Pump adapter for beam device

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Pump adapter for beam device

Postby Doug Coulter » Sat Aug 13, 2011 8:17 pm

I'm slowly putting together a system for doing beam on target and beam on beam work, pictured elsewhere (I think - I'll have to go look and see if I can find a link).
At any rate, the system is a 6 way cross, with the bottom going to the pumps. Two of the sides get beam tubes, the back gets an 8 pin feedthrough, the front that magnetically coupled wobble stick, and the top a window. But! I have to get gas in there somehow, and I wanted a pressure gage on that spot as well, though the system it will go on first has both that and a mass spec.
I plan to make this portable, and to run off one of those surplus Pfeiffer turbos, with a controller I'm designing for the vacuum pump part of things. The controller wants two pirani sensors, one in the foreline, and one on the main system. Sure, at high vacuum, that one will stop working around e-4 mbar or so (we've tested these here and they are a lot better than most commercial piranis). That's fine - knowing foreline pressure and RPM and current on the turbo we can go a couple decades further, witch will be good enough without the expensive ion gage etc.

PumpAdapt.jpg
Pump adapter, now with warts.


I drilled through some 10-32 brass threaded stock 1/16", then shoved a 1/16" steel rod in the hole to take up volume and act as an orifice. The valves have been modified for absolute minimum lost volume past the valve seat, so in a single pulse gas can't just rush through really fast, and there isn't much stored volume to fill up quick and then leak into the system slowly thereafter. Basically the ends of the brass screw are machined to fit up inside the valve and take up all wasted volume in there. I used two because we have that third hydrogen isotope here, and one will be dedicated to that in a scheme I've worked out (I'll show it later), where the glass ampule is sealed up in copper tubing, soldered to one of the valve intakes, and then bent to break the ampule.
The other valve can let in H, D, argon, whatever, or just air to bring the system back up when I need to do that (almost never, and of course, you can just loosen a flange and will anyway if you need to get in there to do work).
The copper tubing holds a special grain-o-wheat bulb we use as a pirani sensor in the circuit shown elsewhere. These are 27.5 ohms cold, and we heat them to the point they hit 47 ohms, and measure the voltage required for that. In use, that will go into a 12 bit a/d on a PIC for readout (and we can linearize it there). The valves are 24v DC and can be driven off the PIC too, with a fet driver. They're pretty fast, and are now on all my other systems as the main gas inlet. They use about an .032" hole, with a tiny button of viton mounted on a thin diaphragm that is pulled off the seat by the coil. About a 5ms pulse will open and shut one (for a lot less than 5ms, most of that is the coil inductance getting to draw current).

I drilled and tapped the adapter for the screws, then soldered them in with Sn/Ag solder, and added Hysol 1-C over that for strength. If you have the right flux, yes, you can solder stainless steel. But it doesn't have great creep strength, and doesn't like being flexed, so the epoxy is a backup for that. The copper tubing was counterbored a little to fit the tiny bulb nicely, and that was glued in. I've found these tend to live longer out of pumpdown turbulence, so that's why it's out on a tube. Also, they fail someday, and this makes it easy to replace -- just cut off the end and glue in another one.

Just one step out of a lot of them, but it's coming along. I have a couple experiments planned for the fusor this is going to replace on the smaller system I've got to do first, but that can happen anytime I've got a couple hours and motivation, then on with this show!

Here's the mockup as it now sits. This is how I might set it up to do beam->target, with a HV FT on the left, the beam tube on the right (not there yet), the wiggle stick on the front, window on top, and a multipin LV FT on the back (which is right now on another system keeping the air out). Should I wish to run the target off ground (probably not) I can mount it on the HV FT. Or I can mount it (or something else) on the end of the wobble stick, which is insulated and has provision to hook a wire to. For example, I could put a disc with an orifice on the stick, then measure beam current on a target hooked to the big FT as I move the orifice around, to plot the beam profile. The stick lets me pull the end completely out of the interaction zone, or can go all the way to out the other side of the cross to be able to reach it and put something else on it.
BeamMockup.jpg
Mockup
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: Pump adapter for beam device

Postby Starfire » Sun Aug 14, 2011 6:31 am

Doug - what is he max working temp of the Hysol? tks

I think beams are the answer.
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Re: Pump adapter for beam device

Postby Doug Coulter » Sun Aug 14, 2011 7:32 pm

Well, I'm pretty sure beams are the way to learn what I think I don't know yet, "the answer" or not. (Does that make sense?)

Hysol (this type anyway, Joe pointed out there are more) is basically an epoxy that is mostly silica added. I'd guess that like most things, there's not so much an absolute limit as it just starts getting weak as the temperature rises. Probably about 200c would be the max temp you'd run it at if you didn't need it to be real strong. Above that most epoxies decompose -- which in this case means gassing, not good. I've never tried it really hot, like charring paper - I expect it would char too and really stink just like printed circuit board material -- it's pretty much the same stuff, though the thermal tempco is lower (for expansion) due to that filler.

And heck, the beam thing gives me an excuse and a way to utilize some serious HV supplies I've not put on the fusors....and do things with the extremely tiny amount of that other hydrogen isotope we don't name often here...Only have a tiny amount of that one.

Finally getting those final experiments done on the little fusor, at least I got a good start tonight, interesting results. When done, that pump station comes free and I can start thinking about putting the beam device on there. The experiments involved essentially floating a part of the outer shell, and looking at signals the particles induce on it, as well as putting deliberate signals on it to warp the field the grid runs in. All I can say for now is "interesting" and it readily generates that pulse mode that has really high Q lashed up like this. More when I know more (and on a fusor thread).
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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