by Doug Coulter » Thu Oct 13, 2011 5:29 pm
The actual problem is a bit worse than that (though the above ideas are very cool!). I need to strike a plasma in a near-vacuum. Initial Z infinite. Then goes fairly low once there are some ions there - say some 10's of milliamps at 50kv (or more). Now that I have ions, they are the load between a central fusor grid, and the grounded shell. Now, I want to be lble to move them around at will -- accelerate them toward the shell, slow and stop them there, bunch them up, then continue bunching them while driving them back away from the shell and towards a central focus. The ions themselves act as a reactive load across the stray C of the grid-shell system as they pass through the grid and out the other side.
This load will vary all over the place within a cycle - and will depend on how good a job of bunching I've been able to do, and how much that gets scattered in interactions at the grid focus in a fusor.
So, not merely swept, but broadband within a single cycle - it won't be anything like a sine wave. I got an arbitrary waveform generator of about the right specs which is essentially a re-purposed VGA card with a computer interface that lets you draw waveforms at will to generate.
I strongly suspect that once such a system is working, the actual demands won't be anywhere near as bad as described above. The problem is, nothing out there can mathematically predict or model very well what this load is going to look like -- the space charge of the ion cloud is comparable to the net charge we put in there "imposing" the driving field, and can even more than cancel it if bunching is done well. The fancy ion-motion simulation programs can't eat this input (so they say, and I believe them). So, until I find out what works, I don't know what it's going to take beyond some fairly broad guesses. That first one is hardest. Might be that once we get it working the first time ever, we find out that a simple waveform with a couple harmonics does it fine - but at this point, we can't even guess at the rep rate or frequency closer than a couple octaves. We can easily guess that with the ions trying to cancel the applied field, the net field will usually be less, and the speeds slower, than what you'd predict using Newton's laws to predict what would work with so few of them their charge doesn't interact much with the other ions.
What little data I have shows speeds as low as the few tens of khz -- but with some risetimes in the tens of ns...broadband, but those modes were "accidental" and kind of "relaxation oscillator-like" and otherwise non-ideal for what we want to achieve here.
What I'm trying to accomplish here is to control a bunch of self-replling ions, letting them spread out in space and time when it doesn't matter, and bring them back together in a focus, tightly focused in space and bunched in time, just when they need to be to interact. Many will scatter out, and defocus in space and time (and velocity), so each cycle gathers them back up again, then repeats bunching them and firing them back through the grid focus.
Think of a triode, inside out, with the plate virtual at the grid center, and playing with transit-time games, with klystron-like time bunching only when absolutely required, just before they hit focus -- ions are hard to cram a bunch of into a little space and keep them that way, so I just don't try for that. Unlike photons, a charged particle beam repels its own contents, and focus can't be maintained over any large time or space (or at least, no one ever has) -- but if you set up the ducks right, you can get it where/when you need it., recover the leftover energy not lost on each pass, and go again at some rep rate. We're just trying to bang some very uncooperative rocks together here.
There's a lot of lore in the fusor world about ion recirculation. Well, actual measurements here show that as very minimal and disorganized at best - not even a good damped oscillation - Q is max of a couple, maybe. You've got the spring(s) and the masses, and a DC applied force. That's not an oscillator (there's no identified gain mechanism, but there is friction), just a fairly oddball tuned circuit. What I want is to drive this thing and get recirculation "with malice aforethought", and I need to be able to reduce the entropy produced by near miss scattering on every pass along the way.
There is nothing currently on the market at all that claims to be able to do this. While outfits like Amplifier Research make amps that are broadband enough into say 50 ohms (or near), there is nothing that can do this at the KV levels into a ??? changing load at all. And a used AR amp at 1/4 the power I need costs more than a new fancy automobile!
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.