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[chrismb]

Some one said a while back that Fusion Research was required to be open by treaty.

What does this bit mean, and what is its origin?

viewtopic.php?p=5142#5142

rnebel:

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.

That's a mis-confubulation of bits. I rather think what was implied that 'fusion research as of 1958' was opened up by the IAEA treaty, but the IAEA is merely an interface between the 'competent authorities' of each member country. I can't speak for US, but the UK 'competent authority' only has interest in tokamaks and has no interest in any other fusion work.

[D Tibbets]

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.

I think you are assuming that the decelleration grid= acceleration grid, depending on the charge on the particle. I'm guessing that this is true only in part as the 'venition blind' arrangenet of electrostatic (and magnetic?) plates allows selective removal of particles based on some range of energy. Thus the escaping electrons would not be accellerated past the entire series of plates but be captured early on. This would represent an energy loss, but it would be less (much less?) than allowing the electron to pass the entire series of plates (blinds). A positively charged series of plates could even be used to decellerate the electrons (as they pass) till they could be diverted sufficiently to be captured, while the alphas or other positive ions gain energy, but this can be recovered by additional decelleration plates downstream.

Dan Tibbets

[D Tibbets]

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.

Chacon? Not relevant:

... 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.

I think assuming 100 % recirculation as assumed by Rider is a good thing is completely ignoring the point. The removal of the tiny fraction of the highest energy electrons is the supposed benificial effect. Obvously, with 100% recirculation this would not be possible and at least partially invalidates the comparison.

Dan Tibbets

[Art Carlson]

I think assuming 100 % recirculation as assumed by Rider is a good thing is completely ignoring the point. The removal of the tiny fraction of the highest energy electrons is the supposed benificial effect. Obvously, with 100% recirculation this would not be possible and at least partially invalidates the comparison.

Ultimately, his argument is about transferring energy from one group of electrons to another. Maybe you can do that with some fancy laser or RF system on a population of electrons with no losses. If you want to physically remove some of the elextrons from the machine and reinject then at a different energy, that accomplishes the same thing. The calculation applies either way.

[Art Carlson]

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.

The Chacon paper gives energy gain for a highly simplified system, providing a stringent upper limit. Since he ignored the coupling of the ions and electrons, and the bremsstrahlung, he never confronted Rider, and I am tempted to say he simplified to the point of dishonesty, but I think he made clear in his paper what the limits of his calculation were. It's the people who use this paper to support claims that Chacon never made who are dishonest, or maybe just ignorant, but probably just overly enthusiastic.

Rick kind of addressed the brem/electron thermalization in p-B11.

Rider's calculation is remarkably general, but no calculation can be perfectly general. For example, he used a rather general class of EEDF, but maybe a distribution not in that class would give more optimistic results. Nebel suggests that relaxing the assumption of spatial uniformity would make things better. I think both of these possibilities are unlikely, but there is no way I can prove that. If Rick would provide just a single example of a non-uniform system that requires less recirculating power than calculated by Rider, then he would have proven the principle and we can all start pushing to find even better ways. This has to be the next step in the dialog, and it is one that neither Rick, nor Chacon, nor Bussard has ever made. (It's fine with me if you want to let Rick off the hook because he is busy getting experimental results for all of us, but I hope you understand now why I say that no one has ever been able to explain to me how you can get around Rider - except for the barest of incipient ideas.)

[bcglorf]

I've got a dumb question, and it's possibly off topic as well, just tell me if so and I'll ask somewhere else.

I've been curious what is supposed to happen if you could create a perfectly spherical magnet field and used it for containing a cloud of electrons. I know, it's impossible, but I'm curious what the expected Maxwellian distribution of the electrons is supposed to be if they are contained in that manner long enough. Is my intuition correct in believing that the center should have lower electron density and/or velocity distributions than the rest of the volume, even over a long time frame? It seems to me from Rider's paper that the 'correct' answer is that the velocity and density throughout the sphere should become uniform over time. I know plasma physics isn't supposed to fit my intuition, but it seems really odd to me that a sphere of electrons wouldn't keep a non-uniform distribution of velocity/density simply by virtue of the electric field created by the electrons themselves.

[chrismb]

I've got a dumb question, and it's possibly off topic as well, just tell me if so and I'll ask somewhere else.

I've been curious what is supposed to happen if you could create a perfectly spherical magnet field and used it for containing a cloud of electrons. I know, it's impossible, but I'm curious what the expected Maxwellian distribution of the electrons is supposed to be if they are contained in that manner long enough. Is my intuition correct in believing that the center should have lower electron density and/or velocity distributions than the rest of the volume, even over a long time frame? It seems to me from Rider's paper that the 'correct' answer is that the velocity and density throughout the sphere should become uniform over time. I know plasma physics isn't supposed to fit my intuition, but it seems really odd to me that a sphere of electrons wouldn't keep a non-uniform distribution of velocity/density simply by virtue of the electric field created by the electrons themselves.

It'll just thermalise and the electrons will take up most of the thermal energy. Just like if it was contained in a toroidal plasma. (And, 'fraid to say, just like if it was contained in a quasi-spherical volume, for any significant length of time).

[bcglorf]

I've got a dumb question, and it's possibly off topic as well, just tell me if so and I'll ask somewhere else.

I've been curious what is supposed to happen if you could create a perfectly spherical magnet field and used it for containing a cloud of electrons. I know, it's impossible, but I'm curious what the expected Maxwellian distribution of the electrons is supposed to be if they are contained in that manner long enough. Is my intuition correct in believing that the center should have lower electron density and/or velocity distributions than the rest of the volume, even over a long time frame? It seems to me from Rider's paper that the 'correct' answer is that the velocity and density throughout the sphere should become uniform over time. I know plasma physics isn't supposed to fit my intuition, but it seems really odd to me that a sphere of electrons wouldn't keep a non-uniform distribution of velocity/density simply by virtue of the electric field created by the electrons themselves.

It'll just thermalise and the electrons will take up most of the thermal energy. Just like if it was contained in a toroidal plasma. (And, 'fraid to say, just like if it was contained in a quasi-spherical volume, for any significant length of time).

I guess my trouble comes in with the electric field that will exist. A thermalized, uniform sphere of electrons should create a negative potential in the center. Doesn't that mean that electron energy and velocity can't both be uniform throughout? If electrons in the center have the same average velocity as electrons outside the core, they will have more energy. If electrons in the center have the same average energy as those further out, then they have to be slower. Or am I just completely out of my mind?

[chrismb]

I guess my trouble comes in with the electric field that will exist.

A [fusion] hot thermalised plasma will have almost no resistance and the electrons will try to neutralise that field by moving away from the centre (if they were ever there). The magnetic fields and thermal motion may cause all manner of transport barriers or enhancements but, just as tokamak researchers have found, you don't really know how it's gonna work out and it's usually chaotically turbulent.

[Art Carlson]

There is a theorem that, given enough time and the unavoidable "classical" collisions, charged particles will always spread across a magnetic field to form a uniform distribution, just as if the field were not there.

[93143]

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.

I think you are assuming that the decelleration grid= acceleration grid, depending on the charge on the particle.

That's a law of physics.

I'm guessing that this is true only in part as the 'venition blind' arrangenet of electrostatic (and magnetic?) plates allows selective removal of particles based on some range of energy. Thus the escaping electrons would not be accellerated past the entire series of plates but be captured early on. This would represent an energy loss, but it would be less (much less?) than allowing the electron to pass the entire series of plates (blinds).

There has to be a negative potential gradient for the electrons to run up, or recirculation won't work at all.

Just make sure the first collectors are close enough to the bottom of that gradient that the decelerated electrons don't gain back a significant amount of energy before hitting them. I guess, anyway...

[TallDave]

The Chacon paper gives energy gain for a highly simplified system, providing a stringent upper limit.

Doing the full bounce-averaged Fokker-Planck simulation is a lot more complex a look than Rider ever took at ion distributions. While it doesn't address brem, it does address Rider's other objection about ion upscattering. Sorry, I'm having trouble keeping track of which of Rider's points you're talking about.

I guess I'm still not sure why you don't accept Rick's explanation that where the electrons go matters. Reading Rider's section on ion-electron energy transfer, it seems intuitively obvious that Rick's explanation breaks Rider's asumptions. Or are you just asking for something better than handwaving?

[TallDave]

Just make sure the first collectors are close enough to the bottom of that gradient that the decelerated electrons don't gain back a significant amount of energy before hitting them. I guess, anyway...

How about shielding strong enough to deflect electrons, but weak enough it doesn't alter the path of alphas very much?

[TallDave]

Doesn't that mean that electron energy and velocity can't both be uniform throughout?

At least for a little while.

I seem to recall this coming up before. IIRC, the electron thermalization time was too short to matter in small machines, but in reactor-sized machines it may be an issue. Maybe I'll try to dig up Rick's post on that.

[93143]

Just make sure the first collectors are close enough to the bottom of that gradient that the decelerated electrons don't gain back a significant amount of energy before hitting them. I guess, anyway...

How about shielding strong enough to deflect electrons, but weak enough it doesn't alter the path of alphas very much?

Do you want to collect the upscattered electrons or not?

If you stick the electron emitters deep enough in the magrid potential well, very few electrons will ever make it out far enough to enter the alpha collector system. That might actually be the best way to go, since asymmetrically bleeding off the high-energy tail reduces the average energy of the distribution, which could be bad. I don't know if magrid losses are any more even-handed, but they probably aren't any worse.

But if an electron does make it out to the collectors, why would you want to prevent it from hitting the first plate? If it misses the first one, it will be going that much faster when it hits the second one. Shield the second one, and it's going even faster when it hits the third one. Shield all the collectors and you have MeV-range electrons slamming into the outer wall. Never mind the fact that the only way to "shield" these things is magnetically, since they sort of have to be positively charged to do their job, and magnetic shielding powerful enough to deflect a MeV-range electron is definitely going to screw with nearly-stationary alphas coming in for a landing.