Fusor Forums

As I was ramping up the power to my fusor the other day, my Spellman supply tripped and I heard some arcing at around 21kV. I couldn't find the source of the problem until I looked behind the lead vest I had draped over the chamber. The custom HV feedthrough I made arced through the insulator, and when the voltage arced, it created a purple glow in the alumina tube. I opened it up and found that several little holes had been blown through the insulator. Doing some quick calculations reveals that the breakdown voltage of alumina this thick is only between 14-20kV, and before the plasma establishes and lowers the resistance of the chamber, the full voltage is imposed across the insulator.

I'll be building a larger version of this feedthrough with a 3/4" alumina tube to 4-bore 0.188" tube for grid wires. In other words, the insulator system will be nearly identical to Andrew Seltzman's. I calculated a breakdown voltage of about 90kV, so it will be more than enough.

I'm posting this a a warning and reminder to those looking to build some version of this feedthrough (or any custom feedthrough, for that matter). Some have expressed in the past that they wanted to replicate my design, and don't want others to make this same mistake.

Holes blown in the insulator


-Liam David

Yes, that is a bit tiny for real HV in the fusor. you need huge thickness and a smallish central bore.

I hope the supply is OK.

Richard Hull

Liam,

How are you achieving a gas tight seal between the center conductor and the ceramic tube?

Thanks.

For the smaller feedthrough, I used a 1/4" Swagelok cap with two o-rings in it since the ferrules I used are stainless steel. On the larger one I'll be building, I'll use a 3/4 Swagelok cap with teflon ferrules, which will allow me to seal against the insulator and electrically connect to the inner grid without using o-rings.

Do you have the part number you used?

Since I have one of these feedthroughs that also is comprised of a 1/4" OD alumina tube, could I layer the current tube with alumina ceramics in order to get the diameter up to 3/4" without having to buy a new tube? I am also only running about 30 kV DC through the feedthrough and am planning to insulate the area between the wire and the tube with multiple layers of wire insulation certified for 40 kV DC.

Telecoping additional larger diameter tubes will definitiely help arc suppression.

Richard Hull