Feedthrough application

[dnthomason]

I have two each of two styles of feedthrough bushings and need some guidance on their application. Each type is for 1” bore holes and are sealed with o-rings. On the unit used on the fusor the o-ring will be replaced with viton.

The bushing on the ceramic feedthrough that would be in the vacuum is plastic (nylon, PE or ….). The creep length on the air side is about 2.4” (60mm).

I could not find anything quite like the units on the left. The only identification is E19001630 Rev 1. The bushing material is rather hard and the gap between the metal and the conductor is filled with a hard blue material. The creep length is about 2.75” (70mm) but has well defined 90 angles instead of the sinusoid of the ceramic ones. My gut feel is that one of these would perform better as the inner grid bushing.

Can someone give me an idea on the DC voltage ratings of the two items? I have read David Cooper’s reply concerning AC vs. DC and will filter and current limit the supply.

Will the ones on the left perform better on the fusor when taking materials and voltage rating into account?

[MarkS]

My feeling is that the 2 on the right would perform best because they are sinusoidal and ceramic, and they could be easily TIG welded / Brazed into place either directly on the chamber, or on a flange like this:

http://vacuumshopper.stores.yahoo.net/158odtube.html

[myID]

Hi-

I think that also the heating of the grid- and in consequence the heating of the feedthrough- could be a problem with the ones on the left side- they might gas out or desintegrate or melt..... (also for the ones on the right side if you use the inner plastic parts...)

Greets

[Carl Willis]

Hi Dennis,

Just to put things in perspective, it is unlikely that either of these types of insulators are vacuum-rated. Of course, that doesn't mean that they can't be made to work, or even that they will not work as-is. It just means that you're covering new territory with them and will have to proceed carefully to determine the potential utility of these feedthroughs in fusor use. I strongly discourage the use of the ones pictured at left, right off the bat. They are probably a resin composite of some kind, liable to char and outgas severely in a fusor environment.

Provided you find that the ceramic feedthroughs have vacuum-worthy seals, they will be limited by the air-side breakdown voltage. My guess is that the fluted ceramic insulators are rated for 30 kV in air and if kept dust-free, dry, and away from metal surfaces, will probably hold 40 kV on most days. If the fluted insulator is submerged in oil, then the upper limit is probably the breakdown of the ceramic itself. No idea what that is, but I'd bet you could hold 100 kV that way.

-Carl

[Jon Rosenstiel]

I ran across feedthroughs like the two on the right while parting out an old vacuum deposition device. They were mounted with the ribbed portion on the vacuum side. (Inside of the bell jar). They're basically current feedthroughs and will only handle a moderate amount of voltage. (Probably around 30 kV, as Carl mentioned).

If you use them with the ribbed portion on the outside (air side) you're gonna' have some outgassing issues because of where the o-ring seal is located.

Jon R

[dnthomason]

Thanks everyone. I will try the ones on the right. Initially 25 or 30 KV will be adequate.

Dennis