FAQ - Minimum Voltage needed to do fusion

It may be difficult to separate "theory" from "application," but let''s see if this helps facilitate the discussion.
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Richard Hull
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FAQ - Minimum Voltage needed to do fusion

Post by Richard Hull » Tue Jan 03, 2012 4:50 pm

This is to answer another on of those recurring questions...........

If one had at one's disposal all the facilities of a national lab and worked with the finest neutron detection gear, using a lot of statistical analysis over long collection times, 1-2kv potentials in glow discharge within deuterium might show fusion.

In the amateur fusion quest, in general, the minimal potential needed to do readily detectable fusion is purely a function of your detection apparatus and little else.

Neutrons are tough to detect. One might seek to only detect protons from the D-D reactions using an internal energy sensitive detector like a PIPs of Surface Barrier detector to check the minimum potential for fusion rather than the far more elusive neutrons.

But, back to neutron detection.........

With an 3He tube of a large size/volume and high helium pressure, utilizing an optimal moderator, fusion might be detectable at 10kv applied. If one is forced to use silver activation, 15-20kv would possibly be a minimum. In skilled hands both these detection methods could be pressed a bit lower potentials

To really condense it all down, If you do not want the hassle of a lot of statistical work and can't produce 25kv across the fusor, then you will be hard pressed to show fusion occuring with any ease or certainty that will make you or many observers feel comfortable without a really good neutron detector. At 25kv it will be all about current, too. The more current delivered at this potential, the better, but 10ma should be quite sufficient to readily demonstrate significant fusion with ease using normal simpler amateur detection schemes.

Again, it is all about your detection method as to where in the scheme of things that you will have a minimal voltage and current needed.

Theoretically, it is totally different issue. However, we are dealing with real fusion and the hands-on doing in these forums and not talking theory.

Richard Hull
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Chris Bradley
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Re: FAQ - Minimum Voltage needed to do fusion

Post by Chris Bradley » Tue Jan 03, 2012 5:41 pm

Richard Hull wrote:
> If one had at one's disposal all the facilities of a national lab and worked with the finest neutron detection gear, using a lot of statistical analysis over long collection times, 1-2kv potentials in glow discharge within deuterium might show fusion.
I think this needs to be viewed, at best, a speculation until someone publishes such a confirmation.

The tunnelling factor for fusion cross-section is, by quantum theory [1] and for DD, e^[-31.4/SQRT(energy-in-keV)]. Assuming energy of 25keV this factor is ~2E-3, and at 2keV this factor would be ~2E-10, a full 7 orders of magnitude lower. If we are talking about at least 10 meganeutrons/s at 25keV [an exaggeration already?], then at 2keV this would translate to 1 neutron/s if all else remained equal.

How long it would take to discriminate 1 n/s over background would depend on the statistics for which we can only speculate on the set-up/equipment/&c.. What can be said, because it has been shown [2], at sea level the thermal background count is ~0.008n/cm^2/s, so if we were to imagine a super-sensitive neutron detector 20 cm away from a 1 n/s source, to discriminate 1 n/s = 0.0002n/cm^2/s, over the background it would be trying to measure a signal 1/40th the strength of the background.

[1] http://fds.oup.com/www.oup.co.uk/pdf/0-19-856264-0.pdf
[2] Variation of Environmental Neutron Flux with the Depth of Water and Soil, Journal of Nuclear and Radiochemical Sciences, Vol. 9, No.2, pp. 45-47, 2008

Perhaps the detection of fast neutrons would provide a more distinct result for such a small signal.

Anecdotally, I think it is worth noting that many folks here with a good set-up have reported in with seeing neutrons fire up at the ~16 to 18 kV level, for a regular fusor, and nothing much below that. (The one notable exception being Jon R's tungsten needle electrode experiment.)

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Re: FAQ - Minimum Voltage needed to do fusion

Post by Doug Coulter » Tue Jan 03, 2012 5:48 pm

Yes, we first saw definite neutrons (a few bubbles in a BTI and some counts on a bonner ball) out of background right around that 16-18kv range. Which was a trick since our gas control was kind of primitive and our power supply not very great for this work - everything was groaning and limiting out, the thing not stable at all. It's only gotten better, but even on the new slick setup, that's about the threshold where there's "no question" and no need for math tricks. Audibly the detectors (2 HE3's and a B10) start to sing more than when its off right around there every time we get to that volts with any current being drawn.

Chris, Richard is right based on current standard model theory. The rates would be VERY low, but fusion would not work even at 50kv without quantum tunneling (it would take 1-2 megavolts without it to bring two nuclei together against Coloumb forces).

So unless you want to posit that some new, provable theory says that tunneling has a lower threshold where it has a hard limit and the probability goes to zero (not just insanely small as current theory says), Richard is correct.
Why guess when you can know? Measure!

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Re: FAQ - Minimum Voltage needed to do fusion

Post by Chris Bradley » Tue Jan 03, 2012 5:50 pm

Doug Coulter wrote:
> So unless you want to posit that some new, provable theory says that tunneling has a lower threshold where it has a hard limit and the probability goes to zero (not just insanely small as current theory says), Richard is correct.
No new theories. The theory says 7 oom lower for 2keV from 25keV. I'm debating if this is detectable?

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Re: FAQ - Minimum Voltage needed to do fusion

Post by Frank Sanns » Tue Jan 03, 2012 6:29 pm

Chris,

Single neutrons per second are detectable. Here was some of my work.

viewtopic.php?f=13&t=6178#p34622

So what do you calculations say about the voltage need to produce 1 neutron/second. Looks like on the order of 4KV to me. Splitting hairs with the Master Richard's numbers is not productive to this FAQ.

Frank Sanns

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Re: FAQ - Minimum Voltage needed to do fusion

Post by Carl Willis » Tue Jan 03, 2012 7:03 pm

Minimum detectable source rate (Smin) is influenced by the following:

-Absolute detection efficiency (E), encompassing the detector's sensitivity and the source-detector geometry ("geometric attenuation")

-Counting time, T (time for which both the detector and the source are operating at the conditions of interest)

-Background count rate (Nb / Tb) and variability

-Degree of statistical confidence required

Generally the experimenter has a lot of control over these variables (but if nothing else, at least the counting times). Glenn Knoll, in "Radiation Detection and Measurement (3rd Ed.)" develops the Currie Equation for minimum detectable activity in Chapter 3, Sec. 6. Assuming 5% false positive / false negative limits and a steady background that obeys Poisson statistics:

Smin = [4.653 * SQRT(Nb) + 2.706] / [E * T]

One of the most sensitive neutron detectors I have ever used in my work was a He-3 slab detector with three tubes at 4 atm., 18" long, surrounded with about 1.5" of HDPE. This detector had a background neutron count rate (i.e. gamma, noise, and high-energy cosmic contributions properly discriminated out) of 0.60 / second. If background is counted for and remains steady over one hour; and assuming the detector can be placed within a few inches of the fusion source for counting times of at least one hour, then Smin lies in the range of 0.1 n / second. Reducing the background by cladding the detector in cadmium would make an important improvement in the Nb term. Reflecting the source would make an important efficiency improvement. Counting background for two or even five hours, or counting the source for a longer period, would likewise improve this number. At some point, one has to think about things like the influence of lead (a source of cosmic spallation neutrons to the background), nearby uranium (fission decay), and the variations of cosmic background radiation over time. But careful experimental design, rigorous statistical tests, and superlative detector efficiency can make a huge difference towards low-level measurements.

None of this translates directly into any credible prediction about what might be the practical "minimum" of voltage or current values in a fusor for detecting neutrons. That is experimental territory, quite particular to individual equipment and approach. But if you have a detector and want to know how to improve it, or conversely, if you want to know what you'd need to detect sources of a particular strength, then the above information should be useful.

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Re: FAQ - Minimum Voltage needed to do fusion

Post by Dan Tibbets » Tue Jan 03, 2012 8:35 pm

The drive voltage is very significant for the rate of fusion, but the density is also extremely important, along with volume of course. The Sun manages to produce significant P-P fusion despite the ~ 20 orders of magnitude lower cross section compared to D-D and a temperature of only ~ 1,500 eV.

In terms of fusors, the record for neutron detection may be the EMC2 reactor made out of a machined copper block. PZLx-1 reportedly produced ~ 1 million N/s at only ~ 300 volts (potential well). I don't know if Tom Logan was there at the time or if the neutron counters had yet been optimized as much as possible, but this represents amazingly small voltage. Compared to ~ 10-20,000 volts the cross section may have been ~ 10^-10 to -12(?) lower, but because of the density possible due to the magnetic field strength the net production was impressive.

Assuming the B field claimed scaling of Polywell devices is true, then at ~ 35,000 Gauss the density would have been scaled as B^2, or compared to WB6 ~ 35 squared or ~ 1000X greater.
If WB6 produced ~ 10^9 fusions / second, then the voltage scaling may have placed the production at 300 volts as ~ 10^-3 fusions / s.
But with the density ~ 1000 times greater (or more as the comparison of density is between WB6 and this device, not a typical amateur fusor). As the fusion rate reportedly scales as the square of the density, the fusion rate would be ~ one million times greater. This would correspond to a fusion rate of ~ 1000 fusions or ~ 500 N/s or higher( or lower depending on the volume , and baseline assumptions.

Note that this refers to raw fusion rates. It has little or nothing to do with Q

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Re: FAQ - Minimum Voltage needed to do fusion

Post by Carl Willis » Tue Jan 03, 2012 9:40 pm

It remains impossible to talk meaningfully about the neutron-production efficiency of the commercial Polywell devices given the paucity and poor quality of the publicly-available data.

Robert Bussard's online-circulated, non-peer-reviewed paper intended for the 2006 IAC conference (this document or anything resembling it does NOT appear in the official AIAA record of that conference, however EDIT: I found it, I think...http://pdf.aiaa.org/preview/CDReadyMIAF ... 2.8.05.pdf) is the ONLY piece of evidence I have seen cited in support of the crazy-high numbers, i.e. 1e+06 n/s (WB-4, 12 kV), 1e+06 n/s (PZLx-1, 300 V), 1e+09 fus / s (WB-5, operating conditions not mentioned), and 1e+09 fus / sec (WB-6, 12.5 kV). Only the last measurement is supported in any detail whatsoever, a salient issue being its basis on three recorded counts. THREE COUNTS. Statistically, that's garbage, not even two standard deviations away from zero, demands enormous faith in a noise-free environment, etc. This has been discussed before here.

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Re: FAQ - Minimum Voltage needed to do fusion

Post by Richard Hull » Wed Jan 04, 2012 6:54 pm

I refer everyone back to my base level statement and to the original old question that the FAQ attempted to answer.

Q "From your experience what is the minimum difference between positive and negative electrodes with which D-D fusion reactions would reliably and detectably appear? "

A "In the amateur fusion quest, in general, the minimal potential needed to do readily detectable fusion is purely a function of your detection apparatus and little else."

I filled in possibles in and around this simple answer as the question came from someone not used to doing fusion or measuring it and was, therefore, rather "loaded" with real ambiguities.

I told what might be the lowest potential that "might" possibly be seen to do fusion in ideal conditions with the best gear and techniques far beyond any amateur's ability to obtain. (tentative low end extreme) I then covered the instrument issues and their range of general detection and usage for a few generalized threshold potentials where"reliable detectability" have been noted by real fusioneers here. I also noted that in good hands, with any given gear, lower fusion onset potentials could be detected based on honed technique than were quoted.

The rather loaded question was answered in a manner that showed the tremendous branching that could occur based on the amount of ambiguity in the question, as stated.

In order of importance the imperatives needed to answer the question are

1. What detection method is used
2. What is the efficiency of detection
3. What is the sensitive volume of the detector (often rolled into #2, (nv))
4. What is the skill of the person making the measurement and applying the instrumentation to the task.
5. What is the time over which the measurement is made
6. The degree of confidence desired to claim fusion. (sigma)

Carl worked to put a sharper mathematical point on much of this in his comments.

Regardless of what machine configuration is used or what the effeciency in doing fusion is in the given device, only the above points can answer the question of "At what applied potential will you know you have reliable, readily detable fusion."

Richard Hull
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Re: FAQ - Minimum Voltage needed to do fusion

Post by Chris Bradley » Wed Jan 04, 2012 7:24 pm

I think the ambiguity was useful when tied with the ensuing discussion and useful feedback from Carl. I don't think it is splitting hairs, but, rather, laying out for the newb the nature of a discussion that there may be no hard and fast 'rules' as to when something is or is not possible.

That being said, I'm just a little concerned that after a newb has read this and the power thread, a question will appear asking - 'so if I have a 1kV, 0.1mA supply, will ebay item XYZtube from Ukraine be good enough to detect fusion?'. I would then emphasise your conclusion; 10mA @ 25kV.

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