RF impedance, no plasma

For Farnsworth type designs.

RF impedance, no plasma

Postby Doug Coulter » Thu Sep 18, 2014 3:18 pm

Well, you have to start someplace. We'd already determined that the capacity of the grid and feedthrough is in the range of 20-22 pf (nicely low given how big it all is). We have been playing with various forms of oscillation, and have found that we get a burst of super high output and Q right at the onset of "lighting off" the plasma. The faster we "oscillate" the better, since this effect happens just at the onset. Evidently, something happens right thereafter that ruins whatever is going on to make this higher gain. If one believes Miley/Murali book (ISBN 978-1-4614-9337-2), that something is a virtual cathode. That's one explanation. Accidental spin alignment, just the wrong way, would be another. We may not care if we can get onsets quick enough, and/or stretch out that time when the Q is high before we fall into "dynamic stability" which looks like worst-case for actual fusion, though it's where most fusor people run and of course is very photogenic. We don't care so much about that - we want to move this field forward to net power as the main goal.

Note the book mentioned above is recent and Springer is the publisher. If you have to ask the price, you can't afford it.

According to Miley, there's a mode nicknamed POPS, which stands for "periodically oscillating plasma sphere" which increased the "vritual electrode" lifetime in his experiments (at far different conditions) from .5 millisecond to 2.5 - a nice factor of five.
Extrapolating from his conditions (low voltage, low RF) from his graphs, which look like sqrt of things - we come out wanting around 800v RF at around 7 mHz for our set of conditions. Since this isn't trivial to add on top of a 50kv DC supply, we're working on a system to do that - it doesn't just fall from the sky all perfect, we have to "make it so". As a starting place, I did some work to create an inductor that would resonate around the right (guessed?) frequency, since the one we'd been playing with before, though it did do something good - was way off for this supposed good mode. I'd already measured the capacity (also with no plasma) and so just needed an inductor we could either drive via a primary winding, or via a Pi network (any HAM will know what that is). The inductor I found and modified is moderately low Q, so we wouldn't get much voltage boost from a PI network, but then, we might not need it - all bets are off until I do the far more difficult measurements with plasma (and therefore high power 50kv DC) present. For this to work, there's no question whatever we're coupling to plasma motion, which is kind of it's own tuned circuit (but more complex, like Mathieu shows). But you have to start somewhere, and we are on unknown, or at least unreported, turf here. And it's a daunting search space - I'll have to find the link to a plot of that function I've put up here somewhere else, but it's hairy, and we add a couple of variables in a fusor to that.
At any rate, here are a couple of scope screen caps on and off resonance, driven from ~50 ohm generator, and looking at the feedtrhough (eg, the top of the capacitor to ground).
OnResonance.PNG
On the resonance

And a mhz off or so (you can get all the numbers off the scope screen if you click and expand the picture)
OffResonance.PNG
Off resonance. The Q (in the RF sense) ain't great, but should do.


I made a movie for the ever-increasing number of subscribers to my youtube channel as well, though this will probably go over most of their heads...you never know, one or more of the 2k+ might actually get it.
http://youtu.be/Yozv3PhzalA


I'll get some curves with plasma present under a number of conditions soon. Stay tuned!

Since it's small, here's the Matieu plot for a simple mass spectrometer. A fusor adds some other variables like the speed of the particles, space charge and so on. Note that the whole useful area of this curve is a small spot where the intersection is.
Anything I can do to reduce my search space is therefore worth it!
Mathieu.gif
Nasty turf for peak finding!
Mathieu.gif (19.23 KiB) Viewed 2728 times
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: RF impedance, no plasma

Postby Donovan Ready » Thu Sep 18, 2014 8:37 pm

Very nice, thanks.
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