Dan, I just want to make sure you understand where fast neutrals come from. (I suspect you may not have appreciated the discussion before, in another forum about another device.)Dan Tibbets wrote:In a fusor, especially a glow discharge fusor operating at typical pressures of a few dozen Microns, high energy ion collisions are going to be with near stationary deuteriums most of the time. Beam- beam collisions are going to be relatively rare. A guess is that beam- background collisions will dominate by at least a factor of several hundred to one or larger. Complicating the picture even further is that the neutral deuteriums are not truly stationary, they are knocked around so that their average energy will be higher, but not by much compared to the ions.
Imagine that a fast deuteron encounters a neutral gas molecule. The probability that it will fuse is based on the cross-section of fusion for those conditions, which will be of the order of ~10^-26cm^2.
However, there is also a probability that instead of fusing with the nucleus of that molecule it will strip the electron off it instead. This results in that fast deuteron gaining an electron and becoming a 'fast neutral' - it has a neutral charge and is now immune from the effects of the electric fields, but will still carry on with the kinetic energy it had when it encountered the deuterium molecule whose electron it has stolen.
That fast neutral is now free to go straight into the vessel wall, at its full accelerated voltage, with no further deceleration or acceleration effect from the electric fields. It will penetrate the chamber wall to some depth, and scatter electrons right there. These electrons will then ionise the local gas and the new positive deuterons at the wall will then accelerate back into the centre of the fusor where they too will soon succumb to the same process.
The thing you now need to understand is that the probability for a fast deuteron to steal an electron off a neutral molecule is based on its cross-section for that type of collision, which is ~10^-16cm^2. Yup, it is a full 10 billion times more likely to form a fast neutral than to fuse with a background gas.
We have a combined effect, then. We have fast neutrals all drilling into the chamber wall at the same point, so one may well find a previous one already embedded in there, and we have the 'star' phenomena in which these 'beams' self perpetuate because each time a fast neutral hammers the shell it creates a flurry of additional deuterons right at the top of the electric field potential, right there at the self-same point.
It is this continual self-perpetuating stream of pummelling the chamber walls at the same point over-and-over by fast neutrals, which builds up the interstitial population of deuterium at those points so elevating the chances of fusion with further fast deuterons, that I am convinced is the main source of fusion in a typical fusor, and that also has lead to confusion over spurious detections in other like devices. It is also easily explains why it is difficult to get any fusion out of glass chambered fusions, and why the neutron flux looks anisotropic if you start with an assumption the fusion is from the centre of the fusor.
Are there any beam-background fusions? Yes, but I do not believe it is a major contribution. Are there any beam-beam fusions? If it were possible to discriminate those from the other types, statistically speaking I doubt you would count any in a 10 minute run in a typical fusor.