Well, this is not quite vacuum tech, but closely related, so it seems to fit here. When doing fusors, there is a need to have pretty tight control over gas pressure and purity, or you just can't make it go very well, not to mention having too much gas makes things hard on your power supply, and too little makes it go out, and the range of "good" is pretty small in most setups.
Most fusor gas handling schemes so far published count on somehow flowing gas through the system, and trying to balance the input rate with the pump out rate. I did that myself at first, and oh boy is that hard, even with some adjustable throttling on both ends of that equation. It's kind of the same idea as connecting two inputs to a perfect integrator (or maybe a leaky one!) and expecting to be able to balance so perfectly that the output doesn't change. As they say on Slashdot, GoodLuckWithThat. I've heard of one guy doing it with a mass flow controller and some feedback from a pressure gage, but small flow controllers are expensive, and that still wastes a lot of fairly expensive gas.
Now, I'm lucky to have a couple of very tight vacuum systems, with turbo pumps on them. So I've found another way to run hands-free for fairly long periods, and not need anything real expensive or hard to get to do it, so I want to share that here. For this method to work, you need a pretty tight system that's been well out-gassed. Running the fusor a bit and pumping down again gets that done pretty quick if needed. I usually just keep the thing down at near base pressure when not in use, which helps.
I have managed to find a small solenoid valve that can go between my turbo pump and the forepump. I bypass this with a normal valve, so I can just have that operate as normal, but for this, I close the manual valve so the only way for gas to leave the system is through that solenoid valve. ( I should say "BillF managed to find" here). As I have a turbo, it will not do anything evil if I close offf the foreline, just draw more power when there's a lot of gas there. This is easily solved by running it at about 1/4 the nominal speed, enough to be good enough to remove gas at this pressure without drawing a lot of current to spin, and building up only a little pressure in the foreline, so that's all happy.
Here's a picture of that side of things:
Yeah, sloppy, clipleads, battery, junky. It's not broke, so I ain't got 'round to fixin' it purty yet, but you can see the AC adapter sitting there to make it nicer. I had to make an RC snubber for across the solenoid to keep from burning out the button I use to pulse it (actually a microswitch I just slap down on the operating desk for an instant to remove a little slug of gas).
I close the big manual valve about the same time I tell the turbo to start spinning down to "standby" speed, which is set lower than normal for that use. Then I start letting in gas, which helps it get to the desired speed a lot faster than just depending on the (minimal) bearing friction to get there. I usually shut off the gas flow when the chamber pressure is about in the right range for running. What actually happens, is that as the turbo goes slower and slower, is that some of this gas is compressed behind it, and manages to get back through it into the tank when it finally hits the slow speed desired. That's fine in this case, and tends to help purify things, as the compression ratio (at any speed) is far higher for air type gases than for hydrogen (of any isotope) so it's kind of a purification step. To enhance that, I generally run a low power glow discharge while this is happening, to blast all the heavier gases off the tank walls.
Once it's all happy and stable, I then turn on the forepump, and pulse the solenoid till things are just right. In my case, I'm using a Pfeiffer PKR 251 ion/pirani gage that doesn't have enough resolution for doing this all that well, so what I really do to know if I'm "there" is look at the current in the glow discharge I'm running, or more accurately, look at the voltage the current liited supply is at, and go for a voltage I know is right for good fusor operation -- this is a much more sensitive way to get the pressure right than reading a 2 digit gage.
Now I can run for quite a long time before feeling the need to do anything about gas supply, and in fact have run long enough to start seeing reaction product lines on my mass spectrometer!
That's pretty cool. Of course, there are times when you need to let in more gas and repeat this process, so now we go to how I do that on the other side of the equation.
Here's a pic of the source end of things:
The gages are still reading from the fact that I used this Friday. Actually, they'll be reading forever -- this system has NO leaks at all now. The day-glo thing is a handle I made for a tiny needle valve from McMaster, but truly, it's not tiny enough to do the flow through thing anyway, and I now just use it on-off. This is soldered to one end of a 1/8" OD, 1/16" ID piece of copper tubing that goes off to the right, around the back, and is shown in the next picture. At the back of the tank, there is a glass insulating break in this, with pyrex expoxied to both of the Cu tubing's involved. The total volume of this is around .1 cc or so (which is important).
Here's a picture of the other side:
There's a lot more going on in this picture (some not related, but good to know about). For now, the Cu tubing comes into the screen from the upper left of the picture, goes through the glass inuslator to another piece that winds up going in the end of my microwave ion source and on into the tank. At that end there is a 2" (roughly) piece of .005" ID cappilary tubing to slow gas flow, not needed now, but very badly needed when I was trying to do this the old way. Letting the gas in through the ion source theoretically means the pressure is a little higher there and letting that work down to lower tank pressures, but it works well enough without that feature, and I now do have troubles with the gas in the glass tube lighting up from the ion source "pusher" voltage, that I'm going to go to just putting it in the tank elsewhere.
The point of some of this is it's no longer important just how long the needle valve is open. It more or less instantly pressurized the Cu tubing, and that then slowly leaks through the cap tubing into the tank, so the timing for the valve opening isn't real critical -- a second or so open lets in one standard slug of gas, which does take awhile to make its way into the tank, but not so slow it's a bother. If that turned out to be too much gas (usually), then I resort to the pump-out solenoid till it's just so again. Done! Boy is this ever a relief from trying to "fly" this thing by balancing in and outflows -- it will stay good for many hours this way without any attention at all.
Since it's in the picture, I'll describe some of the other stuff you see. The main HV feed to the main grid is inside that screen, and my ballast R is the brown thing you can see peeking out from the PVC pipe. There is a plastic hose entering that pipe from a sleep-apnea air pump to keep the R cool and keep ozone from building up. The screen is there to give any arcs a place to go that isn't me, and you can see the cavity tuning screw for the microwave ion source poking out through it.
The glass with resistors in it behind this is my ballast for the 2nd grid, which sits out in the main tank. While pumping down, I set the supply on that to 12.5 ma current limit, and 40kv, but I stop removing gas when it hits about 20-25 kv in current limit, that's the magic spot. At this pressure (around 1.6-1.9 e-2 millibar read on the gage) the main grid in the 6" sidearm won't light off by itself with 53 kv on it -- Paschen's law! So I use the 2nd grid as an ion source (right now, the wire to the puller electrode in the microwave ion source is broken inside the tank, and it's not doing too much for me). There are for sure "interesting" things one can do with the 2nd grid and not-DC that I've discussed elsewhere and will have a lot more on later....
There are some more words on this here is you want more detail on the operational steps. The second post gives a step by step.