Well, I stupidly broke the last one trying to pull the HV feed through out with it still on there, duh. So, time to make another - the old one had one tungsten rod broken inside a hole, and even anodic etching in NaOH wouldn't eat the sucker out of there, so I had to make a new base. In this case, it should be a fairly large improvement, as I'm holding better tolerances (spent a whole day tuning my shop tools), and now, the holes in the baseplate don't go all the way through - just 5.5 diameters worth for the rods. This should eliminate a little issue I had with the rods sticking past the back slightly (even if you machine that flat, there are still sharp edges) and becoming field emitters. Really hard on things when that happens (and it can draw so much power I can't run on top - these TIG rods were designed to be good electron emitters).
So, 20 mil doped-tungsten rods, a new Ti base, made from "billet" (quotes so Jerry won't jump down my throat too hard - I used a piece of 1" Ti rod stock, which ain't, strictly speaking, a billet), and potentially, a new end ring, this time also Ti. We were seeing some hydrocarbons with the otherwise very good graphite end ring, I suppose from very hot D atoms hitting it and knocking off C atoms to combine with in a cooler spot elesewhere in the fusor.
Here's what it looks like:
The base is somewhat special. You'll notice I "concaved" the center. That's because there is a beam of hot, relatively high "pressure" ions that hit it dead center when it's in use. That's where most of the heat is generated. Ti stinks for heat conduction, so the object here is to get that heat generated as close as possible to the copper/BN feedthrough, so it can carry it off most efficiently. I got there with a center drill (1/8th inch at the tip), then a 3/8" drill part way, then a medium angle deburring tool to kind of chamfer the angles and let the beam hit inside the little hole, right where the 10-32 stud begins - so the heat is less than .050" from the sink.
When I get a little further, the W rods will be held in their .020" holes via centerpunching around the edge, like before - that worked quite well. And if I can get that Ti ring round (it's 1/8" wire bent to a circle and butt welded with my TIG welder), and drilled (gotta figure out how to hold it centered - I'm partway though building a jig for that), then the new other end will be that ring.
This time, insead of the circle table and mill, I used the tool-post grinder I made for my lathe, and a big degree wheel stuck on the chuck with magnets to get the alignment (I used one for cams from Summit Racing). The reasoning was I was breaking drills, a real bummer with what they cost and how long they take to get, with the mill, which won't spin anywhere near as fast, and Ti is picky about that when making tiny many-caliber deep holes, or it won't clear the chips. Looks like they came out plenty accurate - all of this is in the 1 mil error range, total. That despite not having grade 7 bearings (not hardly) in the tool post grinder - they are the cheap guys, but have no sensible play at all - the issue is how straight the quill/collet is, and vibration, which amounts to a couple mills when the drill isn't in contact with a hole.
The last time I made one close to this good, I discovered something I hadn't seen before - the beam out of the end was so finely focussed, I was making a screen I'd put inside my view window heat to orange hot, with something negatively charged.
Here's a video that shows the effect better than typed words can - watch the spot move away from the ion source grid on the right - closer to us than the main grid:
This is with the previous accurate grid. The older, all carbon but for the .040" rods is what we ran just before the 2013 HEAS, and it shows no null with ion source voltage variation - this seems to be due to the slight twist on the older grid, which was a mistake in manufacturing (perhaps, it actually works really well, as that video showed, but doesn't toss out this concentrated beam from the end as far as I can tell. Or it's not as well focused when it hits the viewport (about 15" from the end of the main grid - all the way across the big tank).
But, wouldn't you know it - right after I manage to get a really nice baseplate made, without breaking off any drill bits in holes (I did break one, but that was just by touching it a little too hard) - my dremel tool, mounted in an old dremel drill press I use for cutting TIG rods to length, took what is probably its final dump and burned up the shaft coupler (which I've remanufactured several times, but this time - big smoke, plastic all melted, it's probably toast for good now). It melted the coupler that was epoxied (by dremel) to the end of the motor shaft. So now I have a knurled shaft with all the dips still full of epoxy, and now also the wrong diameter to use the "hose trick" for a flexible coupler. Sigh...it's always something, isn't it.
So, I save 20 bucks or so on broken drill bits - and lose 200 to having to buy a new dremel and of course, the new ones don't fit into the old drill press, so...I have to buy one of those too. Soooooo close, but no cigar, yet.
Here's kind of what I was shooting for this time, but I got a little fancier with the recess in the center to really get the heat to happen in just the right place for things to live longer.
Once I get all the parts made, the Ti parts will be anodized in diet pepsi (happens to be just the right strength of phosphoric acid...) to 120v or so, which cuts conditioning arcing way down, and improves Q since TiO2 is a lousy conductor and doesn't attract ions as well - which is a waste for this, at the temperatures this reaches, we can't count on the Ti holding any D at all - it gets red hot at high power inputs.
Here's a detail of the older grid, post being broken - I may still rescue it with more anodic etching - tungsten is etched in an NaOH bath anodically, where Ti isn't affected noticeably by that. It's just a matter of getting the E field in the bath to go down that tiny hole...a real pain to jig for.
So, if you're in the 6" OD fusor world, with a cylinder tank and a 1" OD grid, here's how to make a few million neutrons a second with 50kv and about 20 mA (tops - heat issues - it actually has higher Q at a couple mA, but also fewer neutrons/second).