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Two Stream Instability: Explaination of the fusor's electron beam

Posted: Wed Jan 05, 2011 8:05 pm
by bk8509a
Last year I was able to make a fusor for my senior project for a completion of a physics degree. I also constructed a triple Langmuir probe, which was able to make near instantaneous and repeatable measurements of electron temperature. The process is semi-documented here: viewtopic.php?f=6&t=3022#p12532

My favorite measurements are measurements I got when I placed the probe into the well known 'electron beam' which emanates from the center of the fusor during jet mode. I got excellent data, which I didn't understand at the time. This data is attached.

As you can see, the triple probe recorded a distribution of temperatures in the fusor, which can be placed into a histogram plot, which is very similar to a velocity distribution (temperature instead of velocity, both are basically the same). I didn't understand why my distribution had two bumps until the last week of my graduate plasma physics class this semester while discussing Landau Damping. I have a much better idea right now, one which confirms the theory that this is an electron beam and it contains HOT electrons.

Its called the two stream instability. (Please don't can me for wikipedia, this is located in Chen and I believe is mentioned in Gurnett as well. Its really easy to show things to everyone on wiki.)

'In the case of the two stream instability, when an electron stream is injected to the plasma, the particle's velocity distribution function has a "bump" on its "tail"'

In my histogram I constantly had the tiny bump on the end in all my measurements. I'm very excited about figuring out what it was and I also think that it is a nice addition to everyone's fusor knowledge base. I'm interested in hearing your comments and criticisms (Is Doug still around??)

I also feel that this makes my triple Langmuir probe all worth it; I feel that it really proves its ability to take measurements in the fusor. More of you should make this probe and see if you can confirm my data. Its really easy.

I'll be checking in for a few days, although i'm afraid i've dropped off this forum due to my project's completion.

Re: Two Stream Instability: Explaination of the fusor's electron beam

Posted: Wed Jan 05, 2011 10:59 pm
by Chris Bradley
It seems plausible, on an initial inspection.

My few comments would be; a) bi-modal distributions aren't always bump-on-tail instabilities(!), so perhaps best to be realistic and recognise this as a potential interpretation rather than 'confirmation of the theory', b) as I have seen such descriptions, the bump should be narrower than the bulk velocity distribution, but there again a fusor may have its own peculiarities (high neutral population and low ionisation populations out of the beams) and I am only an 'amateur' reader on such matter so won't pretend to understand the intricacies of plasma waves, c) if your probes are actually measuring the indicated values (electrons of 4eV and 8eV) then they seem *very* low energy electrons, if they are beam electrons. What do you think these measured electron energies relate to, in physical terms, if they are not electrons accelerated from the vicinity of the grid (which'd have much higher energies, presumably)?

Re: Two Stream Instability: Explaination of the fusor's electron beam

Posted: Thu Jan 06, 2011 12:57 am
by bk8509a
Good call on the low energy electrons--they are! Your numbers are off by a factor of 10 though (4 eV and 8 eV wouldn't even Ionize a gas!). So I have a range of 40 eV to 80 eV electrons. The temperatures indicated are produced at a voltage of ~-1000V and a pressure of 165 mTorr. They probably looked like picture one. The jet varies a lot as you've probably encountered. I have data from higher, concentrated jets, that I need to get to. Temperatures of these high jets corresponded to around 10^8 K or about 1/3 of the operating voltage of -30000 volts

It is only a theory, but the data matches and makes sense. I need to read about it in more depth.