Fusor teardown and FT fix

[Doug Coulter]

I'd fried my HV main feedthrough on a really brutal run to activate some new things the other day, and it got so bad I couldn't use it to test the new hornyak detectors, so time to go ahead and break vacuum (I leave the main one on the pump 24/7 normally) and fix and improve some things. Since this also documents some construction details I've been asked about but was too lazy to take it far enough apart to get pix of, well, here they are since it was easy to do this time.

I've been asked "how do you handle the HV around a fusor. "Very carefully" is the trite answer, but also true. I try to keep the stray capacity down, and mount the ballasts right on the feedthrough, blow in cooling air, and shield the entire mess as though I was making a big piece of coax with the ballast as the center conductor. This cuts down ozone, which is minor, and EMI with is anything but. Don't forget that a near perfect 30-50pf capacitor (all the strays) at 50kv can discharge in nanosecond timeframes, providing kiloamps of current - at kilovolts, that's millions of peak watts of RF in the shack - not good for everything else, and before I got more careful, has fried PC computers at 10 feet that weren't connected to this in any way. Serious EMI indeed.
Now it's so quiet it's hard to pick up on a scope probe. But here's what's under all the shielding and cooling:

All the gunk stripped off. Resistors are 225w wirewound, the main one is 50k, the aux one is 100k.

As luck would have it, these fit nicely into fitting for 1" copper pipe, so I use those for mounting and connecting. The ends of the FT's are threaded 10-32 to accomodate this plan, and the HV supplies have banana plugs on the ends of their coax, which just happen to fit nicely into a jack for the main one, or just another 10-32 hole in the secondary one in the background in the above pic.

While I was waiting for the air to get into the tank through the slow bleed valve, I made a shield for the #2 grid (ion generator). Since sched 40 pvc won't quite stand off the 40kv max that one can see (it's usually run closer to 10kv, but "stuff happens"), I made a teflon sheet wrap for the otherwise uninsulated resistor while I was at it. I don't care if that tape melts - it just lets me fit it into the pvc without trouble when putting it together.

Shield for the aux grid HV feedin

The proximate reason to tear this down was some arcing in the main feed through. I'd run it brutally (53 kv, 30 ma, 20 minutes) awhile back, and that toasted it - even my big grid got orange hot in the middle of the rods for a long time in that one, and I almost got the end red hot. I'm using pyrex and quartz for the insulators, either of which can be reduced to its component metals by hot H - and this stuff is as hot as it gets...When I put this together, I'm trying another scheme to protect this. Here's what the damage looks like:

Note cracks in the quartz...

I had a mirror under this so I could watch it fail (this has been an ongoing issue). What's interesting is that while I could see arcing, it was fully contained in the quartz, right about where the pyrex outer tube ends. No spark out to the tank, and no spark into the center conductor either - just inside the quartz. I'd presume that it picked up enough charge on opposite sides of the thickness to just arc through, and the damage was done by energy stored in the capacity of the quartz, but that's a guess. Since more or less nothing can really stand this gaff - zillion degree pre-ionized deuterium hitting it, I decided to simply put a grounded pipe around the entire mess and let it take any hits - but since it's grounded, it doesn't attract ions. This seems to work, using the paradoxical Paschen's law in my favor -- the distance between the end of the grounded pipe and the grid end is too short for a discharge to occur there! Doesn't satisfy p x d

New one with pipe about where it will be.

The copper pipe won't go into the coupler, but it does fit inside the flange hole for the 2.75" CF, and to get it grounded, I just shoved in a little bit of that handy copper screen wire, bent to contract the hole and the pipe. Here's what it looks like installed:

End view, before the grid is added. It's off center now, but when I hang on the ballast, it centers.

The copper pipe sticks into the tank about 3/4" more than the insulators do, so they don't get hammered so hard by stray ions - in tests after I did this, the crazy idea actually seems to work quite well.

I'm running out of pix I can attach to a single post, so I'll reply with more -- and some movies of it in action I'll post somewhere, and at least link to from here - I got some amazing results during the break in running that really surprised me, and which deserve close attention and documentation (maybe I should put them into run data).

[Doug Coulter]

If you look carefully at the last picture of the previous post, you can see a rectangular area of a different color in the back of the chamber. That's there because I had put in a little first surface mirror to be able to see the back end of the grid from the front, and also see the end of the feedthrough. Here's what that mirror now looks like:

Looks funky at this angle, was still good enough in use, but I didn't put it back in this time.

Other things also take a pounding. I've long advised folks to protect any valuable window and themselves by putting a sacrificial piece of other glass between the window and the action. Here's one behind my main door - over a foot from the action -- after about an hour total runtime. Note the yellowing? It's also craze-cracked circularly. Still has some life in it, so I put it back in. I used those little clips I made out of beryllium copper to hold it in the port. In the movies later, you can see it fluoresce a little (or dust on it, or whatever, but it lights up).

Note yellowing...and it's cracking

Ok, now I install the pretty grid. This one seems to have "legs" and since it works so well, I quit fooling with grids for awhile. Those I made of pure carbon for low secondary emission - well, it turns out you need that electron emission to run a fusor unless you have one heck of an ion source (or a bunch of them) and even my secondary grid source isn't enough...nice theory, but...
Here's the grid:

The cool grid. The graphite ends help cool it, have a high radiation for heat. .040" tungsten rods about 2.5" long. Slightly twisted, a mistake in making it, but maybe that makes it work better?

And here it is in place:

Home again!

OK, that was the fun parts. Now to put all the cooling and EMI stuff back on the outside of the tank while waiting for it all to get pumped back down.
Here's a pic of the blower I use to cool the main ballast - it really needs it too. I used an old apap pump before, this one's a little more air, maybe 60 cfm, but through a long skinny hole, it's a real wailer. It pushes air into the 2" pvc pipe centered around the resistor, into the tank end, which is sealed with a foam rubber gasket, so the air has to come out the back and flow along the resistor.

Air pump, high pressure, high flow. Required.

I used a couple of glued on fiberglass rods to keep the resistor centered in the 2" inner pvc pipe, so it wouldn't melt the pipe (usually), and that sits inside an 8" pipe that has the copper screen around it for EMI and safety. Here's the assembly:

The rig as assembled

Since this feedthrough design allows linear motion (like it or not, the vacuum makes it creep), I came up with a clamp for the glass and a shim arrangement to allow me to set the depth into the tank repeatably. In my tank, things seem to work best with the end of the grid roughly flush with the end of the sidearm...for whatever reason.

Clamp and shim

I'm processing the movies I took of the breakin run now, I'll post them too under run data...pretty cool stuff. But full HD is far too big to bother uploading to youtube at 100k bytes a second. So I'm shrinking the resolution etc on my killer fast machine, and will get them up tomorrow. If I didn't do that, they wouldn't be all uploaded by then anyway!

[Doug Coulter]

Well, since the one is already up on youtube, here's the breakin run (part of it). There's plenty of contamination still in the tank for this - that super duper 520l/sec pump could only hold it to 5e-6 mbar before this run, easily and quickly went down to 1e-6 mbar after it - this really blasts the junk off the tank walls. By tomorrow, it'll be down to e-8 again. FYI, the gage I'm using reads almost exactly 2x high on deuterium, but right on air...so when you see me quote numbers to replicate in your lab - it's on you to make any conversions there.
http://www.youtube.com/watch?v=_b0OL1SHAY0



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