Ion microchannels

[TallDave]

I ran across something on this on the Polywell wiki discusson, but I can't find it now. I did Google this up:

http://www.absoluteastronomy.com/topics/Fusor

There are a number of unsolved challenges with the electrodes in a fusor power system. To begin with, the electrodes cannot influence the potential within themselves, so it would seem at first glance that the fusion plasma would be in more or less direct contact with the inner electrode, resulting in contamination of the plasma and destruction of the electrode. However, the majority of the fusion tends to occur in microchannels formed in areas of minimum electric potential(*), which present themselves as visible "rays" penetrating the core. These form because the forces within the region correspond to roughly stable "orbits". 40% or so of the high energy ions in a typical grid operating in star mode may be within these microchannels.

I thought that was interesting.

[chrismb]

I made comment on this in another thread:

viewtopic.php?p=13457#13457

I think there's a Wisconsin paper around where they also simulated this percentage of ions in beams.

[TallDave]

Yes, that paper is linked above.

[D Tibbets]

I made comment on this in another thread:

viewtopic.php?p=13457#13457

I think there's a Wisconsin paper around where they also simulated this percentage of ions in beams.

I'm not sure that is fully discriptive. My understanding is that the electron beams are concentrated- colimated flows from the cathode that would otherwise be emitted in random directions from the cathode surface, some what like a CRT or linier accelerater. These beams contain more electrons and can locally heat the vacuum vessel wall, but I suppose that the current -electron flow is not significantly greater than the flow that would be hitting a larger surface if not focused. My experiance is that there is still ~ several hundred thousand Ohms of resistance in the system (at least in my Demo Fusor). Arching is a different story, the ionization is so heavy that enough charge carriers are created that the resistance drops to very low levels (like lightning) and current goes through the roof. That's why a ballast resister in series is a good idea (or a current limiting transformer).

I'm guessing that the electron beams (micro chanels- who dreamed up that name?), since they form between the cathode wires and stream radially outward, hit and ionize neutral molecules along these paths. The newly born ions then acellerate directly towards the center, missing the grid, at least for a few passes untill they are scattered- thermalized into random orbits . This apparently increases the effective grid transparency from 85-90% to 95-98% transparency. This ~ 10 % increase in cathode grid transparency apparently accounts for the significantly higher fusion rates seen (I'm uncertain of the magnitude of this improvement). If this 10% improvement has an easily measureable benifit, how much more benifit does a ~99.999% transparent virtual cathode like that claimed for the Polywell have?


Dan Tibbets