electric isolation of the feed through on the vacuum side

For posts specifically relating to fusor design, construction, and operation.
Post Reply
ab0032
Posts: 86
Joined: Sun Oct 23, 2011 6:50 am
Real name: Alexander Biersack

electric isolation of the feed through on the vacuum side

Post by ab0032 » Thu Jul 05, 2012 2:43 pm

Hi,
amazon supply and mcmaster are not willing to export alumina tubes to Europe and I was not able to find a supplier here in Europe for them, so I was wondering how important it is to isolate the connection between the feed through and the grid.

Do I need any at all, it the distances are big enough to the chamber walls and the sharp edges of the flanges? Should I put the connection into a wider borax glass cylinder, or wont that do or help? I have a borax glass cylinder about 7cm wide (2-3 inches?). The connection has a wider radius than the wire on the grid, so the electric field should be weaker around the connection than at the grid.

Should I put some aluminum foil into the chamber around the flanges to protect their sharp edges from attracting a spark and damaging them?

Can one of you tell me about experiences with sparking, how likely is it, how often does it happen...

Thanks in advance,
Alex

Tyler Christensen
Site Admin
Posts: 551
Joined: Tue Feb 24, 2009 2:08 am
Real name:

Re: electric isolation of the feed through on the vacuum side

Post by Tyler Christensen » Thu Jul 05, 2012 2:45 pm

A number of people have had feedthroughs destroyed by internal arc-throughs. Usually it doesn't happen for quite a bit of run-time though. Just about anything will work for a short period of time. What's harder is getting a setup that will work for hundreds of hours of runtime.

Some add internal alumina, some don't (like me), and some feel retail feedthroughs don't work at all and build their own. Just about anything that can be done, has been done by someone. There is a lot of documentation of feedthroughs on the site, look through what people have done.

Tyler

User avatar
Richard Hull
Moderator
Posts: 12927
Joined: Fri Jun 15, 2001 1:44 pm
Real name: Richard Hull

Re: electric isolation of the feed through on the vacuum side

Post by Richard Hull » Thu Jul 05, 2012 2:50 pm

An old topic.

The alumina stalk is nice, but not absolutely necessary. Just use a smooth rod to suspend the grid.

The sparking ends when all nicks and loose metal debris are burned off, never to return again once stopped.

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.

ab0032
Posts: 86
Joined: Sun Oct 23, 2011 6:50 am
Real name: Alexander Biersack

Re: electric isolation of the feed through on the vacuum side

Post by ab0032 » Thu Jul 05, 2012 3:07 pm

Thanks, that is good news for me, so I will try without the alumina first and see what happens.

Tyler, what do you mean by "internal arc throughs"? I bought a commercial feed through, so I am supposing if an arc happens at the inside end of the feed through it will only hit the copper wire sticking out of it on the inside. But it I slip a stainless steel tube over it, the arc should hit only that. At least that are my expectations. I am more worried about an arc hitting a sharp edge of a flange an destroying it, or is this something that I dont have to worry about?

Tyler Christensen
Site Admin
Posts: 551
Joined: Tue Feb 24, 2009 2:08 am
Real name:

Re: electric isolation of the feed through on the vacuum side

Post by Tyler Christensen » Thu Jul 05, 2012 3:10 pm

I wouldn't worry about hitting the knife edge, there's far too much metal closer to the inside. Anyways, a fusor doesn't have enough stored energy to cause substantial surface damage (or at least it shouldn't if you're being safe).

The usual fault is an arc drilling a hole through the ceramic, which will then always arc over because where an arc has gone before, there is enough residue to surface track very easily in the future.

People have posted pictures of feedthroughs with holes through the ceramic on the inside.

User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: electric isolation of the feed through on the vacuum side

Post by Chris Bradley » Thu Jul 05, 2012 6:34 pm

Alex,

It sounds like you are discussing a 'stalk' rather than a 'feedthrough'.

These can be, and often are, the same thing. But not necessarily.

The 'feedthrough' is the assembly containing insulation and the electrical pathway into the vacuum chamber at the chamber envelope. Obviously, it has to be both vacuum-tight and can isolate that electrical pathway from the chamber itself (or any other electrical part).

If the 'stalk' is integral to the feed-through, then it is typically a continuation of the insulation so that the electrical pathway projects into the chamber far enough away from the chamber wall that it does not track back across the surface of that stalk

In your case, it sounds like you have an adequate 'feed-through', but that it does not have a 'stalk', or at least does not have one long enough. (If I've read this wrong and you are aiming to manufacture your own feed-through from parts, then forget what I am saying as I think I have no skills capable of making such a feed-through! I'd say better to buy something ready-made. A spark plug or oil-burner igniter plug with a long electrode does just fine! Or just keep a lookout on ebay or Trading Post here for a feed-through built into a ready-made flange.)

In any case, let's say you get to the point of having a feed-through that has a vacuum-side connection you can connect your grid to, but you want that grid to extend into the centre but only to be exposed there at the centre: As Richard says, you can probably get away with just leaving an exposed conductor all the way up to your grid. However, that supporting conductor will generate its own electrical field and you may want to suppress that field.

If this is the scenario you are aiming to discuss, one solution I have used, which generally works, is to use fish-spine beads. I don't have a good picture of fish-spine beads in my own work, but I have prepared the attached pic to explain what you might try with them. Because they interlock, it is simple enough just to fit them around your conductor and this will have the effect of generally screening the wire./conductor up to your grid.

I'll mention that even if you have a feed-through with a stalk, it may be recommended to put some further 'shielding' around that stalk: It is advisable to 'shadow' (from the ion paths) the insulator of the feed-through with some other dielectric structure(s) so that it does not become sputtered with metal atoms that may, ultimately, cause it to become conductive enough to lead to an electrical breakdown.

By using beads like this, they are 'self-shadowing' in that it is very difficult (I think, impossible, infact!) to get them all to develop a contiguous conductive surface under sputtering, so you protect what is threaded through them from ion path sputtering but they still don't become conductive after being sputtered on themselves.

These beads shown are custom and have an ID of 8.5mm. You'll probably not find these for general sale, but you might want ones with smaller IDs anyway, and smaller sizes are commonly available, e.g.; http://uk.rs-online.com/web/c/?searchTe ... pine+beads .

(The ones shown were made on a custom order, and 2kg was the minimum lot they'd make for me. So I have plenty and if you want to try a few, drop me a PM and we can discuss.)
Attachments
macor_fishspine_beads.jpg

ab0032
Posts: 86
Joined: Sun Oct 23, 2011 6:50 am
Real name: Alexander Biersack

Re: electric isolation of the feed through on the vacuum side

Post by ab0032 » Fri Jul 06, 2012 8:13 am

Chris Bradley wrote:
> It sounds like you are discussing a 'stalk' rather than a 'feedthrough'.
Yes, that is right, my title is misguiding, I am asking about the stalk.

> In your case, it sounds like you have an adequate 'feed-through', but that it does not have a 'stalk', ...
Yes.

> In any case, let's say you get to the point of having a feed-through that has a vacuum-side connection you can connect your grid to, but you want that grid to extend into the centre but only to be exposed there at the centre: As Richard says, you can probably get away with just leaving an exposed conductor all the way up to your grid. However, that supporting conductor will generate its own electrical field and you may want to suppress that field.
The aluminia ceramic will not prevent the electric field, it will go through the ceramic, only electrons wont be getting through, so no current, no ionization of gas molecules by electrons flying off etc.

> If this is the scenario you are aiming to discuss, one solution I have used, which generally works, is to use fish-spine beads...
That is a nice idea too, I will consider the new option.

> It is advisable to 'shadow' ... so that it does not become sputtered with metal atoms that may, ultimately, cause it to become conductive enough to lead to an electrical breakdown.
Yes. I will look for a way to put the feed through into a shadow, good point.

> These beads shown are custom and have an ID of 8.5mm. You'll probably not find these for general sale, but you might want ones with smaller IDs anyway, and smaller sizes are commonly available, e.g.; http://uk.rs-online.com/web/c/?searchTe ... pine+beads .

Thanks for all the advice.

User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: electric isolation of the feed through on the vacuum side

Post by Chris Bradley » Fri Jul 06, 2012 11:46 am

Alexander Biersack wrote:
> The aluminia ceramic will not prevent the electric field, it will go through the ceramic, only electrons wont be getting through, so no current, no ionization of gas molecules by electrons flying off etc.
Quite so. Sorry, sloppy language. Of course, dielectrics may prevent *the effects* of electric fields, between conductors.

I have another possibility also for this, which is to use fire cement to mould around the conductor. I'll take some photos later to demonstrate.

User avatar
Chris Bradley
Posts: 2930
Joined: Fri May 02, 2008 11:05 am
Real name:

Re: electric isolation of the feed through on the vacuum side

Post by Chris Bradley » Fri Jul 06, 2012 6:31 pm

[note my 'PS' edit at the bottom]

Alex, here is the other option you could try (which could be alternative to , or in combination with, fish-spine beads).

This electrode is made from stainless M3 studding with Vitcas premium fire cement [ http://vitcas.com/fire-cement-mortars-renders ] hand-moulded around it. It's very good value and they supply through ebay. It is quite an art to make this stuff into the shapes you want so I'll not give much advice on it so you can develop your own techniques, nor can I promise it'll do exactly what you might need it for.

(I expect there may be better ways to form it into shapes than the way I do it - which is, essentially, to roll everything into cylinders and go from there. Not very elegant result, but it usually gets the job done! Let it dry out by itself, don't bake it too early else it just splits apart. Sometimes it splits apart anyway!)

In this case, I am making a feed-through to sit through a metal plate within the chamber. There's actually a picture of it in action on; viewtopic.php?f=14&t=6956#p48561 .

[I'll mention just one other thing about Vitcas premium fire cement - it appears to me that it glows red when struck by fast ions only, green by electrons, and a pale blue when in contact with both (a plasma). I've not got enough to prove that's the case, but that's how it seems to me and makes it very useful for diagnostics!]

Edit - follow-up: as I have only used this cement on electrodes that are positive, relative to their surroundings, or for protecting parts exposed to beams/plasma, I thought I might best do a test to see if they do OK for negative electrodes. My tests suggest they don't. By whatever mechanism, it seems the electrons are far more ready to pass through the cement. So I can't recommend this cement for use with negative electrodes.... feel free to experiment with it if you like, but I don't think it is suitable for a fusor.
Attachments
fire_cement_electrode_cs.jpg

Post Reply

Return to “Fusor Construction & Operation (& FAQs)”