Feedthrough repair

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Jim Kovalchick
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Feedthrough repair

Post by Jim Kovalchick »

I moved on to a home made feedthrough made from a glass tube and ultra-torr fitting. It has out performed my traditional feedthrough except for vacuum tightness. My home made one leaks like a seive.

Longing for leak tightness and D savings, I wondered if I could enhance my conventional feedthrough. While making those plans I found a pinhole short in my feedthrough. Obviously I pushed it too hard at some point. The failure point was at a small bulge in the metal seal. Note the black spot on the alumina in the picture.

My question is about repairing the fault. Does anyone think this point could be repaired? My first thought is maybe alumina paste and/or packing the inside of the alumina with a alumina paste or a quartz tube. Thoughts?
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Mark Rowley
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Re: Feedthrough repair

Post by Mark Rowley »

Jim,
If I’m looking at it correctly, the hole looks darn close to a metal sleeve (silvery line between the two blue marks).

F03E381B-56DC-46B9-8BC3-F12AA916E946.jpeg

I’m assuming the metal sleeve is grounded to the flange. If so, then the alumina paste may be the best bet. However I doubt it’ll hold up past 30-40kV. Unless you could drill out the pinhole like a dental cavity and backfill it with the paste once all the carbon is gone. It’s probably anyone’s guess how far you could electrically push it after that.

If the sleeve isn’t grounded your quartz tube idea seems like the best bet. Maybe a slightly oversized quartz tube with alumina paste between it and the pinhole.

Regardless of outcome, I think the idea of attempting a repair is worthwhile.

Mark Rowley
Frank Sanns
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Re: Feedthrough repair

Post by Frank Sanns »

If you are not going to heat it too high, indium solder is the normal hard seal for gas laser ceramic to metal bores.
Achiever's madness; when enough is still not enough. ---FS
We have to stop looking at the world through our physical eyes. The universe is NOT what we see. It is the quantum world that is real. The rest is just an electron illusion. ---FS
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Richard Hull
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Re: Feedthrough repair

Post by Richard Hull »

A burn spot at a critical point is always bad.

I would try to save the thing by taking a fine ball point diamond bit in a Dremel tool and boring into the alumina until no burn is seen and then back fill like a dental filling with alumina paste. You figured this out by yourself. The bit is critical though. It must be diamond and a tiny ball bit. It should be rather easy, but as to whether a viable fix, it is merely a good effort at it.

As we push the voltage limits in our fusors with better power supplies, the insulator is the fail point facing advanced efforts.

As to a good vacuum seal to prevent leaks, pure Indium is great and will adhere, with proper treatment, to most any metal, glass and porcelain. Its rather low melting point limits it to 100 degree C applications. It can take more than that but one must err on the side of caution. Indium is readily available in small amounts a very good prices at Rotometals. (see the trading post special sources posting)

viewtopic.php?f=75&t=13887

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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