~1.4MeV, I guess, judging from the data sheets.

So we will have alphas at 5.7MeV hitting the magrid. 'Deceleration means' or not, it won't be sitting in front of the magrid, so the magrid will get a direct hit. There will be 10,000's of ions and electrons liberated in that one collision, and there will be order of 10^20 per second of these collisions. The 4He's will be halted, then, presumably, accelerated into the reaction space along with the magrid material ions, and the reaction space will be heavily contaminated reducing the reaction rate.

Just think about it for a moment - the target is 800MW. If the magrid occupies just 5% of the total steradian coverage, that'd be 40MW*(5.7MeV/8.7MeV) of alpha particle power being pumped straight into the magrids. So you would need of the order of 10's MW of cooling on the magrids. And that is not beginning to contemplate this issue of contamination.

I have been asked to try to provide solutions. I cannot see any. Sorry. This appears to be insurmountable as the 4He reaction products will be isotropic and will therefore bombard the magrids uniformly.

best regards,

Chris MB.

[TallDave]

Converting alphas is relatively simple (it's nothing more than a particle accelerator run in reverse). Alpha sputtering may be a problem.

There are some more detailed anaylses of the conversion issue in the Design forum.

Alpha sputtering is very worrisome and I don't think there's a good model for how that can work yet.

It's not so much the cooling that's a problem (we already envision 1 MW/sq inch cooling for a D-D iirc, hence the need for trinary concentric cooling pipes) as the fact those alphas are going to knock little pieces of the Magrid off.

Of course, if fusion were easy we wouldn't be here in 2008 still trying to figure it out.

[Mike Holmes]

Chris, as a non-technical outsider, let me propose what seems to be the problem. You ask questions: great. Then people post links to threads where this stuff has been discussed previously. You seem to then come back and say that nobody has answered your question.

Worse, then you say that there is no answer, and that we all must simply be dreaming. As if somebody has proposed that we already know how to build a net-power machine.

What it sounds like, even if it's not, is that you are unwilling to go and read the stuff that people have already put hard work into laying out previously. A record like this forum exists, in part, so that such discussions do not have to be rehashed again and again for each new person who comes along. In theory it's a labor-saving technique.

Same with Simon's blog.

You coming back and saying "I can't find it there, can you point it out for me" sounds, again, like you're not trying very hard, and are demanding special attention.

Now, I use the word "sounds" specifically and intentionally, because this may well be an erroneous perception on our part. But be aware that this is how you come off. To more than one reasonable person. You may want to adjust your tactics slightly, if, in fact, your goal here is to become more informed.

In any case, on both sides here, we're seeing a lot of behavior that I can only quantify as identity politics. Frankly, I'd love to see ITER work, as much as I'd like to see Polywell work, or any other net-power fusion solution. Are we really in competition for funds? Is that what's going on? Why do we have to compare probabilities at all? The only reason to do so is because you're identifying yourself with one or the other project, and an attack against it is feeling like a personal attack. As if you're foolish to defend the project.

These two projects are where they are in their life cycles. They each hold their own promise, and their own challenges. Why don't we leave it at that, and get back to the subject at hand here, of what we can do to help out with the Polywell project (if you're looking to promote ITER, I'm sure there are plenty of sites devoted to that subject).

I'm going to assume that you're not here to malignly sabotage Polywell, but are legitimately interested. Even criticism is welcome here, as a way to get to understanding - we love that Art Carlson has spent his valuable time trying to dissect the project. But we see his contributions as being genuinely inquisitive, and the discussions end up with everybody being better informed. He's rapidly becoming as much of an expert on the nature of the Polywell as any of it's direct proponents.

Your contributions... well, again, perhaps we're being too sensitive or something, but your contributions feel like they're disingenuous. Like you're trying hard not to understand, but to react from a position of as little information as possible in order that you can launch criticisms that will be tirefully laborious to answer. Like you're trying to wear us down. Like you've got an axe to grind.

Could well be just our own identity issues here. But that's human nature to some extent, and you'll have to pardon us if we're somewhat invested in the project, given the work we've done (not really myself, but those who have put in lots of hours discussing these subjects). That's not to say that you have to be especially sensitive or something, and certainly not that you shouldn't be critical, or ask questions. But merely that you try to do so in a way that doesn't seem like badgering. You'll get better responses, I'm sure.

I hope that helps.

Mike

[chrismb]

Alpha sputtering is very worrisome and I don't think there's a good model for how that can work yet.

Thanks, TallDave. Now that seems a sensible start to addressing the issue I tried to raise, and from here I can then possibly make contribution!!

Regrettably, this may mean that the device will have to run with a neutron emitting fuel so that there is less sputtering at the surface. Unfortunately, it would still rule out DT, because the 4He from that still peels out of the reaction at 3.4MeV.

p+6Li might work a bit better, at least keeping the maximum particle energies down (~2MeV for each of the 3He and 4He, still not great, but usefully lower). It peaks in cross-section at somewhat over 1MeV, so after climbing back up the potential, if you ran it around that well depth, then the sputtering would be much reduced. Obviously more engineering challenges with the higher drive voltage, but maybe it's the only way.

Another advantage would be that there aren't any secondary reactions from that, whereas for p+11B there are, e.g., follow up reactions like 4He+11B which is neutron emitting.

The maximum power output would have to be kept within a 20MW/m2 limit, which is the absolute limit of known W based divertor materials, as far as I am aware. For a device with structures at 1m distance, this would therefore be an absolute limit of 250MW but I think it would have to be notably cooler than that again, really, to be able to get the coolant through the magrid.

[chrismb]

Chris, as a non-technical outsider, let me propose what seems to be the problem. You ask questions: great. Then people post links to threads where this stuff has been discussed previously. You seem to then come back and say that nobody has answered your question.

well.. Ok I see this perception. But I keep coming back to the detail. it's the detail we need to get right, the hand-waving will sort itself out.

Worse, then you say that there is no answer, and that we all must simply be dreaming. As if somebody has proposed that we already know how to build a net-power machine.

I'll keep an eye out for giving that impression. I mean to denigrate no-one and apologise for such occasions.

You coming back and saying "I can't find it there, can you point it out for me" sounds, again, like you're not trying very hard, and are demanding special attention.

But I did do this! I can just go and keep quite and pretend to have found it, but I didn't find it.

I do not expect to be able to demand an answer, but it was a response to my comment that said such a thing exists. I am simply looking for that.

In any case, on both sides here, we're seeing a lot of behavior that I can only quantify as identity politics. Frankly, I'd love to see ITER work, as much as I'd like to see Polywell work, or any other net-power fusion solution. Are we really in competition for funds? Is that what's going on? Why do we have to compare probabilities at all? The only reason to do so is because you're identifying yourself with one or the other project, and an attack against it is feeling like a personal attack. As if you're foolish to defend the project.

These two projects are where they are in their life cycles. They each hold their own promise, and their own challenges. Why don't we leave it at that, and get back to the subject at hand here, of what we can do to help out with the Polywell project

I couldn't agree with you more. I have never raised ITER/tokamak. They have appeared in the threads because others chose to use it as part of their arguments. To progress that debate, I therefore need to close down that discussion.

(if you're looking to promote ITER, I'm sure there are plenty of sites devoted to that subject).

I send similar questions to the tokamak communities. JET at Cluham are exceptionally open and friendly and always have answers to these types of questions in spades, and ususally much more than I had thought about. ITER seem cagey. The Japanese never reply.

I'm going to assume that you're not here to malignly sabotage Polywell, but are legitimately interested.

I'm not sure why I am here yet, either. I thought I would run through a few baseline quantitive questions first to form my own technical assessment of where the project is at.

Thanks, Mike.

best regards,

Chris MB.

[Mike Holmes]

I'm not sure why I am here yet, either. I thought I would run through a few baseline quantitive questions first to form my own technical assessment of where the project is at.

That's great. You seem to be a competent physicist, and it would be great if it turned out that you could see some potential in the project. I hope you can be convinced.

Mike

[MSimon]

chrismb,

I have a sticky post up at Admin that may be of some use to you. It starts with a bit on etiquette and gives a list of basic learning resources.

I had an Air Force guy tell me a year ago that my IEC Fusion blog was the place to go to get up to speed. He told me he sends all his newbies there to get an education. AFAIK not one of them has had to ask any questions after reading the material. And I have improved the resources since then.

I cover everything from basics to advanced topics.

I envision that all the basic research up to and including a 100 MW research reactor will be done with D-D. There are ways to make the SC coils immune (relatively) to high neutron flux. Good for at least 100s of hours of operation before the coils need annealing to remove neutron damage. Possibly as long as 50,000 hours with the right design.

BTW Bussard's design for the 100 MW reactor envisions a heat flux of 1 MW/sq m. At the high end of non-exotic cooling. A good place to start until the concept is worth more engineering. And even then if necessary it could be throttled back to 30 MW giving 1/3rd the heat load and absolutely ordinary heat transfer conditions.