Fusion due to fast neutrals in a typical fusor?

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
Post Reply
User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 7:05 am
Real name:

Fusion due to fast neutrals in a typical fusor?

Post by Chris Bradley »

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.
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.)

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.
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Fusion due to fast neutrals in a typical fusor?

Post by Richard Hull »

I have held Chris' view for some years now. Interstitial deuterium atoms, (wall buried deuterium), contributes mightily to fusion in a fusor this is fast particle on target fusion. The target is continuously bombarded and, thus continuously loaded while at the same time electros from crashing energies are helpfully released at the walls to ionized slow or static neutrals so that they may attain full acceleratory energy in possible beam-on-beam fusion. The loaded "tasrget" shell wall is stationary and has a giant surface area. Again, as I have stated, many tiomes in the past, the fusor has many modes of doing fusion there is no one way, no theoretical certainty, for assuming a single mode of operation.

The fusor environment is a grand mix up of postive and negative processes. The positive processes, the ones doing fusion are foiled by a myriad of negative processes in the device. Yet, these same negative processes create new opportunities for alternate, competing, positive fusion processes a kind of aid-hinder mix that results in a net output of fusion, but at extreme costs and inefficiencies. These losses are put up with so that the amateur can do nuclear fusion. Not very elegant fusion, but net, measurable fusion, nonetheless!

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
User avatar
Dennis P Brown
Posts: 3160
Joined: Sun May 20, 2012 10:46 am
Real name: Dennis Brown

Re: Fusion due to fast neutrals in a typical fusor?

Post by Dennis P Brown »

I do believe someone who posts on the forum (Carl?) has coated their inner chamber walls with Palladium. If so, such a chamber would absorb and hold far more deuterium along the walls and one would suspect (but far from being a definitive test) such a chamber might produce measurably higher fusion rates. The reason I say "suspect" is that the deuterium might migrate too deep and too rapidly in Pd for this test to maybe work (i.e. no surface accessible deuterium.) Since I believe someone does have such a chamber, it would be interesting to find out. Search's results for me have been null so far on this topic - maybe I read it at their web site ... .
In any case, the wall "fusion" theory sounds valid - deuterium will collect along steel walls very readily even without significant KE. So, considering the extremely small cross section of fusion for two ions and low density in the plasma chamber of these ionized deuterium, and high number of deuterium atoms on a chamber wall, just by hand waving arguments this theory strongly supports the idea that the most likely fusion location is the chamber wall.
User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 7:05 am
Real name:

Re: Fusion due to fast neutrals in a typical fusor?

Post by Chris Bradley »

My opinion, for the limitation that it is worth without experimental justification, is that it would make little difference in the 'equilibrium' condition of an operating fusor, because fast neutrals being embedded into the metal wall will still penetrate, whatever the material composition. In operation, I don't really see that a metal's ability to intercalate deuterium would make much difference if the deuterons are being embedded deeply, under keV worth of energy.

Where I think lining of the chamber, or putting pieces of sheet material at the ends of the beams, would help is the initial start-up conditions, in which a chamber could be 'soaked' in deuterium and one might then expect the equilibrium condition (of deuterons being embedded to them being ejected) to be reached much faster because the metal already contains a significant population of deuterium from the outset.

I guess it must make some contribution in the running state, but I'd not put money on it being a big difference.
User avatar
Richard Hull
Moderator
Posts: 14992
Joined: Fri Jun 15, 2001 9:44 am
Real name: Richard Hull

Re: Fusion due to fast neutrals in a typical fusor?

Post by Richard Hull »

Large hydrogen thyratrons have a hydrogen or deuterium reservoir that requires a separate heater voltage to outgass the titanium or paladium storage device within. This re-establishes the operational, design gas pressure in large tubes prior to operation. There is often a 5 minute warm up on the filaments and gas reservoir, prior to applying the high voltage to the anode. So the idea of pre or re-conditioning a hydrogen gas discharge device is quite common in vacuum tubes. In this manner the tubes never go "hard" as gas is naturally buried in the tube elements. The heating outgasses the hydrogen. Note that many such larger hydrogen thyratrons are running at 40-60kv and even there at those voltages, the gas atoms are not buried so deep that simple heating can't chase them back out into the inter-element volume. I don't think we bury the deuterium so deep that they are not constantly moving into and back out of the interstitial spaces near the surface via both electron and deuterium neutral bombardment.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Post Reply

Return to “Fusor and/or General Fusion Theory (& FAQs)”