I've done a couple really rough and ugly ones. I believe Rick gave us a number for electron losses at one point. Anyways, you asked for an explanation, not a calculation.
A calculation generally trumps hand-waving. Rider has provided a quantitative calculation. That's how high the bar is now.
TallDave wrote:
How this energy is captured and recirculated?
AFAIK electrons just recirculate by bouncing off of the B fields. I'm not sure what you're looking for beyond the WB effect.
You apparently haven't understood the essence of Rider's argument, which applies even if you have 100% "recirculation" in the sense that electron loss rates are negligible. In order to reduce bremsstrahlung, you need to reduce the energy in the electrons, but in oder to produce fusion, you need sufficient energy in the ions. In a thermal plasma, it is easy to calculate how fast the cold electrons are heated by the hot ions. The answer is "too fast". That is where non-thermal distributions come in: By reducing the number of electrons with velocities similar to those of the ions, you can reduce the coupling between ions and electrons and therefore keep the electrons colder. But you have added a new problem: The electrons, left to themselves, will relax to a thermal distribution. Again, it is easy to calculate how fast they do that, and again the answer is "too fast". If you want to go that route, you have to continually remove energy from some some groups of electrons and add energy to other groups. Now, there are ways this can be done, but Rider showed that we are talking about a lot of energy, so however you do this, you have to do it very efficiently. This has nothing to do with electrons bouncing off B-fields and is the point that has never been addressed by Bussard or Nebel or anyone else.
Rider has provided a quantitative calculation. That's how high the bar is now.
Well, again, you didn't ask for a calculation, just an explanation. A calculation doesn't necessarily trump handwaving unless they're working from the same assumptions, but if you want to raise the bar to calculation, I'll raise you again to a full bounce-averaged simulation and point you to Chacon et al.
You apparently haven't understood the essence of Rider's argument
He makes a couple arguments. One has to do with losses to the grid. The brem/electron thermalization issue (I assume we are only talking about p-B11 now) was addressed here by Rick a year ago:
As near as I can tell, the arguments being made here are the same as those made by Rider many years ago. The problem is that you can’t study the polywell physics in isolation, you have to study the system as a whole. This is the mistake that Rider made and it appears to be the mistake that is being made here. You can’t just look at the coupling between the ions and the electrons, you have to look at where the electrons go.
The electron temperature in a polywell is set by the potential well and it is largely unaffected by the ion-electron collisions. The reason is that electrons which are upscattered by ions are removed from the system. These upscatterd electrons will rattle around in the polywell until they exit through a cusp. Since they have been upscattered, they will have more energy than the applied potential between the coils and the wall. Consequently, they will leave the system and go to the wall. As they proceed to the wall, they will lose almost all of their energy through direct energy conversion (this is what happens when a particle runs up a potential hill). Consequently, the electrons located inside the polywell don’t heat.
...
I would say that the following is true:
1. P-11B is a very different beast to optimize than D-D or D-3He due to the Bremsstrahlung.
2. If all you had to do was to light the plasma against transport (and the theoretical scaling laws work) then one could get very large Qs from P-11B. This is the upper limit possible assuming 100% direct energy conversion.
3. The Polywell allows you to select whether you want the power to come out in charged particles or Bremsstrahlung. The lower the Bremsstrahlung power the more recirculating power you are going to have. The tradeoff is whether you win by direct energy conversion of electrons or by higher temperature electrons giving off Bremstrahlung.
4. Polywells often have virtual anodes which means that in a single orbit through the Polywell the coupling direction between ions and electrons may change, as Hanelyp suggested.
5. The devil is in the details. You have to do the energy balance on the entire system to see what will work. Sweeping statements like those made by Rider are overly simplistic.
I'm not sure you're going to get too much more of an answer due to NDA considerations, but it sure sounds like EMC2 has simulations that contradict Rider.
If you have ”direct energy conversion” from electrons and ions going in the same direction the effect from the two particles should cancel each other. The effectively in direct energy conversion will drop.
Torulf2 wrote:If you have ”direct energy conversion” from electrons and ions going in the same direction the effect from the two particles should cancel each other. The effectively in direct energy conversion will drop.
I think that would apply if the ion and electron escape flows through the cusps were ambiplor which they are claimed not to be. Many more electrons (especially the important upscattered electrons) are escaping via the magnetic leaks than ions because the ions are confined both by the mafnetic fields, but more so by the electrostatic field produced by the excess electrons.
If the escaped electrons have an energy above the potential on the magrid, then they will not recirculate and will fly to the wall. I had not concidered that these escaped electrons would be climbing up a potential well outside the magrid and thus surrendering a least a part of their energy back to the potential well. That decreases the energy losses, while still permitting these evil high energy electrons to leave the system.
I have seen calculations by both sides, reputed to conclusively prove it will and will not work.
I have seen Dr. Bussard run calculations on a blackboard and not come up with the same answer twice ... the primary reason being each time he did the calculations he was trying a new assumption, so I have a pretty good idea how grey some of those assumptions were.
The Polywell has never been adequately modeled. I've seen people try and I've seen the difficulties. I applaud the attempts as they undoubtedly produce insights, but it is a devilishly complex system for such a physically simple machine. For that matter, even modeling a Fusor is a fairly complex task, and I've seen criticisms of several models that have been tried. My own attempt worked perfectly on my first try using the conditions of a run on my own fusor ... then fell apart on all other conditions.
If you wish to do calculations to prove it will not work you need only choose a model in which it cannot work. For example, you assume it is a tokamak.
If you wish to do calculations to prove it will work, you need only choose a model that disregards the reasons it won't.
If you wish to prove it will work, not just calculate that it will, you have to build it. Since WB7 has been running and a new machine in on the way, I'll wait for the results I trust.
I will also be delighted if Focus Fusion works. Or Tri-Alpha's project.
For that matter, I'd be delighted to hear that ITER was routinely producing net power.
You'd have to have a way to pick up upscattered electrons before they got past the trap grid, or whatever structure it is that maintains a negative potential relative to both the magrid (tens of kV) and the wall (MV). Electron direct conversion using the magrid potential well is all well and good, but if an electron gets out into the high-voltage alpha converter it's going to slam into the ash collectors at very high energy, somewhere in the range of a MeV.
Detailed design of the system may smear out the sharp distinction I've drawn, but the upshot is that you can't use the same gradient to decelerate alphas and electrons at the same time. You have to do one first, then the other. And it strikes me that the potential for catastrophic energy drain is much lower if you do the light, relatively low-energy electrons first.
Rider has provided a quantitative calculation. That's how high the bar is now.
Well, again, you didn't ask for a calculation, just an explanation. A calculation doesn't necessarily trump handwaving unless they're working from the same assumptions, but if you want to raise the bar to calculation, I'll raise you again to a full bounce-averaged simulation and point you to Chacon et al.
... optimistic conditions (i.e., spherically uniform electrostatic well, no collisional ion-electron interactions, single ion species)
And as for an explanation, do you mean MSimon's comment?
Bussard claims that the discovery of what he terms the Wiffle Ball effect and by circulating electrons escaping from the Wiffle ball at high efficiencies he can get the total electron circulation efficiency into the 99.999% to 99.9999% range, making machines of his proposed design viable for power production.
Again not relevant (although it may be an accurate description of what Bussard said). Rider effectively assumed 100% recirculation. Problem still there.
Then I guess I can't really tell what you're asking, or are saying is unanswered. The Chacon paper gives energy gain (i.e. power balance) for these kinds of systems in general, at least in regards to D-D. Rick kind of addressed the brem/electron thermalization in p-B11.
Last edited by TallDave on Thu Oct 22, 2009 3:28 am, edited 1 time in total.
If the understanding of the contract info is correct, the results on WB8 will be delivered end of April. If it's a flop, we shouldn't expect funding for WB8.1. Maybe we get an annoucement its a flop, maybe they get a few more months to try and make it work first. If it doesn't flop, and they get the follow on funding for WB8.1, probably no announcement. Longer silence would mean they've moved on to PB11 experiments. They probably wouldn't want to announce WB8 success in case WB8.1 flops. The closer they get to finding out, the more reason to keep quiet. If we hear nothing by end of July next year, we can probably assume the scaling and confinement time worked on WB8.
Wouldn't saying WB7 is (was) running like a top be the riskiest thing to say, of all points in that chronology? Saying (e.g.) that "WB8 is looking good so far" seems like less risky an admission than that WB7 was running like a top back when nothing was sure.
I don't discount that things may change to forbid such a communication, but I'd expect some word or other when WB8, WB8.1, etc, are thru (or their respective projected R&D cycle as in the contracts at least).
choff wrote:If the understanding of the contract info is correct, the results on WB8 will be delivered end of April. If it's a flop, we shouldn't expect funding for WB8.1. Maybe we get an annoucement its a flop, maybe they get a few more months to try and make it work first. If it doesn't flop, and they get the follow on funding for WB8.1, probably no announcement. Longer silence would mean they've moved on to PB11 experiments. They probably wouldn't want to announce WB8 success in case WB8.1 flops. The closer they get to finding out, the more reason to keep quiet. If we hear nothing by end of July next year, we can probably assume the scaling and confinement time worked on WB8.
I have come to a somewhat similar conclusion. On my optimistic days (most of them) I'm thinking "the Navy wants a year or three head start".
Some one said a while back that Fusion Research was required to be open by treaty. And a Fusion Powered CV will be hard to hide in any case.
On top of that BFRs would alter the job the Navy needs to do. Cargo ships in the 30 to 50 knot range would cut way back on piracy. And "unlimited" energy would go a long ways towards reducing the incentive for war.
Engineering is the art of making what you want from what you can get at a profit.
They probably wouldn't want to announce WB8 success in case WB8.1 flops.
A WB 8.1 flop isn't the end of the world. A viable D-D/D-T Polywell reactor would still be a huge deal even if p-B11 turns out not to be viable.
I'm not sure they will announce anything at all before getting WB-100 to produce megawatts of fusion (obviously, there would be attention just to the attempt, but I expect they would be fairly quiet and reserved). Dr. Nebel has cited the cold fusion embarassment as reason to be cautious. It's important not to overpromise.
Even with good WB-8 results, this is still a longshot.
If you go back and read what he actually said, Dr. Nebel's comment that WB-7 was "running like a top" was simply a statement that the apparatus was robust and worked repeatably. This is compared to WB-6, which did run repeatably at lower power in electron transport studies, but blew on the fifth fusion attempt.
Saying the apparatus functioned well is not the same as saying it was producing data that proved the concept would make power plants. He also warned us the results were "nuanced." Our last word from him suggests he is still cautious in his claims.
What we can draw from this is:
1) Dr. Park knows what he is doing in the lab, which raises my hopes that, one way or another, we'll learn if this thing can work. The data may or may not be what we want, but the data should be reasonably abundant and probably trustworthy.
2) The "nuances" were sufficiently encouraging to move ahead.
It's not in the national interest of the US to keep this technology from going commercial. Furthermore, this project has never been classified. Fusion research world-wide was declassified in 1958 by international treaty.
Engineering is the art of making what you want from what you can get at a profit.