More progress to report on the beam device that's about to get real here. As with a lot of projects, most of it is getting ready, not doing the simple and fun parts right off.
So, I drug the little Pfeiffer pump station out from under the fusor-2 table, and took off the old tank, substituting the new stuff. This system didn't yet have a valve in the foreline, so I adapted one from a refrigeration unit to work with the 1/4" ID plastic hose this one uses for foreline - it's only a diaphragm pump, not a great forepump. To do this I turned out some little hose nipples on the lathe, from brass, that were a solder in fit to the .325" ID of the valve copper pipe. The valve is sitting on the station on the left front, yet to be plumbed and wired in. I'll put a little 24v transformer in there across the power to the forepump, and some sort of switch/pushbutton arrangement to allow me to have the forepump on, but the valve off - for pressure adjustments. Or, of course, just have the valve open whenever the pump is running. I suspect that having that mode will cut down how often the forepump has to run - seems like it's the thing that has any back-leak at all.
- Basic Mockup
The long sloping pipe towards the camera is what I use to pump down tanks for gas transfers, glass blowing, making detector tubes and so forth - useful accessory. Has it's own pirani sensor, inlet valve, and there's a gas manifold (with a gage) down there on the bench. You can pull the manifold to zero, then let in this many mbars of this or that gas to get a mix you want, then let the desired amount into that sidearm, for filling gas tubes and detectors with a specific mix.
The lower cross has ports for that, the main pressure gage, and the mass spectrometer for this system. The upper 6 way cross gets a fixed target and feedthrough on the side away from the camera, the back is the push-pull-twist toy for the movable target, the top gets an 8 pin feedthrough for low voltages (faraday probes and target current monitor) , the front gets a window (which I've got to make a mica sacrificial shield for) and the side with the Al plug will accept the beam tube when I make that - the easy and fun part probably. But none of it works till it all works.
I figured I could use a target with a bunch of thermal mass, that I could either measure current on, or put voltage on and also measure current. Eventually, the idea here would be to evaporate some Zr or Ti onto this, then impregnate it with one of the hydrogen isotopes I have available. At the moment, it's just a big chunka copper.
- Final target/beam stop
I cut the main feedthrough stalk to position this right flush with where that sidearm joins the cross.
Now, the movable target is, well, movable yet still wants to have a connection to the outside world. Insulated. Well, stranded teflon didn't seem like a good idea, fish beads not so great either, as the way this is laid out, the target connection and the feedthrough are very close when the target would be in the beam, but it would still need a long wire to allow the target to be pushed or pulled out of the beam - what's the extra wire going to do then, get in the way somehow? And it's required to be able to be "pushed" out of the beam, so I can get it hooked up in the first place - that needs around 3", but in position it's more like half an inch. After a lot of head scratching and searching, I finally found a real weak and stretchy spring. Not in my vast explicit spring collection either - but on a gasoline engine, the little spring they use to take up lost motion in the governor linkage. So, I had a kit to make into what I want:
- Parts for the target feedthrough and connection
I took an 8 pin 5kv FT and shortened the inside vacuum wires to make the 8 pin ceramic octal tube socket be flush in the cross, soldered that on (yeah, that's a no no, but I hope it won't get hot enough to give me a significant lead vapor pressure), and bent the pins on the other side to fit into another ceramic 8p tube socket. I needed one wire for the target connection, and from experience, I knew I'll need some sort of shield for the insulators in this - there's always at least a little sputtering. So, making a virtue of necessity, I decided to fab a copper sputter shield on its own wire, to be a kind of offside faraday probe in the bargain. I just needed a hole for the spring wire/ceramic tube insulator to pass, and a rod welded on the back of the copper disc to hold it and connect it to the outside world. It could I suppose also be used to deflect a beam some - but also from experience, I know that the 5kv rating is pretty much the limit, and a 125kv beam isn't going to be moved much by 5kv over an inch or so...but I do have 6 more connections I could do something with later if I'm willing to take apart this ship-in-bottle to do that.
- Assembled
Now its time for lunch and a beer...after which I'll try to get this somewhat assembled and hooked up - going to take some finesse to get that spring end under a screw while it's stretched and all tightened up. Then I can see if this will work OK and not short to something when the target is in position. Once I put in the foreline valve, and wire up a supply for it, I can at least test for leaks - I have a plug to put where the beam tube is going to go for that one. Then, ion source and focus tests, then the real thing. Baby steps, but those are sometimes the fastest way to get somewhere, as if you get them all right, there's no need to go back and check/fix each part - just move forward. Eyes on the prize!
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.