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

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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 Jan 28, 2016 1:05 am

Andrew,

Nice work and AFAIK a first here on fuser.net, I echo Richards concerns, you have quite a few loops happening there, so I guess you need some kind of hysteresis to compensate for delays. If anyone can do it, I am sure you are the one.

Looking forward to your next run..

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

Post by Andrew Seltzman » Thu Jan 28, 2016 5:33 am

I have the control system successfully holding a constant neutron flux based on feedback from the neutron detector. At the lower rates that the fusor is producing the integrator time constant has to be fairly long to prevent the system from oscillating. For these tests the ion injectors are just holding at constant emission without feedback control from current since that part has not been built yet.

Video of operation
https://youtu.be/BiEOAbmksow

Circuit schematic
SAM_4810a.jpg
Control system close up
SAM_4815a.jpg
Control system and neutron detector
SAM_4813a.jpg
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Richard Hull » Thu Jan 28, 2016 4:12 pm

This appears to be a first in electronic fusor control that appears to be rather successful. Do you have a current fusion level at which the system seems stable over about 20 minutes or longer? What is your longest stable, controlled run? Just curious.

Really a super effort.

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 » Thu Jan 28, 2016 5:26 pm

Based on comparison between the old BTI(it finally did fail) I had and the BF3 detector, I would estimate neutron rate in the 2e5 n/s range. More fine control will be possible with higher neutron rates as the effect of a single count becomes less. As of now, the control system is running with mostly integral feedback with a very long time constant to prevent oscillations. If the neutron rate was higher, the time constant could be decreased.

At high proportional gain or lower integral time constant the system does start to "slinky" rather noticeably, but with low Kp and long Ki, it runs stably.

The longest run would be in the 2-3min range. I'm still having problems with out gassing of the boron nitride grid insulator and possibly the ZnSe viewports as things heat up which limits run time as the pressure controller reduces gas flow to maintain constant pressure until no deuterium is being admitted to the chamber. Removing the ZnSe ports and upgrading the grid insulator design is on the to-do list.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Dennis P Brown » Fri Jan 29, 2016 2:39 pm

I am a bit concerned by the ZnSe window - powerful electron bombardment could release Zn into your system and cause long term contamination - Zn, if it does deposit will re-vaporize again and again - not good for electronic sensors, I'd think. I assume you need that ZnSe window as an IR clear window but if you aren't using it at the moment for that purpose I'd suggest replacing it with a glass window for now. Why keep an unknown that might be causing issues until you have the system fully operating in the manner you want and then installing the ZnSe window? Just a thought.

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

Post by Andrew Seltzman » Fri Jan 29, 2016 5:36 pm

That is one of the things I worried about with the ZnSe viewports. There are defector magnets to prevent the electron beam from hitting the surface, but it doesn't really deflect the D- ion jets.

I have removed the ZnSe viewports and still have the problem. It's most likely the BN on the grid insulator, the binder has a tendency to soak up some moisture.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Seltzman » Wed Mar 09, 2016 3:43 am

The ZnSe windows have been removed and the damage analyzed. Both top and bottom windows showed deposition from sputtering considerably more visible then that on the glass viewports.
SAM_4818a.jpg
SAM_4817a.jpg
SAM_4863a.jpg
This is likely due to the deposited material interfering with the BBAR(broad band antireflective) dielectric coating on the window causing considerably more change in transmitted light. Examination of the window surface under a microscope showed pitting of the BBAR coating where the D- beam was hitting the window.
top-lens-d-ng-beam-centera.jpg
Subsequent cleaning of the window was able to remove most of the BAR coating(and the deposition with it). Post cleaning the window is transparent again.
top-lens-d-ng-beam-center-cleana.jpg
None of the pitting or damage extends into the ZnSe material, it appears only the BBAR coating is strongly affected by D- bombardment, however to err on the side of safety, the ZnSe windows were not re-installed to prevent any potential sputtering of zinc from the now unprotected window surface into the vacuum system.

The viton o-ring sealed glass disk viewports were replaced with AR coated vacuum rated viewports.
SAM_4859a.jpg
Due to the previously observed interaction of the sputtered material and D- beam with AR dielectric coatings an internal shield system was designed to prevent exposure of the viewport internal surface with plasma or sputtered material.
SAM_4852a.jpg
A stainless adapter holds a 1.25" diameter 1/8" thick glass disk inside the vacuum system in front of the the viewport. The glass disk is held in place with a internal retaining ring. No groove in machined in the adapter, the friction force of the ring is sufficient to securely hold the disk in place.
SAM_4857a.jpg
SAM_4854a.jpg
The adapter allows the glass disk to hang off of the free space inside the copper o-ring between the two conflat flanges.
SAM_4858a.jpg
A pump out duct in the side of the adapter vents the volume between the shield and the viewport.
SAM_4853a.jpg
The small distance between the shield and viewport, and large aperture of the shield prevents any restriction in field of view.
SAM_4883a.jpg
The lead glass x-ray shield is held in place with an o-ring but a plastic holder will be machined to securely hold it in pace.
SAM_4884a.jpg
Dimensions of the shield holder are as follows
Glass disk is McMaster 8477K28
Retaining ring is McMaster 92479A790
SAM_4864a.jpg
Additional upgrades included the design and installation of a faraday cup to measure D- beam current reaching the end of the CF nipples on the vacuum chamber
SAM_4843a.jpg
SAM_4848a.jpg
SAM_4845a.jpg
SAM_4846a.jpg
The collector plate is recessed inside a secondary electron suppressor shell. The shell is also isolated from chamber ground so that both the collector and shell can be biased for ion energy analysis.
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Re: Mark3 operation 3e-4 torr / quad ion source @ -40kV, 1mA

Post by Andrew Robinson » Wed Mar 09, 2016 5:16 am

Outstanding work as usual Andrew!
I can wire anything directly into anything! I'm the professor!

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

Post by Andrew Robinson » Wed Mar 09, 2016 5:17 am

Ha... Sounds weird saying that. Feels like I'm patting myself on the back lol.
I can wire anything directly into anything! I'm the professor!

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

Post by Richard Hull » Wed Mar 09, 2016 2:35 pm

This is the kind of advanced work that we would hope to see more of here. Again, thanks for the continued sharing.

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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