Electrostatic fine-control valve?

[chrismb]

Here is my thought-contruction; I take a metal vacuum part with a little volume to it (say, a tee) and I plug one opening with a high temp plastic with a fine hole running through it for about 1". I then fit a fine metal tube, into the hole facing the volume, in one end of that plastic part, but only half-way through it. I then attach electrical connections to that fine metal tube and feed them out, say through a feedthrough made through a blanking piece in one of the other ports of the tee.

Now I hook this tee up to my vac system with the chamber on the side of the plastic only, and the gas supply facing the side with the metal tube sticking in/out a little. I then apply an ionising +ve voltage to the metal tube, with respect to the tee.

What I am trying to do here is to control the flow of gas through the tube by ionising and electrotatically repelling ions away from the tube. Some ions will enter the tube, become neutralised (no e-field within it) pass out of the other end of the tube and on to the vac system. Turn up the voltage, less flow, turn down the voltage, more flow.

What could possibly go wrong!?!?

[johnf]

Chris
There is nothing stopping the neutrals from going through the tube.
Also any negative ions will race to and through it.

Time for an experiment??

[chrismb]

The way I was thinking of it, John, was that it would be a fine needle such that a high electric field would form around the tip. I am then presuming that there would be some optimum pressure (around the Pachen minimum, I guess) where this would work best, ionising neutrals as they made their way to the tip. Only those dead in line would have the likely benefit of flying straight through, the rest would pick up a charge rather than bounce through the tube so easily.

As you say... an experiment seems in order here, given the simplicity of it.

I was half-planning to do this kind of setup anyway - I think my leak valve leaks (but not in the right way!) so I was thinking of alternatives. Here, I would use a hypodermic needle between the gas feed high pressure (and the lower pressure behind a manual valve venting into the chamber) so as to regulate total flow. The extra electrical feed would then be the only addition on top of this plan, so if I go this route then it should be easily implemented to see if it makes a difference.

A little way off yet as I have to get my new chamber setup commissioned.

[Doug Coulter]

I have been looking at a little Pfeiffer leak valve (solenoid operated) and auto fuel injectors for this too. I don't think the thing you propose will work, or at least not unless it's not much pressure on either side of the valve. I may just try to make a solenoid/needle valve from scratch here, actually, we'll see. On-seat leakage is an obvious concern unless almost pressure welding forces are used, which has its own set of issues. It's still in the "I'm pondering the design" phase here. And "how to handle stuff so tiny, as it has to be to get the flow rates desired".

I did make a cool little ion source that would have looked a lot like that. I had gas coming out of the end of a .005" capillary tube, and a hole in a flat plate about .010" away, about a .020" hole.
At that point, the gas was still at fairly high pressure (didn't have time to expand to the tank pressure yet), and just 500v would light it off and ionize it, even at tank pressures in the e-5 mbar range. Looked like a little torch flame in operation, and trying it with N2, you could see physical separation between where the various spectral lines were emitted, as well as shock diamonds in the "flame". It looked about as cool as you could imagine (and probably was quite cold in fact). This was hard to keep working due to the tight spacings and sputtering though, so I went with the other one.

I also got a weird effect with diatomic gases that kinda freaked me out. After running that thing for maybe a minute, with maybe 5 watts in, the whole tank contents would produce a bright flash and the tank would go "tink" as if hit by a hammer -- at e-4 mbar pressures! I quit doing it, as the EMP that produced also killed things nearby, how, I haven't a clue -- wasn't much of a way for it to get out of the tank but it did -- a long chain polymer of N's that collapsed into N2? Not a clue -- It was a little too exciting for me though, and expensive when a PC a few feet away went up in flames. At any rate, I stopped doing things that way.