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

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

Post by Daniel Firth » Wed Nov 25, 2015 6:41 am

I don't have much to add... just wanted to say it's been interesting watching your progress in this thread.

I think this got buried in a thread about my construction, but I based my grid off of yours. It's not water cooled, but I used your bending method with the slotted pipe.

Also, my BTI bubble detector was done in about a year. I got it in 2012, and now it has many gigantic bubbles. Do you keep yours stored compressed, or uncompressed?

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

Post by Bob Reite » Sat Nov 28, 2015 12:31 am

If you got a full year out of your bubble detector, you did well. Mine was gone in 9 months. They are supposed to be stored compressed in the factory supplied tube between 15-20 degrees C. I was shocked when I took mine out of storage and found large bubbles in it, even though it was stored compressed. That seems to be the failure mode. The bubbles from the last measurement "grow" even while under compression.
The more reactive the materials, the more spectacular the failures.
The testing isn't over until the prototype is destroyed.

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

Post by Andrew Seltzman » Sat Nov 28, 2015 2:13 am

I think there might be a lot of variation based on how well the dosimeter seals when assembled.

They used to manufacture the dosimeters with an anodized aluminum cap and compression system, but then switched out to a plastic one which seams to seal a lot better, probably since the epoxy that they fill the threads with bonds well to the plastic. The first one that I had was made with the aluminum handle and always had a "fishy" smell inside the tube. It lasted about 1year range before it was leaking and would not recompressible. By 2 years it had huge bubbles as well
viewtopic.php?f=13&t=6121&p=34565#p34565

The second one that I bought had the plastic handle and it looked very well epoxied in place. It never had any hint of a "fishy" smell in the storage tube. I got it in 2009 and it would recompress until 2012, with the built in compression chamber, and after that it would still recompress by placing a mechanical pencil eraser in the compression chamber before tightening it down to increase compression.

I always stored both in the tube, compressed. It's possible there is a lot of variation in life due to problems with the sealing system.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Sat Nov 28, 2015 3:59 am

Here is an excellent publication on leak rates for viton/kalrez/teflon o-rings for UHV systems(atom optics traps), the vacuum tests start on the 3rd page of the publication
viewport-oring-leak-rate.pdf
(440.42 KiB) Downloaded 243 times
And a reference of various vacuum compatible ceramics, I'm thinking of making part of the grid insulator out of macor, or LAVA, does anyone have any experience using these in a fusor under ion/electron bombardment?
http://www.technicalproductsinc.com/ceramics.html
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Sun Dec 06, 2015 8:19 pm

All 4 ion sources have been cleaned and rebuilt and baked out at 200C before instalation. They now use the angled anode rings and SmCo magnets instead of NdFeB magnets for compatibility with Deuterium with out disintegrating, and to allow high temperature operation(up to 300C) without loss of strength. I decided to go with the original style pole piece for now since it provided a near parallel beam with the angled pole piece but may switch over to the other style later. The SmCo magnets have lower field strength though and the ion sources require higher voltage to operate and produce a less intense beam which is fine since they are still significantly over powered for the fusor.

Ion sources before cleaning
SAM_4686a.jpg
In addition, one of the glass disk viewports was replaced with a conflat viewport eliminating one more viton o-ring (5 removed in this modification)
SAM_4691a.jpg
They now use copper o-rings instead of viton and have pump out ducts to avoid virtual leaks.
SAM_4692a.jpg
SAM_4689a.jpg
One of the swagelok valves was outgassing something when it was getting hot and was also replaced, and a 0.5um filter was added to the dry air admit line for venting the fusor to atmosphere for servicing
SAM_4694a.jpg
Some burning on the inside coating of the ZnSe viewports was observed when running with deuterium(doesn't happen with air). There seems to be some charged particle coming from the grid that is not readily deflected by the magnetic beam deflectors. I'm assuming it's a negative deuterium ion
SAM_4695a.jpg
I've doubled the field strength on the beam deflectors by adding a second magnet but it doesn't help much. I may have to remove the ZnSe ports for deuterium runs or re-design them or the grid to deflect the ion beam away from that area.
SAM_4697a.jpg
A feedback control circuit has been built to regulate fusor pressure by feeding back from the vacuum gauge analog output, It will stabiily control pressure from the 1e-4torr range to the 10s of mTorr. This considerably reduces workload during fusion runs and increases neutron output by tracking the optimal pressure. It's a very simple circuit.

Pressure controller circuit
SAM_4698a.jpg
An NP10 boost converter similar to this, or other boost converters used to drive nixie tubes:
http://www.ebay.com/itm/SHV5-5Vin-400Vo ... SwwE5WVDQB
provides 87V from the 24V supply, then an LR12 adjustable regulator steps the voltage down to 70V to feed the positive side of an OPA445 high voltage op amp which is configured as a gain 21 non-inverting amplifier. A recom RS3-1212D DC-DC converter provides -12v to the OPA445 and an AD826 that is used as an input differential amplifier. The vacuum gauge output(quattro 999, 0-10v, logarithmic to pressure) is connected to the negative side, while a potentiometer supplying 0-12v is connected to the positive side. The valve will now accurately control fusor pressure. There is still some drift and overshoot since it's only a proportional feedback controller, but the final version will be a full PID with valve dither ontop of the analog signal to improve performance.
12-5-2015-pressure-control-close.jpg
The fusor will now pump down to the low E-4 range in about 5-10 minutes and max out at about 5e-5 torr after several hours.
12-3-2015-pumpdown2.jpg
The cleaning of the ion injectors and removal of 5 viton o-rings seems to have really helped, though the pressure still climbs when running as the grid insulator heats up. It's made of boron nitride with a boric oxide binder which is fairly hydroscopic and will soak up water every time it sees atmospheric air. It will have to be replaced with a combination of macor/alumina/fused quartz.

Fusion rate is now at 3e5 n/s
12-5-2015-calculations.jpg
Star mode at ~3mTorr during fusion fun
SAM_4704a.jpg
Last edited by Andrew Seltzman on Sun Dec 06, 2015 10:19 pm, edited 1 time in total.
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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 » Sun Dec 06, 2015 9:19 pm

More nice work Andrew, efficiency is improving you are now in the E-9 range and it should get better with use. If you can get unto the -65kV range I thing the numbers will really shoot up.

If I understood correctly you are using feedback to control a vacuum valve, another possibility is set the vacuum valve and use a combined pressure and mass flow controller, to regulate the pressure with the gas inflow. Something like an MKS 649 or similar.

I look forward to see the more run data.

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 » Sun Dec 06, 2015 9:39 pm

Actually it controls the inlet gas flow by controlling the voltage on the piezo valve.
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Richard Hull
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull » Sun Dec 06, 2015 10:21 pm

Getting "betterer and betterer"!
Good work

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 » Sun Jan 10, 2016 7:29 pm

The gas control system has meed modified to include a full PID feedback system and in instrumentation amplifier to buffer the input.
SAM_4780a.jpg
This results in better long term pressure stability and lower overshoot
12-18-2015-PI-feedback.jpg
step-responce-log.jpg
A dual layer circuit board has been designed in expressPCB for the final configuration of the control system
SAM_4790a.jpg
gas-pid.jpg
The circuit board holds 2 independent PID control circuits, for the gas valve control only one will be used, the version with both in use will be used to control the HV power supply and in injector current. The one connected to the power supply will compare the grid current to a setpoint and adjust the ion sources accordingly to allow operation at any pressure/voltage/current point with all 3 adjustable independently.

The ion source control will use this circuit to control the input voltage to a set of emco F40 proportional HV supplies driving the ion sources:
http://electronicdesign.com/power/simpl ... ers-output

And assembly is in progress
SAM_4781a.jpg
SAM_4782a.jpg
The boards will be housed in Hammond 1590BB boxes, one for the power supply/ion source control, and the other for pressure control
SAM_4783a.jpg
SAM_4789a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Roberto Ferrari » Mon Jan 11, 2016 12:12 pm

Hi Andrew

Your comment about the SeZn screen and its damage when running on D2 is consistent with the reductive power of hydrogen.
Please comment on any chemical alterations.
Congratulations for your excellent work!
Roberto

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