Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA-9mA

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Sat Jul 30, 2016 11:56 pm

I've tracked down the source of x-rays coming from the fusor. I had initially thought the 15mR/h was some of the higher energy x-rays coming through the lead glass, or stainless shell, however it turned out to be coming from the viewport near the vacuum gauge. Some of the secondary electrons from the grid were getting up into the conflat cube and generating x-rays. A lead foil cap over that viewport has now eliminated any measurable(on a ludlum 17 ion chamber, minimum resolvable rate ~0.5mrem/h) x-ray flux from the fusor. I'll still need to add lead shielding around the vacuum chamber when I start running at 70kV, but for now the x-ray issue is solved.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull » Sun Jul 31, 2016 6:20 pm

The shell starts to go transparent on the standard .060 inch thick SS shell at about 35kv applied. At the shell, working at 40kv, using a victoreen ion chamber, I get about 10mr/hr. It will only go up from there. Any window or view port will be much higher at lower voltages as the lower energy x-rays produced will pass through the glass as electrons strike the shell on the opposite side. This is much the same as old x-ray tubes.

The lead cap trick over the view port was used by Joe Zambelli in his 2001 fusor. I avoid it by pointing my view port straight down into the concrete floor through the video camera. View ports are the most dangerous emitters of radiation in the amateur fusor and all that danger is x-radiation. Neutrons from fusion are of no real consideration for the average successful amateur fusioneer as few will use more than 40kv applied.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Tue Aug 16, 2016 5:46 pm

I recently scored an URSA multichannel analyzer on ebay and am hooking it up to do neutron activation analysis
SAM_5159.JPG
The system was calibrated with a Cs137 check source
SAM_5161.JPG
Cs137.jpg
And an Am241 source(smoke detector)
Am241.jpg
All of which showed distinct peaks.

Spectra was also taken of the x-ray emission of the fusor
At 25kv (with the lead shielded cap off)
fusor-25kv.jpg
At 30kV (with the lead shielded cap on)
fusor-30kv.jpg
At 40kV (with the lead shielded cap on)
fusor-40kv.jpg
It's interesting to see how the 30-40keV x-rays are just starting to get through the 1/8" stainless shell, while the 25keV ones are blocked entirely unless the lead cap is removed.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull » Tue Aug 16, 2016 7:56 pm

Great work Andrew! I have noted this above and for some years. Your work puts a real fine point on this. Thanks for the excellent report.

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Wed Aug 17, 2016 1:48 am

Additional progress is being made on the liquid cooled grid to address the heating of the field control cap that covers the lip of the ceramic by brazing it to the cooling lines on the grid. Cusil brazing alloy (72% silver and 28% copper eutectic) was used in the brazing with a boron modified flux(Superior No. 601B/3411)
SAM_5190.JPG
The allow has reasonable wetting properties on stainless and only contains vacuum compatible materials(no zinc or cadmium)

A test braze was completed on an older grid, shown after sandblasting and some cleaning
SAM_5191.JPG
SAM_5192.JPG
SAM_5194.JPG
The braze seems structurally and thermally sound and generally wetted ok on the stainless, though the gap may need some tuning to get good flow down into the area between the cooling lines and the cap.

Additionally, the neutron detection system is being upgraded to use He3 detectors instead of the older BF3 detectors that were previously in use. The new detector is about 7x more sensitive then the BF3 tube on the system. This should allow the time constant on the constant neutron flux control system to be reduced for tighter regulation while still remaining stable.
SAM_5195.JPG
SAM_5196.JPG
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Steven Sesselmann
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Steven Sesselmann » Thu Aug 18, 2016 1:27 am

Andrew,

It's really good to see you make these small incremental improvements it means the technology is being refined and becoming more mature.

By the way I have one of those LND He3 tubes, and I'm interested in knowing where you got the connector. Does it come like that with the threads or is it something you have custom made?

Steven
http://www.gammaspectacular.com - Gamma Spectrometry Systems
https://www.researchgate.net/profile/Steven_Sesselmann - Various papers and patents on RG

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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Thu Aug 18, 2016 3:08 am

The connector is a TNC bulkhead mount adapter to clamp on coax, the clamp on section has the same 7/16-28 as the TNC and the LND tube. the other fitting is an Amphenol 79675 Adapter TNC to BNC adapter, a small spring connects the pin on the end of the He3 tube to the center pin on the other adapter
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Mon Aug 22, 2016 7:03 pm

Pictures of the molybdenum cap glowing under ion bombardment after about 40sec of operation at 17mA
SAM_5200.JPG
Pictures of the molybdenum cap glowing under ion bombardment after about 40sec of operation at 17mA
SAM_5201.JPG
The old grid was removed and inspected prior to the upgrade to the version with the brazed cap. The macor ceramic has held up extremely well with no signs of thermal or arcing damage.
SAM_5210.JPG
SAM_5211.JPG
A cooled grid with a brazed field control cap has been installed and is currently being tested
SAM_5217.JPG
SAM_5218.JPG
The new grid was tested for several minutes at ~2.5e6n/s with no discernible heating of the field control cap
SAM_5220.JPG
The current limit for operation time is now the heating of the vacuum vessel which will require some additional fans to keep cool .
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull » Mon Aug 22, 2016 7:08 pm

I have never really worried about the shell heating which tends to reach about 200 deg F after 1.5 hours of running. (electron bombardment and neutral crash area).

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
Retired now...Doing only what I want and not what I should...every day is a saturday.

Andrew Seltzman
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Mon Aug 22, 2016 7:37 pm

Mine seems to have localized spot heating near the ion sources. I think the fraction of the deuterium beams that is neutralized or charge exchanged at the focal point is being accelerated outward and striking the shell near the injector ports on the opposite side.
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