chrismb wrote:I've never really bought into the argument that Polywell would be "a sphere - but with holes in it". It would be, I rather think, a ball with spiky bits. They are all, and one of the whole, the same thing. Looking at it that way you suddenly find yourself with a set of much simpler questions - what happens at the termination of the spiky bits? The nature of 'the cusps' then becomes somewhat irrelevant.
I do not see why the electrons wouldn't migrate and be 'held' in the same topology, even if there is the claimed radial polarisation that goes on. If the sought-after central polarisation occurs in a stable fashion, I would then suppose that the higher electron density would simply be a facimile of this spikey ball, and the spikes of increased electron density would run up into the cusps.
This is good. Go on.
Well, if I was to be told that "a shape" of plasma (of as-yet undefined topology) could be so configured that by controlled injection of electrons and ions into that shape that you could create a charge gradient towards its centre, then I can't really say that isn't possible. And I guess this is where the idea of Polywell springs. However, if you view that shape as being incomplete with holes, you're now struggling with whatever boundary conditions those represent. Both in terms of the added complexity of the problem, and also for the unlikeliness of big discontinuities in such a plasma, it would seem reasonable to suggest that whatever shape the magnetic fields impose on this shape, that the basic structure of a core of differential charge exists throuough that "shape". The shape is clearly going to be a ball with spikes, what seems to have been a point of argument is that the spikes have a fundamentally different structure. Maybe they would be just the same - a spike with a central core of higher negative charge, just like the sphere.
Ambipolar diffusion would mean that this effect would diminuish the further away you are along a spike, from the source of the constant ion/electron injections. The only questions left, in that case, are; how long does a spike need to be so that it is 'neutral plasma' at its end interacts as a sheath structure when in contact with a surface, and whether the losses across that sheath are tolerable to the operation of the device. As mentioned elsewhere, the 'fast' electrons at the end of the spikes (those that exceed the Bohm velocity on account of their own energy) would self-expel themselves whilst there is also a constant loss of ions of a very regular and preditable energy (Bohm velocity + system pd, presumably?). The pd of the wall might even be managed to optimise the behaviour.
At this point, you'd only have to feed in the lost ions and replacements for the high energy electrons. However, if this is all you fed in, a further question arises; does the charge gradient remain?
So far it sounds like I'm being rather helpful to Polywell theory. But, as always, I have to drop a small bombshell somewhere: If a charge gradient structure can be maintained in a plasma 'shape' by electron/ion injections, then why use a Polywell shape? Why not just use a mirror scheme with two cusps? And if you don't need the cusps at all to maintain this process, then why not use a toroidal shape?