Getting high voltage into a vacuum

Tales of woe that teach. We learn best through failure sometimes
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Getting high voltage into a vacuum

Postby Doug Coulter » Sun Aug 08, 2010 12:24 pm

Edit - I moved this topic here because it fits better here - this design series "almost works" and doesn't give a long life due to reduction of the glass or quartz from hot D ions, the unequal division of E field in a stack of varied (or identical) insulators with non-zero spacing (so you can fit them at all - none of this tubing is precisely round) and so on. These work, for awhile, and are easy to fix, but now the new BN one I did, which I will soon document better, is the "way to go" and no mistake about that, even though it cost a lot more - it cost less in the long run...literally. It's funny how you spend all this time documenting attempts and what turns out to be temporary success, then forget to give the real answer that solved it for good and all - and it's not purely any sort of especially amorality. It's just that once a problem is solved, it's solved, and you move on. And yeah, no one likes showing off their failures, sure. But the point of this site is to instruct, to write the book on how it is really done - right. And also to show how that idea that seemed good at the time - and was - was supplanted by a better one. This saves everyone effort - no need to try (or suggest) what others have tried and found the failure mechanisms in, thinking you've thought of something new. Nope, it's probably been done, but not documented due to human and other failings. I'm hoping to correct that here, at least a little bit.


I had enough expensive commercial HV feedthroughs fail that I finally bit the bullet and started making my own.

There are some pictures and words here.

This design works well for DC and RF, but has a problem similar to the commercial ones -- it doesn't like hot plasma much, and in the case of hot hydrogen, the quartz gets reduced to metallic silicon after some running. It does solve some of the problems with the commercial ceramic designs around the fact that the conductor is exposed far back into the open volume in those.

But I've recently had to cut back the quartz/pyrex end inside the tank and add a ceramic insert that is easy to replace -- it gets reduced too, but not so quick, and if I can find some pure alumina, is supposed to reduce into something volatile that the pump will just take out over time. The thing is, I've not found a good source of the pure stuff as decent size tubing yet. The stuff at McMaster is mostly 84 Al, which has a binder that reduces to conductivity. Specifications of things are a weak point with them -- most don't care, and it gives them more flexibility to change suppliers on the fly.

Quartz is by far the best insulator you can use for most of this, but on further analysis, that's not always what you want. I've gone to pyrex for the outer tube in that design, as the higher bulk conductivity keeps most of the glass nearer to ground -- so it doesn't attract ions so fiercely, with a quartz inner lining so it's not touching the high voltage electrode.

The tubing couplers I use are surprisingly good for high vacuum -- they don't seem to have enough linear dimension to have much permeation through them, at least compared to the 7" viton O ring around my big door/window. I just didn't notice a change when I went from none, to one, to now, 3. They pass a He leak test with flying colors.

A major feature, or bug, is that they don't hold the tubing very well along the in-out axis. This means you have to keep the tube from creeping into the tank under vacuum, but also that you can adjust that as you want without breaking vacuum, or even making much of a leak while you do it. Since we have been playing with what happens when the end of our cylinder grid sees the field gradient near the end of the side arm it's running in, this is a "feature" for us, not a bug. Having the grid simultaneously occupy two places where Paschen's law works out differently seems to help us run better under some conditions as the ionization is easier to get going in the big part of the tank than in the sidearm. Things seem to happen quick when just a little of the grid end is exposed the the larger tank, so I machined a clamp for the glass tubing, and some shims to place in there so I can position it precisely and repeatably.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Getting high voltage into a vacuum

Postby Doug Coulter » Sat Sep 18, 2010 8:39 pm

So, I've now come up with a best and final (I think() design here. The outermost tube is 1" OD thickwall pyrex, and is cut so roughly 1" of the copper inner conductor (3/8" diameter for heat conduction) sticks out the end. A piece of 3/4" OD thickwall quartz inside that that comes out about 1" less than the pyrex. For that last 2" I put a piece of alumina tubing over the inner conductor, so it fits inside the pyrex and butts onto the quartz tubing end. This has now taken hundreds of hours of severe pounding at 2.5kw/50kv with no signs of damage at all.
Though I do admit to putting a little homebrew first surface mirror under it so I can see if things are starting to act like a failure and nip it in the bud. So far, it's just a nice way to see the back end of the fusor grid and visually see the temperature when we run hard enough for things to get incandescent -- which is rare. We run into outside of tank heating issues before that happens usually, and are using a squirrel cage computer case fan to cool that part in the gap under the lead covering -- and that's not really enough for a long run, only some tens of CFM just doesn't take away a couple kW before our neutron oven begins to melt.

Now we are cookin, and no troubles, no worries. It will probably fail someday, but with this construction (detailed better in the link in the previous post) fixing it is just making a new piece of cut tubing on the lathe-toolpost grinder with a cheapo diamond wheel. We are forced-air cooling the air side of this to keep the temperature there down to where the viton O rings can live, which works pretty well. We are using a little APAP pump (sold for sleep apnea) which is quiet and medium pressure to bring the air into a piece of 2" PVC pipe that covers the air side to prevent arcs to the 8" diameter grounded screen on that end of the tank -- which is for our safety. If anything arcs, there's a nice ground for it to hit instead of me! That Spellman is kind of like a higher voltage output welder in sheer power.

I have machined a piece of 2" pvc rod as a "hose clamp" to keep it from creeping into the system, and in fact to set protrusion length, I use a stepped PVC shim between this and the tubing coupler, so I only have to tighten that to the point of not leaking, not super tight any more. And with the shim, it's real repeatable.

This is a real winner on price/performance all the way around. The only hard part to make is that inner conductor rod, which will never die, and the rest is just cut tubing of various sizes and types. Boy did that problem ever dog us -- now it's history :D
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Getting high voltage into a vacuum - next rev

Postby Doug Coulter » Tue Apr 05, 2011 8:29 pm

OK, after a few of these, and wrecking a few in various ways, I now think I finally have a design that will live. This turned out to be a LOT trickier than I'd thought. You see, most insulators are oxides, and nearly all oxides are reduced when hit by high velocity hydrogen ions. This makes them conductive (the underlying metal becomes a metal again), which attracts more ions....downhill you go, and rather quickly. Also, anything not a vacuum has a higher dielectric constant than a vacuum, which means that as the high voltage is applied, the outside takes on the same charge as the inside, until that gets neutralized by, you guessed it, being hit by hydrogen atoms and reduced. Further, no insulator is perfect, and some are worse than others. The only thing not reduced by hydrogen in common use is pure alumina; which actually isn't that common, it's much pricier than the AD-84 most think of as alumina. The binders in AD-84 happily reduce to metals themselves, negating any advantage.

I've found that you can't just layer dielectrics and get the full additive voltage -- it refuses to divide equally among them, so when one arcs through, so does the next layer and so on, cascade failure again (perhaps I should link my earlier efforts in "it almost worked?). Since all these things have a dielectric constant, you're making capacitors, like it or not, even if there aren't explicit electrodes -- the surfaces will do fine. The upshot of this is that something like say, a thick alumina rod will hold enough "charge" to explosivly shatter it if there should be an arc through it as well. Ditto pyrex and quartz, though not quite as bad.

No wonder the commercial feed throughs stink for fusors! This is a genuine hard problem they simply don't address -- they are made for really high vacuums, not the soup we run of hot ions that are attracted to the thing we'd like to have survive.

Well, I finally got one that doesn't seem to have any of these problems, finally, and right now it's in the big tank where I can't easily take its picture. But we are preparing another fusor on the upstairs smaller system, so it needed a good feedthrough too. Made one (or did most of the work) today, and since it's not mounted anywhere, I have pictures.
HVFT1.jpg
Feedthrough parts

HVFT2.jpg
The other side of things


There are three layers here. The outermost is pyrex. It has kind of a high dielectric constant, but is also a lousy insulator as these things go. That turns out to be a "feature" rather than a bug. On the outside of the tank, it acts like a voltage divider, obiviating the need for corona rings to make sure the voltage gradient doesn't get too high somewhere. On the inside of the tank, the whole thing sits at ground so it doesn't attract ions. Just inside that is a tight fitting thick wall quartz tube -- the main insulation. It only touches high voltage at the outside of the tank end. It does not contact the central rod. It also extends further than the pyrex into the tank. That part can become conductive (due to chemical reduction) and it doesn't hurt a thing, but extending it past the pyrex protects the outer layer, and keeps losses due to ions just smacking the conductor down. Tightly fitting over the center conductor is another piece of quartz tubing, kind of a belt and suspenders arrangement. It would be quickly ruined (shattered) if it extended out to where the ions would hit it, and it would arc through at the voltages I want to run anyway, due to the dielectric constant and charge collection. I know, the usual way -- seen them explode and try to dump shards into my turbo pump.

The conductive parts of this are 6061 aluminum, which will be anodized before use. I've found that this adds a few KV worth of goodness, and prevents high current density sparks to a point on the surface. I'd suppose a coating of carbon black might do that about as well, but anodizing is easy and more permanent.

The main "plug" was turned on the lathe to be 20 mils or so smaller than the ID of the pyrex tubing (in this case, about .79") and O ring grooves turned into it. Only the one near the vacuum end actually seals, the other one is just to help things stay straight. There will be a third larger O ring cushioning the end of the plug and pyrex so it doesn't see too much stress. In this case, I might not use an O ring, but a big chunk of teflon with a hole punched in it, maybe about 2-3" diameter, to insulate the HV air side from the tank area, and prevent (or attempt to) arcs down the length to the tubing coupler/clamp on the tank.

My upstairs tank, originally designed for beam on target work, is 6" diameter and 2.2 feet long, with the feedthrough flanges coming in through the sides, in this case the top of the main tank, which is lying horizontally itself. This means I need a 90 degree turn at the inside end to mount a grid on the tank axis. So, as you can see, I milled some flats on that end of the 3/8" rod and tapped through it sideways with my usual 10-32 hole for a grid mount screw. The other end of the rod is tapped 3/8-16, as is the end of the plug it screws into. I milled a flat on those threads so as not to have a trapped gas pocket in that part. I tapped the other end of the plug 10-32 for the HV lead to bolt onto.

So, all this goes in the ultrasonic cleaner overnight, and tomorrow I anodize the rod (at least), and make the clamp/collar needed to keep it from creeping into the tank through the tubing coupler, and we're done with this. Or almost. There will be some tiny dabs of high temperature silicone to keep the glass pieces from sliding around, up near the plug end of things.

The beauty of this design is that it costs less, works better, lasts longer than commercial ones, and on top -- to fix it when it breaks (if it does, these are getting pretty good) all you need is to cut some more tubing. The $70 tubing coupler lasts forever, as do the machined parts, and fixing one of these is a snap and costs very little should you need to.

This has been almost as long and hard a chase as the gas control stuff, but the other one I made just like this (except it's tapped into the end of the rod for the other system) has already gone more hours with less trouble than anything else yet tried, and looks like living "forever", at least in this context.

This is something you machinist guys can copy with some confidence it's not a waste of your time and materials. With this, you only need a 1" tubing coupler for the tank, which is also something a machinist can make (I have, but it's less trouble to buy one).

This one is expected to stand off 60kv on the little system, and the aluminum should help it carry off heat. Should I get back to the beam-target stuff (likely) I only need to make another like this to fit the flange on the other end of the tank, put the other polarity on it, and have 120 kv for that....

Now, we think we have a 160kv supply coming. I kind of doubt this design will do that as is. Stay tuned!
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Getting high voltage into a vacuum

Postby Joe Jarski » Tue Apr 05, 2011 10:02 pm

Doug, thanks for the update on your latest feedthrough. That's one of the things that I've been putting off for a while, but will have to make soon. It looks like this new one is a lot easier to make - no more quartz blowing, just cut the tubes and stick them together.
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Re: Getting high voltage into a vacuum

Postby Doug Coulter » Wed Apr 06, 2011 9:47 am

The only parts hard to get right are machining that plug, and I forgot to mention that I chamfer the inside of the pyrex at the plug end to make pushing the plug in easier and not nick the O ring.
I'm using about a 15 degree angle, and cutting out about half or a third the pyrex thickness there, to about 100 mils into the pipe. I always do that ad hoc after measuring the actual ID of that piece of pyrex, as it varies a little.

I very gently flame polished that chamfer with an attempt at annealing after grinding it on there. If you make the O ring groove too tight or too shallow, you break the glass, if too loose, it doesn't seal well. You can't really polish it smooth with flame, or you wind up shrinking it, but you can round off all the really sharp little corners due to the uneven diamonds on the stone you use to do the chamfer -- kind of dull it a little so it doesn't cut viton and doesn't have an easy place to break under insertion pressure. I use my grease, which is 80% beeswax and 20% forepump oil to lube the O ring, or you can't put it together. It gets gone after awhile...

I've also done the plug and rod with OHFC copper and just brazed the rod through the plug -- works great (this is what's on the big tank) but is harder to do. The other variant on that is to turn the whole thing out of one chunk of aluminum -- also works great, but wow does that fill the shop with lathe swarf. This last one wastes less stuff. So far, I've not manage to melt the Al versions I've made, even with the head of the bolt screwed into it orange hot -- carrying out the heat is a moderately big deal and all these do that well. I've been known to add a little surface area (drilled pipe cap held on by the main connection bolt) to the air end to help with that. The big tank now has all that inside a PVC pipe with an air feed that blows over it. Else you can get the end so hot the viton starts to gas.

The real trick/magic here is at the end where the rod gets into the tank. You just have to leave a little of it completely bare it seems. I'd tried having a tight fitting piece of alumina there, but it was nothing but trouble. In this design, the outer quartz doesn't touch the rod, and the inner quartz ends fairly far back in there as it's not thick enough to handle the full voltage (and it may not even be needed, actually, but...seems good to have as much insulation as you can around the area that goes through the grounded clamp).
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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Re: Getting high voltage into a vacuum

Postby Doug Coulter » Thu Apr 07, 2011 5:13 pm

One final picture. Here's what all those parts look like assembled. The blue gunk is some special silicone from the autoparts store. This is the good permatex stuff, medium high temperature, and it doesn't give off acetic acid on cure -- it's some different chemical from the normal goop. The real high temp stuff (orange) does give off vinegar, and takes forever to get hard, this is a test compromise. It's in there to keep the inner two pieces of quartz from sliding out, as this one will be used vertically.

NewHVFT2.jpg
Final assembly, ready to put into use.


New HV FT for the new fusor build.

I suppose this might also be appropriate in the vacuum tech forum, but it had to go somewhere.
Posting as just me, not as the forum owner. Everything I say is "in my opinion" and YMMV -- which should go for everyone without saying.
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