plasma diagnostics

[hellblazer]

Have ordered two books on the subject, but I'd thought I'd throw the question out.

A langmuir probe, if I'm not mistaken, will be vaporized in the plasma we're playing around with, so that leaves an interesting question as to how to measure core densities.

Garret, did you build your laser interferometer, or did you obtain it otherwise. What about microwave interferometry or faraday rotation (sounds really delicate).

Anyways, looking for some ideas as I await my books on the subject.

[LehighUBoy]

Hal,

I've used langmuir probes, laser interferometers, optical emission diagnostics extensively with the torsatron experiment.

Using a stepper motor and langmuir probe mounted to a linear motion feedthrough (bellows) one could rapidly transverse the plasma core with minimal damage (designed and built similar setup for the Magnetic Nozzle Experiment at PPPL several years ago).

The laser interferometer system I employed was salvaged from a device at PPPL and modified to operate with my torsatron, thus my knowledge lies more on application side rather than design.

For optical emission a host of possibilities exist including:
· http://wwwrsphysse.anu.edu.au/~smc112/r ... /paper.pdf
· Helium line relative intensity ratio comparison
The optical diagnostics were performed with a donated ocean optics spectrometer, however my fusor will use a gated CCD spectrometer acquired from a fellow researcher at Tokyo University (unable to resist another plug – fusion is fun in groups!).

-garrett

[hellblazer]

From my limited understanding, the langmuir probe is useful for only low temperature, low density plasmas. 10 KeV would vaporize it, wouldn't it? I'll think about this a bit. I think at 40 KeV it would be pretty vaporized unless the stepping was pretty darn quick... (don't know, just guessing)

Very, very cool paper. Many, many thanks.

As to the plug, I whole heartedly agree. Rock on.

[LehighUBoy]

The nature of a langmuir probe is to establish a equilibrium of ion and electron flow for a given electric field, therefore surface interaction is partially controlled with driving circuitry. Secondly, density is a crucial factor and extends the lifetime of materials. I have experience with langmuir probes designed to handle 10 Megawatts of auxiliary heating in tokamak diverter systems, and I'm fairly confident that the current fusor arrangement isn't unreachable. The questions of diagnostics really end in what's practical and accurate instead of what is possible.

-garrett

[hellblazer]

Aha. I see.... Many thanks! Again, did you build yours (and the associated circuitry) or was this acquired as well.

I know the principle of the probe is simple, but that doesn't always translate into engineering simplicity.

[hellblazer]

Well, spent last night looking for info on the net for this. Will likely have to wait until my books arrive.

So, to compensate for the temperature, does one just raise the bias of the sweep voltage in the probe? This could get very ugly if that is the case.

[LehighUBoy]

http://wwwsoc.nii.ac.jp/aesj/division/f ... hikawa.pdf - A paper that briefly mentions laser induced florescence (LIF), a diagnostic I previously mentioned.

I've engineered several langmuir probes myself including: planar, triple and single probe. As stated before the diagnostic may be possible, however not practical or accurate. After considering the approach for fusors, my recommendation is attempt a related concept called retarding grid energy analyzer (RGEA).

Furthermore, for density and voltage distribution measurement, a laser interferometer and optical emission diagnostic (i.e. LIF) probably will yield acceptable results.

-garrett

[hellblazer]

Many thanks for another wonderful paper.

I'm looking into the methods you suggested, ordering by difficulty and expense.

Again, thanks for the help here. I certainly do appreciate the time and suggestions in my battle with ignorance.

[Richard Hull]

No fusor yet operated could vaporize a langmuir probe. While the plasma may be hot by electron temp standards, it is ice cold based on density which is our normal sensory concept of hot and cold.

The problem with the Langmuir probe is the distortion in the plasma in a purely spherical system that it would introduce by its very presence. It is not like jamming it in a long continuous race track load o' plasma and taking readings.

Most everyone doing this uses laser interferometry. Most have returned densities in the center of 10-30X the inter-grid density, from what I have heard or been told.

Richard Hull

[LehighUBoy]

Retarding Grid: http://wwwrsphysse.anu.edu.au/prl/probht.html

-garrett

[hellblazer]

Thanks. I think I was taken to caution by reports of vaporizing diamonds in these beasts.

I've just received one of my plamsma diagnostics books: "Principles of Plasma Diagnostics" by I.H. Hutchinson. (http://www.amazon.com/exec/obidos/ASIN/ ... 76-8118224)

It's a great book, chock full of extremely useful stuff. Clearing up a lot of questions I have.

[guest]

"mil" is one 1000th of an inch (.001 in.)

-garrett

[hellblazer]

Many thanks.