by Doug Coulter » Sat Mar 05, 2011 8:42 pm
What type of gage is it? Thermocouple?
At any rate, yeah, I don't like handing out bad news to anyone, much less you guys -- I'd rather see you kicking butt totally, frankly, and I think it might be possible. The intent is to make it so you get to fusion with less fooling around, is all that's motivating me, and while I was doing those tests today, saw some benchmark numbers on pressure vs what I saw. Figured I'd just pass that along. I have a guy coming down at some point with pro movie gear to do a documentary on us, and I'll have him do a split-screen off the tank view and our gage during a pumpdown that should actually be some useful data for all -- I don't have the stuff to do that just yet myself.
What you're probably going to find out is that once you have a couple of "touchstone" points, the fusor itself will be your best possible gage anyway -- being able to see what's going on in there is key, whether by camera or mirror (or that really neato chunk of thick lead glass like I have). Due to how steep that side of the Paschen curve is, most gages just aren't going to have enough resolution to show much in the range of interest, which is pretty tiny. For example, in my own fusor, at first the pressure I could run was about 2.2 e-2 mbar. Now with ion sources and all that other jazz, I can get down to 1.4 e-2 -- not exactly an enormous difference, and at say 9e-3, I can't get it to draw any current over microamps (plenty of X rays from field emission, but basically no fusion). At this point, I just use the volts of a current limited supply (that second grid) to tell me when I'm "there" and its far more accurate than any gage I've seen.
All the thermal type gages read way high on H (and isotopes) compared to air, sometimes almost as much as the ratio of atomic weights -- big number -- "air" is normally called Z=29. My opinion is that you don't really care that much as long as it's repeatable so you know when to turn on the power and so on. The hassle is, when you have leaks (I'm still hunting mine) you don't know the composition the gage is seeing, so that tosses another big variable into things. Ion gages (the PKR-251 has both) have their own weirdnesses with different gasses, depending more on how easy they ionize than how thermally conductive they are. Nothing's perfect. The pressures here are too high for a mass spectrometer to work right.
Getting a better pump before you get the leak isn't going to do a ton of good, from my experience. Due to what happens in the transition from viscous to molecular flow, even my super duper 530 l/s turbo can't take on even a ridiculously tiny leak -- a pinhole would be huge compared to what I have, yet it takes me from e-9 mbar to 2e-5 for example after an entire day of pumping down. Well, that doesn't tell the whole story. With no leak I get to e-6 mbar in a couple minutes, e-7 in an hour or so, e-8 overnight, and e-9 in a couple days with some baking here and there.
I'd bet you can find something surplussed at Tech -- we go to the auctions, but you guys get first crack at stuff like that, I believe (the auctions are frequented by resellers who bid everything to 1/2 to 2/3 retail, so we don't buy much at them). If not, well, you know who to call....we have some spare stuff here, some of which you've seen and we didn't pay much for it - we can let it go at cost. But it's not time for that yet. You should be able to get to fusion, albeit kind of lousy, with what you have, and it's more important to fix those leaks than have a killer pump for now.
Once you get there, and have the other issues hammered out, then....a good secondary pump is going to take you real far, but not until then.
Oh, for those of you reading along -- we're both being lazy here -- Lee is talking torr, I'm talking millibars. They're not all that different (760 torr == 1 bar), so it kinda works anyway as long as we don't start using things like microns (of which unit?) and like that. All our gages have more error than the conversion factor, depending on what gas is being measured!
We have a saying in stock trading -- "the ticker is the truth". Same goes here -- what you see in the tank is the truth, the gage is more of a guess or estimate unless you go real crazy and have one of those spinning levitated balls measuring friction or a manometer, both of which don't work real well down in this range anyway.
What you'd like to see is this -- the ability to completely turn off current drain by having a good vacuum. That will happen at not real far below the ideal pressure for a fusor. If you can get your gage down to 1/10 or 1/100 of that, you can start talking about having fairly pure fuel -- and it does matter, we've tested that. Up to about .5% impurity, you get one class of behavior, and better neutron output. Above that level, you get better stability of the fusor, but much less actual fusion. It seems a little impurity helps the thing stay "lit off" -- but makes it draw lots more current too, and a lot of that is wasted in ionizing the various components of air, which can really eat the power and heat things up -- probably because things like O, N, Ar, can be multiply ionized and transfer ionization to the D on collisions. The higher the Z of the impurity, the worse this effect is. They found at JET and ITER that just going from tungsten to carbon "beam stops" they reduced losses tremendously (big integer factor), even though in either case the amount of the high Z stuff that was in the plasma was a fraction of a percent...This isn't the same conditions, but the same effect does seem to occur with a fusor.
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.