Getting HV into a fusor

[Doug Coulter]

Different than a vacuum by far, the issues are mostly on the inside because there are gas and ions on that side too. The more or less standard HV feedthrough design does NOT work well here, for a number of reasons. The one I'm thinking of when I say standard is the one you get from say Lesker, where there is a wire or rod attached to the end of the ceramic, that then "floats" inside the large hole in the ceramic and eventually enters the tank. There are (at least) two reasons that doesn't work in a plasma set of conditions. One is Paschen's law, and that extra length back to the "top" of the insulator may be the longest path in the system -- so the gas/arc happens lengthwise down the central rod, and fries the ceramic near the bolt -- or fries the ceramic where it joins the flange with an arc from the top to the inside at the flange. Another issue is that with that long rod, it attracts all the ions, neutralizes them, and gets hot -- just what isn't wanted. So one has to eliminate both that long path, and any exposed extra conductor, a modification that turns out to be really hard to make on a normal feedthrough design.

I have learned a few things empirically, that I will share here, but I'm looking for more input from others who have fought this particular dragon.

One is that to insulate the central rod, it's not enough to insulate it "pretty well", it has to be really good, and able to stand off the full voltage, as otherwise the outside will attract ions, and they hit hard enough to reduce any oxide chemically --. Emprically, pyrex is pretty conductive by these standards, and so far, only quartz has been good enough for that. But near the end where it meets the fusor grid, it gets hit by high speed ions anyway, and tends to become metallic silicon...leading to quick failure there (pyrex fails immediately, shatters, ugh). So the only thing I've found that will do for that last inch or so is alumina, which when reduced evaporates off, and stays insulating for a lot longer (until some sputtered grid metal gets on it).

For the outermost part, I have found that the conductivity of pyrex is a good thing indeed, achieving much the same effect JohnF was mentioning on another thread of spreading out the voltage differential nicely along the length. So when that outermost piece is grounded in the middle (via the tubing coupler that lets it into the tank) we have no lengthwise arcing on the outside side, and on the inside side, it stays at near ground too, and doesn't attract hot D+ ions so bad, and therefore lives a long time -- and even if it becomes a little conductive on the end, it's insulated by the inner quartz tube well enough to not cause trouble.

Ok, those tricks, described elsewhere on this forum, work fine at up to my current 53kv limit, and the fat copper rod I use helps with heating issues, being able to carry heat out of the tank nicely, even though it's nearly a foot long. The problem is, now I want to go up to the hundreds kV, and there is no source of quartz tubing thick enough for the middle insulator I can find, they just don't make almost solid rod with a tiny hole in the middle (thick wall), and in general, due to tolerances, you can't just get telescoping sizes.

So, a new design is needed -- I already know there's nothing I can buy that solves all this, and my friends at CERN and ITER haven't solved this one either (private communications and thanks Joe S for hooking me up with those experts).

I calculate that a roughly 1" diameter piece of teflon will do the insulation fine...but...it won't like exposure to vacuum one bit, and especially not hot ions, and the stuff flows as Jerry points out elsewhere. So I began with the idea of using a pyrex outer, with teflon insert, but now I have to make the glass/metal seal on the in-tank end. Since pyrex is conductive (compared to quartz or teflon) I then have the problem of the in tank end no longer sitting at ground and it will attract ions with the usual destruction. McMaster doesn't sell alumina tubing large enough to slide a piece over that....it's extremely hard to make a quartz/metal seal or a quartz/pyrex seal -- the expansion tempcos are too far off. I suppose I could try the same geometry I'm using now, but using teflon instead of quartz for the inner insulator, but -- it would be in vacuum even if I use the alumina extension, and it would get hot, which would make it give off various nasty gases on the inside of the tank, not good. Probably not so hot as to ruin it, just everything else that got hit with the fluorine it gives off in that case -- out on the air side, no troubles in other words.

So I'm stuck between what I think I need, and what I can find materials to actually do here, and any thoughts are welcome. This is just a plain bizarre kind of application which no one is designing hardware to handle. It kills commercial feedthroughs right quick, well under 100 hours of running, sometimes inside 20, and at those prices, well, I can't do this. We are running at a pressure point where a spark doesn't take the shortest path -- on the left side of Paschen's law curve, and I don't think many other applications ever do that -- if any. Were this a vacuum, I'd be fat, but of course a fusor doesn't run in a vacuum, it needs fuel!

FWIW, I am running at 1.6 e-2 millibar indicated, or thereabouts. At 1.9 and above, I get fewer neutrons and the current tries to rise without limit, below that too much it just goes out and I can only run currents more or less equal to what my ion source provides, it's not self sustaining in the 6" diameter sidearm. If I put another grid out in the big part of the tank to act as an ion source, it will light off down to about 1.4 e-2 mbar at 40kv or so, but no lower and actually, I'd like to go lots lower than that if I can find a way (working on that, separate project). The reason is -- higher Q when the mean free path gets long so I'm mainly getting fast on fast collisions and boy does it ever go up quick -- double the output going from 1.9 e-2 mbar to 1.6 with the same input volts and ma.

[chrismb]

OK, so my post here is not quite the topic you are discussing, so this won't help (!) but hopefully might get the thought-juices flowing, perhaps...

In my setup I get a lot of help from the residual magnetic fields floating around. Mag fields appear to suppress undesirable discharges - but only so far.

My setup needs lots of connections and, though smaller voltages, potentials (V/m) that in some locations are not so different to fusors.

I got discharges all over the place from the feedthroughs I had. I made teflon threaded standoffs from the baseplate, but I still got discharges coming off the interconnections at the top, even when screened by even more teflon. In other words, even just a tiny gap left in a dielectric still won't help much, and those wily electrons will find a conduction path.

Here you can see where I ended up, simply out of practical convenience. This would be unsuitable for you, but the problems I am overcoming here might be relevant (to avoid!). I had made up 2kg(min order) worth of 8.5mm ID fish-spine beads for general purposes inc to use as 'shadows' around M8 parts. I used these to drop over the feedthroughs (which are M5 stainless studs) and then passed the wire through a peek insert that passes through the top most bead. Still conduction paths right through where the beads touch - and that was tenths of mm. So I then wrapped kapton tape around those beads to cover the gaps up. Also, I wrapped PTFE tape around the wires (those are Kapton wires too) and that 'bundle' of tape can then be slid down the wire to bung up the remaining hole left in the top between the wire and the PEEK insert

So I think there might be a couple of pointers that could still apply to any thinking on feedthroughs; i) howsoever you arrange dielectric material around the feedthrough, you can't leave even the slightest gap exposing a conduction route because it'll always find a path through, ii) however, you don't actually need a great thick insulator to do that and, for example, (rather than using kapton tape) you could wrap a thin sheet of teflon around the standoff and then push ceramic beads over the teflon wrap to hold it in place which'd shadow the teflon from ion exposure/deposition.

I'm not sure I've explained all this quite well enough, but fire back to me on anything if I've been unclear. And I'm sorry that this probably doesn't help that much but you're running square into the issues of why 'fusors' are so limited in their scope of being recognised as likely to answer any 'serious' fusion questions.

[Doug Coulter]

Right, I've found (replicated!) all the same stuff as you. I didn't mention that I'd tried to solve the issues on a regular feedthrough by say, jamming a glass tubing that fit over the center conductor back in there to where it was a tight fit on the inside taper of the commercial piece ceramics. It nearly worked, but not quite -- glass on ceramic meant it never fit perfectly. Various dances with teflon on the glass to eliminate that kind of worked for awhile (teflon back in there far enough to not get hot) but not very long, as you say, if there's a sneak path, something will find it. I did find that with close enough tolerances around the center conductor, I no longer had these weird lengthwise arcs down it at least. But a teflon spacer put in "the back" to hold things aligned would occasionally explode from one. Interesting that it showed fracture faces, as if it were brittle. Mass spec starts to see fluorine, time to quit.

Further, either glass or quartz (which is e3 less conductive) carried near the tip at the grid attracted enough hot ions to be chemically reduced to the metals, and once that happened really got going with ion hits and heated to the point of shattering even quartz!

I've not drawn the current design I'm using yet, which seems to be OK for 50kv range, but I'm about to tear the thing down so I'll get a picture of it and put it up in the next few days or so. The main flaw with that re higher voltage is that it's near to arcing from the center rod, through the glass and quartz, to the tank where it passes through the tubing coupler. All the other issues seem fairly well solved with that one, but it won't scale (as the computer guys say) with materials I've been able to find so far. Would be nice if someone made a quartz pipe 1" OD and 0.1" ID that would hold that much voltage off laterally, but I'd still have the issues with the end getting destroyed unless I could also find some pure alumina to guard the tip near the grid from ion bashing.

Those lengthwise arcs really threw me for a loop for awhile, till I studied up on Paschen's law and figured out why the electricity wasn't taking the shortest path! Now I understand (I think) but that's not much help if I can't put my hands on the right materials and a good design.

Teflon would do except for all the things we know -- it flows, and it's not happy in vacuum getting hot. So I'm thinking I have to put the vacuum seal at the business end inside the tank, instead of the easy way I do it now, (and then stuff the glass pipe full of teflon) but....that means direct metal contact with the glass, which gives me other problems -- the glass will conduct some and then attract ions farther back towards the flange and need more protection from hot ions. The same issues with alumina exist as with the quartz -- no one seems to make a fat rod with a tiny center hole, and I have doubts that would seal well with the tubing coupler (viton O ring) as well... My glass to metal sealing skills are such that only pyrex/tungsten works for me and it's still kind of a pain to do at replacement time. And even pyrex can heat-shock shatter under these conditions -- that end gets HOT. I really want to avoid that right over the gaping intake of a new bought turbo! Dodged a bullet once on that (the fine screen caught all the pieces) but...

At least the power supply is coming along nicely so far, mostly a lot of dumb shop work to do at this point, soon to get some full power tests, I hope. Should be impressive if nothing else, but I hope it's not impressive in burning up parts I want to have live awhile!

Some of this might magically go away as a problem, or so I hope. The reason is -- I've found already that as I go down in pressure (longer mean free path) to the point where the fusor won't self start or run on its own without a large supplemental ion source out in the big tank, that output goes up like a rocket -- almost vertical output and Q vs pressure reduction, as if I'm finally getting fast on fast interactions. So if I push that a little farther, it's possible I won't have some of these problems (assuming I can get a copious source of ions that works at those lower pressures). At that point, it's a "vacuum" again, and the old designs should work, perhaps.

And yes, when i described what I'm trying to do to my friend at ITER and my friend at CERN, they said, nope, you're never going to get that to work, but after discussing possibilities we came up with something that does work, kinda. Hey you guys -- you can post here yourselves!

I may shove a magnet in there sometime for another reason I have in mind (controlling electrons without messing up ion trajectories too much) and see what effect that has, just for giggles.
It wouldn't live long as it would get hot, but it'd be an observation worth making, and they are cheap anyway.