Possible Sputter Conductivity Issues

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Mark Rowley
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Possible Sputter Conductivity Issues

Post by Mark Rowley » Sun Oct 13, 2019 2:59 am

After quite a few runs I've noticed a gradual rise in current draw over the past couple weeks. As of now it's not even close to it's peak operating potential as the current draw becomes excessive past 20kV. The alumina grid stalk has gone from bone white to dark silver as well as the supporting ceramic hardware at the base. Fwiw, the grid is tungsten.

Other than sputter, the only other thing I can guess would be an internal fault in the feedthru. Either way I'll pull the feedthru / grid assembly, clean things up, and see what I can find out.

Maybe due to the small size of the 2.75" cross, I'm thinking sputter conductivity problems may become an issue before the larger spherical systems.

Has there been other reports in the past due to sputter conductivity? Only thing I could find was viewport concerns.

Mark Rowley

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Richard Hull
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Re: Possible Sputter Conductivity Issues

Post by Richard Hull » Sun Oct 13, 2019 5:30 am

I can't remember any major concerns with sputter coatings drawing extra currents in the many larger spherical fusors in the past. I have run a few electrometer ohmic checks way back around 2001 as I saw the alumina stalk deposition as a possible shorting issue. I could not detect anything in the under 500 megohm range. the electrometer can sense much deeper than this, but attachment and electrostatic issues without heroics become an issue as the gigohm range is approached. I found no reason to worry and have not noticed increasing current needs in my 6" fusor IV. I have not cleaned my grid stalk alumina tube since 2004.

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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Mark Rowley
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Re: Possible Sputter Conductivity Issues

Post by Mark Rowley » Sun Oct 13, 2019 8:42 pm

After removing the assembly it’s condition was far from the worst I’ve seen. However the alumina stalk cracked and had a sputter/carbon track within it. The ceramic insulator ring really took a beating as it’s almost black (it was thoroughly cleaned before installation). I ended up replacing both and cleaning all the sputter material off the main insulator with 40% hydrogen peroxide. After reassembly all was fixed. No more excessive current draw and a very slight vacuum leak was addressed as well.

First test run this morning netted aporox 150k isotropic neutrons per second. Far from my best run. I’ll post a YT video later today.

Mark Rowley.
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Mark Rowley
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Re: Possible Sputter Conductivity Issues

Post by Mark Rowley » Mon Oct 14, 2019 1:33 am

The first video is of this mornings run after maintenance. The second video is from about 10 days ago during a high output run. It was during that run where the sputter issue started to become most notable.

https://youtu.be/KT5VrEAqe30

https://youtu.be/L6_7tlhK7_I

Mark Rowley

Jerry Biehler
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Re: Possible Sputter Conductivity Issues

Post by Jerry Biehler » Mon Oct 14, 2019 6:58 am

On deposition system all the power feedthroughs are covered with a cap that kind of looks like a copper pipe end cap with a hole in the center. The conductor passes though and the concave side covers a portion of the insulator. This creates a "shadow" and prevents material from coating the insulator and shorting it to ground. Easy to make. The ones I have are stainless though.

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Mark Rowley
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Re: Possible Sputter Conductivity Issues

Post by Mark Rowley » Tue Oct 15, 2019 3:33 am

I'm having a hard time picturing that. Wouldn't a copper end cap lessen the insulative spacing between fields?
Has anyone tried this in a Fusor?

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Re: Possible Sputter Conductivity Issues

Post by John Futter » Tue Oct 15, 2019 5:47 am

Nope
Pull a ion pump apart, they use this system to stop the anode / cathode spacers from stopping it working due to Ti build up

i'll take some pics and post in the next couple of days of 10kV ion pump insulators and associated hardware
why not learn from those that have gone before

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Re: Possible Sputter Conductivity Issues

Post by Mark Rowley » Tue Oct 15, 2019 6:28 am

This sounds interesting. And advance thanks John for the upcoming pics. It's definitely something I'll need to see to understand because incorporating what I'm visualizing seems electrically problematic....so obviously what I'm thinking is wrong.

Unless I've missed it, I don't recall anyone else employing a cap like this on a fusor. Sounds like a phenomenal idea that's underutilized in the fusor world.

Mark Rowley

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