Is fusion caused by D+ D2 collisions?

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Jeroen Vriesman
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Is fusion caused by D+ D2 collisions?

Post by Jeroen Vriesman » Thu Apr 26, 2012 2:47 pm

Here are links to two very interesting presentations about a wonderful "six ion beam" experiment:
http://fti.neep.wisc.edu/static/TALKS/1 ... ianegl.pdf
http://fti.neep.wisc.edu/presentations/ ... ec2011.pdf
(has been posted on this forum before)

In the presentations it's stated that only a small fraction of the fusion seems to happen in the centre where the beams collide, and neutron production drops with better beam focus, a de-focussed wider beam generates more neutrons than a focussed beam.

Made me wonder, is fusion in a fusor caused by D+ D+ collisions in the centre, or are D+ D2 collisions everywhere in the fusor responsible for the fusion?

Should be easy to measure, if the grid in a fusor is replaced by a simple smaller solid sphere, would there still be fusion? (less fusion, because ions would always stop at the sphere, accelerated D+ ions would have less time to hit D2's, but still some fusion would happen if the fusion is caused by D+ D2 collisions).

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Carl Willis
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Re: Is fusion caused by D+ D2 collisions?

Post by Carl Willis » Thu Apr 26, 2012 3:26 pm

Well, you can have:

1) D+ / D+
2) D2+ / D+
3) D3+ / D+
4) D / D+
5) D / D
6) D2+ / D
7) D3+ / D
8) D2+ / D2+
9) D3+ / D2+
10) D3+ / D3+
11) D / D2
12) D+ / D2
13) D2+ / D2
14) D3+ / D2

Molecular ions are probably more common than atomic species. Neutral species like D and D2 are very abundant. You can have fusion throughout the device volume, including in the implanted deuterium in electrodes. The Wisconsin experiments have shown that most fusion in the classic self-sustaining IEC reactor is peripheral rather than central, and that has been an established fact for quite a long time.

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Chris Bradley
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Re: Is fusion caused by D+ D2 collisions?

Post by Chris Bradley » Thu Apr 26, 2012 4:58 pm

Jeroen,

I believe I now have strong anecdotal material showing that the way an amateur's fusor works (that is, for >90% of its neutron production) is down to fast neutrals crashing into the shell at the ends of the beams, where other fast neutrals have already embedded themselves.

I argue the beams become visibly clear because the fast ions, accelerated by the fields, snatch an electron off a background molecule and leave behind a 'cold' ion, so what you see as 'beams' are those denuded, cold, ions recombining. The fusor works well to accelerate ions because where the neutral beams impact the shell, the cascade of electrons at that point ionise the background right at the top of the electrical potential.

This is a positive-feedback process, whereby the ions will align right on the same spot of the shell, thus greatly enhancing the probability of further neutral-embedded reactions.

This is why, I would say, no-one has scored the same neutron counts with a bell-jar - because a bell-jar neither supports embedded deuterons as well as the metal, nor rejects as many secondary electrons for further ionisations.

It seems to me, therefore, that fusors *rely* on all that lossy collisional stuff at the shell. That's why a fusor has to 'light up' before the neutrons appear.

(I make an exception, in this description, for the 'needle cathode' approach. I think this operates in a subtly different way.)

Unfortunately, I'll never have access to the level of diagnostic gear that would be necessary to track fast neutrals in the chamber, so I don't see how to argue for it, excepting by theory and photos that are not inconsistent with that theory.

I used to think that most of the reactions were fast ions colliding with the background, but there are several reasons why I have now concluded this is only a minor source of reactions. I'll be pinning up my experimental observations as to how I have formed this conclusion over the next few months.. this is just a 'spoiler'.

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Re: Is fusion caused by D+ D2 collisions?

Post by Jeroen Vriesman » Thu Apr 26, 2012 7:41 pm

Thanks Chris for pointing to the "needle cathode"

viewtopic.php?f=6&t=2799#p12309

That answers my question, the spherical grid isn't needed at all, and the plasma focus in the centre might only be a side-effect.

Since D2 is everywhere in the reactor it seems to me that the most likely source of fusion is "something positive hitting neutral D2", where "something positive" is D+, D2+, D3+, and the neutral D2 can also be inside the cathode material.

Chris, I don't understand why there would be any "neutral beams".

Carl, with peripherial, do you means just outside the grid, or near the chamber walls, or everywhere outside the grid? Do you more info about experiments showing that the fusion is peripherial? Interesting, thanks!
Somehow the plasma focus in the spherical grid made me think that fusion was happening in the centre....assumptions are easy to make.

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Chris Bradley
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Re: Is fusion caused by D+ D2 collisions?

Post by Chris Bradley » Thu Apr 26, 2012 7:48 pm

Jeroen Vriesman wrote:
> Chris, I don't understand why there would be any "neutral beams".

The charge exchange cross-section is orders of magnitude higher than the fusion cross-section. When a fast ion comes across a neutral atom, it is far more likely to 'steal' the electron and become neutral (and keep on going fast), than to fuse with it.

So if we are looking at a collisional system where ions are bumping into stuff, then we have to figure out what is most likely to happen during those collisions.... and it *isn't* fusion!!

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Re: Is fusion caused by D+ D2 collisions?

Post by Richard Hull » Thu Apr 26, 2012 8:55 pm

I think Jon Rosenstiel is the only one who has done the no central grid experiments within the community. (Needle and solid sphere cathode tests) In none of these experiments has he reported anything like the number of fusions taking place as with the central grid in place.

I have discussed at length, in several past posts the many possible methods by which fusion might occur. (recent exposition FAQ)

viewtopic.php?f=14&t=6607#p42290

Theories abound, but all and many are the methods by which fusion occurs in the fusor. In the past, measurements in reports by U of W and U of I it was found that approx 20% of the fusions within a fusor are occuring in the central grid region. Considering its relative tiny volume compared to the rest of the chamber, I would think it is a good place to do fusion per unit volume.

In addition to the many methods of fusion I have discussed in the past, Carl has added 14 totally different fusion possibilities involving deuterons and complex fuel molecules, both neutral and ionized. I am sure that we are still not totally inclusive on all the processes and fuel opportunities.

There is just so much more volume in the "velocity space" fusor and a massive surface area for possible target fusion that fusion opportunities by many other good offices only makes the central region look as if it is a poor stepchild.

This relegation to poor stepchild status is especially true in light of the disappointment that comes over us regarding our idealistic and seemingly optomistic, yet satisfying ion-on-ion collisional theory assumption within the center of the grid. We thought we had it all down pat.

Again, per unit volume of velocity space, the central grid region is doing OK, regardless of what methods of fusion are taking place there. Can any one of those other processes outside the grid be optimized? Doubtful, as most are thermal-collisional in nature, but they do add up to 80% of what we get out of the common amateur fusor.

Richard Hull
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Fusion is the energy of the future....and it always will be
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Re: Is fusion caused by D+ D2 collisions?

Post by Dan Tibbets » Sun Apr 29, 2012 1:06 am

I think stating that most fusion seems to occur outside the grid, while true is misleading. The critical consideration is the fusion per unit area or volume. The U W papers have shown ~ 10% D-D fusion occurring in the core- region within the grid. A later paper revised this to as much as 50%. This does not mean much unless you also consider the volumes involved. An assumed core area of 20 CC and a peripheral area (outside the grid) of ~2000 CC would result is a per CC volume fusion rate of ~2000/20 or 100 times higher in the core if the measures was ~50%/50%, or ~ 20 times higher per unit volume in the core vs the periphery if the 10% of total fusions number is used.

I used dimensions (1.25 grid diameter and a ~6 inch fusor diameter). I suspect the proportionate size of the U W fusor has a larger core, based on pictures, so the comparison may be less dramatic. But, still fusions per unit volume still dominates in the core. And this is in a glow discharge fusor where neutrals may outnumber ions by at least an order of magnitude. If the neutrals are reduced (ion guns and/or lower pressures) this core dominance may be mildly to much more significant. I assume that core dominance would be due to the more favorable fusion crossections and possibly density found with some degree of beam - beam convergent behavior. That this is seen (?) in this highly neutral polluted systems suggests that beam - beam fusions (or at least beam- neutral fusions) are significant and possibly even dominate. This would seem to implthat things that favor beam- beam collisions will significantly improve performance. Of course there are confounding experments- like why D- He3 fusion dominance on the grid itself was seen. And I'm still not sure of the significance of the U W Hirsh Fusor comparison. I'm stubborn about being suspecious of this work as it is not a straight duplication, but a modified version and the details may result in a much different enviornment.

What I find also revealing is Japanese (and others?) work that seams to show beam - beam fusion starting to dominate in ion gunned fusors when the pressure drops below ~ 0.5 Microns. This means that the ion population vs the neutral population is starting to reverse. Methods that might result in even greater ion densities without increasing the neutrals much will operate in this mode much better. Polywell fanboy plug here- the claimed ~ 1000X increased ion density does not apply to neutrals.So long as vacuum pumping keeps up, ion- neutral collisions will be proportionately much less common- both scattering collisions and charge exchange collisions between ions and neutrals.

Dan Tibbets

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