Polywell In Europe Raising Funds

[Skipjack]

I thought the point of the polywell was the the leaks would not equal total losses. To the stay with the bucket analogy, you would have a small hose attached to one "hole" and it would lead the losses back into the bucket through another "hole". That is at least what I understand.

[TallDave]

Skipjack,

Electrons confine ions, so the WB only has to confine electrons. But without recirculation, even electron losses are still hopelessly high because the surface is considerably inferior to a tokamak's toroid. The recirculating (or oscillating) WB is, we hope, the answer to limiting electron losses to something that allows Q>1.

WB-8 will tell us something about how losses scale with B and perhaps R. That's the key challenge here.

[Art Carlson]

I thought the point of the polywell was the the leaks would not equal total losses. To the stay with the bucket analogy, you would have a small hose attached to one "hole" and it would lead the losses back into the bucket through another "hole". That is at least what I understand.

Homework:
Assuming

1. that the width of a cusp is one electron gyroradius,
2. that there are no additional coils outside the magrid,
3. that electrons follow field lines,

calculate how far out an electron escaping through a cusp will travel before it is turned around to reenter another cusp. Feel free to make reasonable approximations and additional assumptions.

[alexjrgreen]

calculate how far out an electron escaping through a cusp will travel before it is turned around to reenter another cusp.

Judging from the WB7 photo, the corner cusps are essentially closed by the box-corner reflection mechanism that kcdodd's simulation revealed. The electrons leaving the central cusps return back to the same cusp.

chrismb says that makes each central cusp recirculation jet a tokamak but, since the torus must be twisted, perhaps it makes a stellarator.

[KitemanSA]

By the by, Polywell isn't a spherical magnetic containment system for plasma.

So you would be happy if we said "A spherical MAGNET cannot confine electrons without leaks"?

Heck yes! Then I would want you to say, "thank God the Polywell isn't a shpherical magnet confinement system for electrons!"

The confinement system is the positive grid. The magnets just protect the grid; oh, and improve the grid confinement a thousand(?) fold.

[Art Carlson]

By the by, Polywell isn't a spherical magnetic containment system for plasma.

So you would be happy if we said "A spherical MAGNET cannot confine electrons without leaks"?

Heck yes! Then I would want you to say, "thank God the Polywell isn't a shpherical magnet confinement system for electrons!"

The confinement system is the positive grid. The magnets just protect the grid; oh, and improve the grid confinement a thousand(?) fold.

That may what you'd want me to say, but what I'm gonna say is that your understanding of how a polywell works is sure different from everyone else's here.

[Art Carlson]

calculate how far out an electron escaping through a cusp will travel before it is turned around to reenter another cusp.

Judging from the WB7 photo, the corner cusps are essentially closed by the box-corner reflection mechanism that kcdodd's simulation revealed. The electrons leaving the central cusps return back to the same cusp.

Good. Then we've dealt with that silly picture of 'a small hose attached to one "hole" [leading] the losses back into the bucket through another "hole"'.

To stick with the analogy, the question is whether the hose can be squeezed off so nothing much gets out of it. Squeezing it with magnetic fields would just be shifting the cusp physics from one place to another, so I assume you have electric fields in mind to stop up the hose. Of course, the problem with an electric field that turns back the electrons is that it would pull out any ions that are there.

Next assignment: Making reasonable assumptions on the geometry, density, and electric potential, calculate the maximum possible value of | n_i - n_e | / n_e .

[TallDave]

Of course, the problem with an electric field that turns back the electrons is that it would pull out any ions that are there.

I haven't seen anyone try to quantify the competing electric forces at the edge of the well (negatively charged plasma ball wants to push electrons out the cusps, electrons want to pull ions with them). I suspect this is difficult to characterize without a detailed simulation. And I'm not sure I would trust a simulation anyway.

Hence, WB-8.

[alexjrgreen]

Of course, the problem with an electric field that turns back the electrons is that it would pull out any ions that are there.

It's a little more complicated than that. Gauss's law means that electrons and ions don't see the magrid until they pass through it.

A slowly outward moving ion at the edge of the wiffleball sees only a point negative charge at the centre of the well, decelerates and accelerates back towards it.

An electron at the edge of the wiffleball sees the same negative point charge and tries to accelerate away from it, magnetic field permitting.

At a central cusp the electrons accelerate away and pass through the magrid, only then becoming aware of the positive charge which causes them to decelerate and accelerate back towards it. They return to the magrid with the opposite velocity they started with and continue on (decelerating against the negative point charge of the well) back to the wiffleball.

Ions see a bigger central cusp, but mostly still only see the negative point charge at the centre of the well and so still decelerate. Any of these that pass the magrid are repelled by it and keep going. Ions that are close enough to the electron jet follow it out.

[chrismb]

we've dealt with that silly picture of 'a small hose attached to one "hole" [leading] the losses back into the bucket through another "hole"'.

This made me think of a cartoon of my youth which I seem to recall was called "Chilly Willy". It was about the [mis-]adventures of a plucky little penguin of that name. In one cartoon he find himself in the sea in a bath without a plug (for reasons well beyond this post, and long forgotten). So he pulls off the shower pipe and head that happened to be attached to this bath, jams the lower end in the plug hole, at which point the water coming through the plug now vents out of the shower head, which Chilly Willy simply points to the stern of his bath for free propulsion at great speed because, of course, the water coming through the hole will recirculate itself through the shower head and back into the sea with an almost limitless density and a high fraction of recirculation.

The thing is, if only rnebel were to simply say "this will happen" then it means it will, beyond question, and we could then power naval vessels by the Chilly Willy propulsion system instead of worrying about whether Polywell will work. I therefore pause in eager anticipation that rnebel might see sense with the idea and utter words of general confirmation of the principle, which would then [by his act of saying it] turn the idea into a certainty beyond any worthwhile critical examination.

(I wonder how sarcastic I can get before being moderated!)

[TallDave]

Gauss's law means that electrons and ions don't see the magrid until they pass through it.

This would be true if it were a sphere. I'm not sure it's true for the Magrid.

[MSimon]

(I wonder how sarcastic I can get before being moderated!)

Not even close.

[KitemanSA]

Heck yes! Then I would want you to say, "thank God the Polywell isn't a shpherical magnet confinement system for electrons!"

The confinement system is the positive grid. The magnets just protect the grid; oh, and improve the grid confinement a thousand(?) fold.

That may what you'd want me to say, but what I'm gonna say is that your understanding of how a polywell works is sure different from everyone else's here.

Maybe that is what has been causing all the confusion around here. My description is as simple and accurate as can be, I think, though you may be yet again shifting the discussion back to the ions. My statement is in reference to the electrons which were the topic of discussion when I made the statement. The MaGrid contains the electrons as I stated above. The electrons create a potential well that contain the ions. Ok?

Would it have been more acceptable to you if I had said "The confinement system for the electrons is the positive grid"?

[Art Carlson]

Of course, the problem with an electric field that turns back the electrons is that it would pull out any ions that are there.

I haven't seen anyone try to quantify the competing electric forces at the edge of the well (negatively charged plasma ball wants to push electrons out the cusps, electrons want to pull ions with them). I suspect this is difficult to characterize without a detailed simulation. And I'm not sure I would trust a simulation anyway.

Then you haven't been paying attention. If you are not up to solving the homework, you can cheat by looking for the post where I showed that the cusp plasma will be quasi-neutral. It's not as complicated as a simulation. Back-of-the-envelope estimates are perfectly adequate.

[Art Carlson]

A slowly outward moving ion at the edge of the wiffleball sees only a point negative charge at the centre of the well, decelerates and accelerates back towards it.

If you had done the homework assignment, you would know that it is not sufficient to consider only the interaction of the ions with the spherically symmetric component of the electron distribution. The contribution of the electrons in the cusp fan is huge.