Remind me - why 10T field?

[D Tibbets]

Why do colder electrons leave via the Magrid?

Its the only place they can go. They don't have the energy to reach either the core virtual cathode or the chamber wall. Eventually they leak thru the B field to the positive MaGrid.

Cross field transport is the dominant loss mechanism in non cusp systems like tokamaks (if edge instabilities can be controlled). In a cusp system like the Polywell, the leaks through the cusps dominate- except with efficient recirculation this loss mechanism can be mostly eliminated. The Polywell then becomes somewhat similar to a tokamak, at least for electron losses.
I believe the dominate cross field transport mechanism is a random walk process where a charged particle trappped on a field line is hit by another charged particle and jumps one gyro radii closer to the magnet. This process eventially leads to the particle working it's way through the magnetic field and hitting the magnet.
Within this cross field transport dominate regime, the polywell has a tremendous advantage because only the electrons need to be confined thus. The ions are confined due to secondary electrostatic forces from the excess electrons. The tokamaks try to contain neutral plasmas. In tokamaks the ions have much greater gyro radii compared to electrons at any given energy. Thus, in order to contain them long enough, the distance they can 'random walk' before they reach the magnet must be large enough to accommodate the confinement time needed for useful fusion. This is what drives tokamaks to such large sizes. This begs the question- why don't tokamaks use electron rich (non neutral) plasmas? I don't know.
This might improve confinement times for any given size, or perhaps help to control edge instabilities (like might be (sort of) like what Tri-Alpha is possibly doing with their FRC approach) . Also, I'm not sure why tokamak plasma densities are more limited than Polywells. It might have something to do with arcing concerns. also the effects mentioned in my prevous post might play a role.

Dan Tibbets

[TallDave]

What Kite said.

That makes me wonder now -- if the cold electrons are rattling around the bottom of their well, that puts them inside the cusps, which is the closest they can get to the Magrid. I wonder if that shows up as a larger current at that area on the Magrid.

It would really be neat to have a detailed map of the currents across the surface of the Magrid. Maybe someday...

I wonder how much Rick's fast photography shows?

[TallDave]

I believe the dominate cross field transport mechanism is a random walk process where a charged particle trappped on a field line is hit by another charged particle and jumps one gyro radii closer to the magnet. This process eventially leads to the particle working it's way through the magnetic field and hitting the magnet.

Thanks, I hadn't seen this described before. Here's a nice wiki on RW:

http://en.wikipedia.org/wiki/Random_walk

This begs the question- why don't tokamaks use electron rich (non neutral) plasmas? I don't know.

Wrong shape, I think. The force would be tend to point toward the inner wall (Gauss). I believe this issue came up a couple years ago whem someone raised the idea of a torus Polywell.

Also, I'm not sure why tokamak plasma densities are more limited than Polywells.

Low beta because of the bad curvature, and (I think) ion pressure.

[icarus]

TallD:

Are you being purposely evasive or is that just how it comes across? To avoid any possibility for further circumlocution, let's try and pin down one simple point here.

Q: Do we agree or not that a Polywell of a certain radius (say 1m coils) will have a maximum ion density that it can operate at?

(Accepting the assumption that the density across the entire radius is most likely some smooth functional relationship.)

[MSimon]

TallD:

Are you being purposely evasive or is that just how it comes across? To avoid any possibility for further circumlocution, let's try and pin down one simple point here.

Q: Do we agree or not that a Polywell of a certain radius (say 1m coils) will have a maximum ion density that it can operate at?

(Accepting the assumption that the density across the entire radius is most likely some smooth functional relationship.)

I do believe there is a maximum ion density. Now the question is: what is the limiting factor? Compression ratio (reaction space vs non-reaction space)? Space charge? Magnetic field (a compression ratio factor)? Or something else.

I wish I was in a position to do experiments.

At this state of knowledge I think coming up with a number is dicey because there is very little experience with oscillating beam machines. It is all by guess and by gosh with numbers all over the place.

[TallDave]

Q: Do we agree or not that a Polywell of a certain radius (say 1m coils) will have a maximum ion density that it can operate at?

Short answer: no (or at least, not limited by the annealing range; obviously the overall limit stops somewhere short of infinity). As Rick pointed out, a Polywell can operate with a Maxwellian plasma. It just doesn't have ion convergence and a monoenergetic (or at least only partially relaxed) distribution without the annealing.

Long answer: depends what you mean by "operate." There are probably edge densities at which we won't have ion focus, because the annealing process doesn't work at those densities. But PWs can also operate as unfocused, magnetically confined ETWs with a shielded anode, which can be run at high beta and thus still outperform toks by a few orders of magnitude even without focus. That may not be the preferred mode, though, given the gains from convergence (hence my last question there, about the tradeoff). I think whether the range of densities that allow ion convergence are suitable for power generation is an open question. It might even be dependent on the application. (And a thermal tail is bad news for advanced fuels, of course.)

I think this notion is confusing, because we're so used to thinking of IEC machines as being, basically, ion focus machines. Rick's comment seems to indicate that the WB is actually much more important.

[chrismb]

Rick's comment seems to indicate that the WB is actually much more important.

You guys crack me up! All that talk somewhere about writing down what Rick has said so you can repeat it later.... you do realise this is how religions start, don't you?

[TallDave]

Heh, I didn't realize all those people attending scientific lectures and taking copious notes were actually doing religion, not science. No doubt your revelation will shake the very core of the scientific establishment.

I wish I was in a position to do experiments.

I'm increasingly hopeful we'll see some data in the next year or so, because if Tri-Alpha, Focus Fusion, General Fusion, etc. start producing more newsworthy items as their schedules seem to indicate, it may reduce the perceived pressure the sponsors seem worried about.

[chrismb]

They take notes because of what is said, not because of who speaks.

[MSimon]

Rick's comment seems to indicate that the WB is actually much more important.

You guys crack me up! All that talk somewhere about writing down what Rick has said so you can repeat it later.... you do realise this is how religions start, don't you?

Reminds me of the cult of Einstein. People are still quoting the dude.

[D Tibbets]

TallD:

Are you being purposely evasive or is that just how it comes across? To avoid any possibility for further circumlocution, let's try and pin down one simple point here.

Q: Do we agree or not that a Polywell of a certain radius (say 1m coils) will have a maximum ion density that it can operate at?

(Accepting the assumption that the density across the entire radius is most likely some smooth functional relationship.)

Indeed there is a maximum limit. Based on what is mentioned in the patent application about the modest B fields necessary to maintain dynamic pressures of 10^22 to 10^23 particles / M3, the magnetic field strengths obtainable should not be limiting. Machine size and thermalization issues may force some limiting compromise.
But, from what Bussard, and Nebel have said, the Wiffleball confinement factor or gain is the critical issue. Due to the limit allowable density outside the magrids before Pashin arcing occurs ( ~ 5 X 10^-6 atmospheres (~ X 10^19 to 10^20 particles / M^3. This sets a density limit similar to Tokamaks, or I assume any machine where some mobile charge carrier separation from ground is required*. The Wiffleball confinement, if real allows a density differential of ~ several thousand above this limit, and this is what limits any edge density within the Wiffleball to ~ 10^22 or 10^23/ M^3. The picture becomes more murky with confluence and effective densities above this in a core smaller than the Wiffleball radius. Nebel mentioned several orders of magnitude were possible. That would set a hard upper limit for the core density at ~ 10^25 charged particles / M^3 within a small confluent core.

* Why does the Wiffleball allow increase4d densities? I speculate it is due to multiple factors. Smooth surfaces so there is not charge build up. Gauss law properties such that the internal plasma does not see the positive charge on the magrid. Magnetic shielding against electrons. This is a major difference from tokamaks. The electrons will not "slowly arc" to the magnetically insulated surfaces within the magrid as rapidly as ions due to their smaller gyroradii. The ions being contained electrostaticallly by the excess electrons allows for this dichotomy that I do not believe would be possible in neutral plasmas. Even with these advantages, I believe the Wiffleball confinement could tolerate edge densities much above 10^22 to 10^23 charged particles / M^3 before arcing would occur within the machine. It all boils down to what tricks you can play to delay the onset of Pashin arcing as the density increases.

Dan Tibbets

[chrismb]

Reminds me of the cult of Einstein. People are still quoting the dude.

Not sure why it reminds you so. Einstein's work is reproducible, and *provable*, from first principles. Polywell is a mishmash of hopeful claims - a bit like a religion, in fact.....

[TallDave]

Heh, if anything reminds me of a religion here it's how chrismb religiously compares interest in Polywell to religion. Not even very good trolling. The proof of Einstein's work was empirical, just like the proof one way or the other will be for Polywell.

They take notes because of what is said, not because of who speaks.

And we aren't taking notes because of what Rick is talking about? Hello? You do realize this place is called TalkPolywell? He's the guy running the experiment, and we're interested in how it's going.

Anwyays, bleah, enough feeding...

But, from what Bussard, and Nebel have said, the Wiffleball confinement factor or gain is the critical issue. Due to the limit allowable density outside the magrids before Pashin arcing occurs ( ~ 5 X 10^-6 atmospheres (~ X 10^19 to 10^20 particles / M^3.

Good point Dan. It's apparently not a simple Paschen curve, but that probably is another factor limiting edge density, one that applies with or without annealing. Can you show your work on that number? I'm curious how you get to that. Or is that in Valencia? Sounds familiar.

[kcdodd]

From running simulations, so far, electrostatic confinement at densities above 10^17 are untenable. I have yet to be able to make a wiffleball effect form in the simulation at high beta. The plasma simply pushes the magnetic field out in the cusps, and the density in the cusp becomes on the same order as the core, in some places even higher then the core due to the grid voltage (that is with plasma self e-field simply turned off). You would need a biased voltage of about a billion volts to counteract the plasma self field at ~10^21.

[TallDave]

That's interesting. Joel had trouble modeling WB as well. Apparently it's very difficult. He did find cusp-plugging, though.

I wonder if Bussard was able to model the effect he observed?