Cross section vs keV

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David Kunkle
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Cross section vs keV

Post by David Kunkle »

Want to make certain I'm reading these cross section vs keV charts correctly.

Using this chart: http://iec.neep.wisc.edu/images/crossSections.jpg

At 10kV, the cross section for D is about 6e-32 sq.meters, and at 100kV, it's 6e-30 sq. meters. All else being equal in a fusor, going from 10kV to 100 kV, the reaction rate would go up 100 fold? (And, no , I''ve no interest in going to 100kV. Just picking nice, round numbers off the chart.)
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
prestonbarrows
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Re: Cross section vs keV

Post by prestonbarrows »

Yes that is the jist of it.

The abscissa on the plot is center of mass energy, so 10kV on the DD curve corresponds to two D atoms headed toward eachother with 10kV energy each.

For an IEC, this is roughly equivalent to 10kV drop between the vacuum chamber and grid. However, in the real world, when you apply a given voltage you will end up with a jumble of ions with some energy distribution and most of the ions having energy less than the applied voltage due to energy lost to collisions and ionizations. Also, not all ions will be born at zero volts potential so will not fall through the full applied voltage before reaching the center. Not all fusion happens directly at a point source in the center either, some finite amount happens while the ions are still being accelerated and in beam-on-target reactions in the grid.

The true answer would be some integral accounting for the energy distribution function and other factors. Given the slope of the DD cross section between 10 and 100 kV, the yield would go up something less than 100 fold, with 100 fold being more or less the theoretical upper bound of improvement.

For example, in a thermal plasma (not IEC) the 'reactivity' <σv> is often used instead of cross-section where the reactivity is the cross-section integrated up and averaged over a thermal distribution of energies.
David Kunkle
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Re: Cross section vs keV

Post by David Kunkle »

Got it all- thanks. I originally started looking closer at the charts after re-reading some posts about how most amateur fusion isn't detectable until around 25kV. According to the graph, the difference between 10kV and 30kV is about 20 fold- theoretically. ;) I had always assumed that if fusion wasn't detectable at 10kV, then there must be several orders of magnitude difference by 30kV. So, 10 or 15 fold difference between 10kV and 30kV is enough for most neutron detection setups to go from 0 to obvious proof of fusion?

Also, with my gear(Ludlum 2000 scaler and a 12" He3 tube) what would you expect the lowest detectable neutron cps to be? Not sure I phrased that correctly and possibly a dumb question since I know my nuclespot and Be shows up at about 70 cpm above background. Humor me.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
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Richard Hull
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Re: Cross section vs keV

Post by Richard Hull »

Dave, If you have a 3He tube of 12" you ought to be able, with good electronics and good smooth operation, to detect fusion at even 20kv! Poor operation at lower pressures of D2 under 6 microns will be more difficult at 20kv even with good 3He gear and electronics.
So very much depends on the would-be fusioneer's operational skills and the degree to which his detection gear is hummed in to just detect neutrons only.

The best test is a ten minute run at more or less fixed fusor operational conditions in moderator and then another ten minute run with the detector tube out of moderator with the same operational regime. That will tell the tale 100% of the time. This is, and will forever remain, the ultimate test.

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
David Kunkle
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Re: Cross section vs keV

Post by David Kunkle »

Richard Hull wrote:The best test is a ten minute run at more or less fixed fusor operational conditions in moderator and then another ten minute run with the detector tube out of moderator with the same operational regime. That will tell the tale 100% of the time. This is, and will forever remain, the ultimate test.

Richard Hull
Hadn't thought of that, and I don't recall that in any of the detection FAQs. Good to know. Thanks.
If your experiment needs statistics, you ought to have done a better experiment.

Ernest Rutherford
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