Using an RGA to detect fusion

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Sarvesh Sadana
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Using an RGA to detect fusion

Post by Sarvesh Sadana » Sun Oct 30, 2016 9:52 pm

Could an RGA be used to detect fusion?

For example, if I attach an RGA onto my vacuum chamber doing D-D fusion, would it be able to detect the He3 and tritium produced? I'm thinking it won't, due to the very small amount produced, >100000 fusions/sec.

How could this be done?

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Nick Peskosky
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Re: Using an RGA to detect fusion

Post by Nick Peskosky » Mon Oct 31, 2016 1:24 am

In theory yes it might be possible but the gear required is probably scant available to the amateur Fusioneer. Open Ion Source (OIS) RGAs that couple a quadropole element with an electron multiplier are the most common devices on the market. Unfortunately, the minimum operating pressure required for the ion sources in these analyzers is usually <10^-4Torr (well below the standard operating pressure of a Fusor). At pressures higher than this the higher mass constituent gasses will adsorb to and sputter the Ti/Mo/Pd/Au electrodes that comprise elements of the ion source and quadropole. Closed Ion Source (CIS) RGAs can sample in the 10^-2-10^-3Torr range but come with a hefty price tag. It might be possible to sample the gas stream entering the throat of a turbo/diffusion pump but it would require a very complex differential pumping design (ion pump/Ti sublimation pump on the RGA arm) and I doubt the He/Tritium markers would even present as parts-per-trillion (PPT) in the 'dirty' atmosphere of a typical home-brew Fusor. A lab grade all-Conflat system with ultrapure D2 and ultrapure flushing gasses might fit the bill. The chamber would need to be evacuated to <10-7Torr ranges (days of pumping/heating) just to get all of the 'krud' out of the gas stream prior to sampling. Even this would be a longshot once the plasma is ignited in the millitorr range.

Unless you have access to a local physics lab or semiconductor cleanroom (mass spec or leak detector) chances are you'll need a rather deep wallet to buy the required RGA/connectors/processors/software required for a complete setup.
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Dan Knapp
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Re: Using an RGA to detect fusion

Post by Dan Knapp » Mon Oct 31, 2016 1:51 am

The amount of tritium produced is so small that it would be unlikely to be dectectable by an RGA, not to mention that it would be obscured by the H-D ion.

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Re: Using an RGA to detect fusion

Post by Jerry Biehler » Mon Oct 31, 2016 7:23 am

Doug Coulter has done it on his system and has detected T. You you need a pressure reducing valve and separate pump to sample the chamber since the chamber runs at a much higher pressure than the RGA can handle, the RGA needs to be at least in the -4 range, preferably better.

RGAs are all over ebay, you can find Inficon transpectors all the time. I picked up another Dycor Dymaxion 200M (200AMU with MCP) for less than $200 if I remember. Stuck it on my SEM to check it out and it works fine.

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Re: Using an RGA to detect fusion

Post by Dan Knapp » Mon Oct 31, 2016 11:47 am

Did Doug post data on this finding? The usual RGA can't resolve T from HD.

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Richard Hull
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Re: Using an RGA to detect fusion

Post by Richard Hull » Mon Oct 31, 2016 6:37 pm

Stock RGA's will not detect T in an average amateur fusor which never reaches 250,000 T/sec. However, a properly operated and reduced pressure RGA fitted with an additional, optional, electron multiplier and a fusor operating far in excess of 2 million T/sec, might, just might, see T.

Based on the gas pressure and number of gas molecules in an operating fusor, the T produced is such a tiny fraction of the whole that the average RGA stands no chance of detection. We have done all the calculations in past posts here.

The average yahoo with an average fusor and the finest RGA available will never see T.

Richard Hull
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Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

Sarvesh Sadana
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Re: Using an RGA to detect fusion

Post by Sarvesh Sadana » Tue Nov 01, 2016 12:14 am

What about He3? Since helium is noble, would a stock RGA detect its accumulation in the chamber?

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Re: Using an RGA to detect fusion

Post by Dan Knapp » Tue Nov 01, 2016 8:20 pm

He3 is detectable with an RGA but suffers the same problem as detecting tritium in the presence of a large background of deuterium and water, which will produce a mass 3 HD molecular ion (unresolvable from T and He3 by the usual RGA analyzer). Detecting the TT molecular ion (mass 6) would be statistically highly improbable given the extremely low level of tritium compared to the high background pressure of deuterium in a fusor. The best chance of seeing tritium in a fusor would likely be as the DT molecular ion at mass 5 where there would be fewer interferences.
Another problem with quadrupole RGA's not mentioned here so far is the so called "zero blast" where neutrals going straight through the analyzer obscure the low mass region. MKS/Granville Phillips touts their ion trap-based Vacuum Quality Monitor as the solution to this problem for observing the low mass ions. The best shot at observing tritium as a fusion product could be by observing DT with the VQM. Unfortunately, the VQM is a relatively new product, and there are few of them on the surplus market as yet.
To summarize, tritium detection by mass spectrometry is not normally used as a diagnostic for fusion even in big time experiments. Ian Hutchinson, author of the definitive text on plasma diagnostics in fusion research, doesn't even mention it

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