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Old 05-15-2013, 12:41 PM #1
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Default Measuring the temperature of plasma using laser ?

Hi all,

I am a high school student who is working on an inertial electrostatic confinement fusion reactor for the past 2 years. (The attached photo is the picture of the device). Not long ago I have read that the Joint European Torus and many other fusion reactor uses laser to read of the temperature of the plasma in their chamber by measuring the difference of wavelength that enters and exit the plasma. I wonder whether is possible for an amateur scientist to create such apparatus. Any reading material or text will be very much appreciated !

Jack Puntawong



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Old 05-15-2013, 08:32 PM #2
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Default Re: Measuring the temperature of plasma using laser ?

We could tell you all about the light shining into the plasma, but the light coming back, well that's a horse of a different color.
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Old 05-16-2013, 12:22 AM #3
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Default Re: Measuring the temperature of plasma using laser ?

Jack,

I'm away from home and my books are there as well.

Do you know the name of the effect this lambda shift has?

I'm think it could be from Rayleigh scattering or from Thompson scattering..

Both rely on the density of the plasmas, and require VERY complex optical circuits to be able to process the light. After that, you'd need a lab grade interferometer and maybe a spectrometer.

If I want to make a laser induced plasma temperature measurement, I have to produce a series of shots in the spectrometer over which a good piece of graphing software can integrate through to get electron temperature.

So... basically, I capture the direct plasma emissions, send them through a lab grade spectrometer, and let the software calculate temperature after I get a clean chunk of data points.

That's how I do it. Get me the name of this lambda-shifting effect you describe and I'll see if I can find any papers for you. I'll be back on campus in a couple weeks, so I could freely download them there for 'educational use'.

My initial guess though, is that any shift in wavelength that could possibly occur in a hot plasma, might be very slight. Remember its hard to interact with light - its fast and self inducing. IF an amateur could produce a shift, he might need a spectrometer that can measure with hundreths or thousandths of a nm in precision. Just think triple gratings, a dozen doublet lenses and a fiber. Not fun.

Just a guess.

But I still recommend finding a way to capture plasma emission directly. If you know the chemical composition inside the chamber at any time, it COULD make plasma measurements much easier for you this way. Not simple - but easier than Rayleigh or Thompsons would offer.

BTW are you using a B field for an "exhaust" system?
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Old 05-16-2013, 10:38 AM #4
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Default Re: Measuring the temperature of plasma using laser ?

Hello Fiat Lux,

Yes, I believe it was a Thompson Scattering setup as you have mentioned. One of the fusor.net have attempted to create this setup but has also failed. This is his link: http://www.rtftechnologies.org/physi...rk3-optics.htm

Honestly, I had no idea that the Thompson Scattering setup is going to be very COMPLEX and expensive. I'm glad that I have ask this community for advice before jumping into the unknown. However, I really would like some reading material on this just for fun. I really have no access to public library or university library because I live in Thailand Looks like I'll have to look for another way to measure the plasma temperature.

Inside the reactor chamber, I'm planning to use air for plasma demonstration and deuterium for neutron product. Also, there is really no "exhaust" in this system because it is base of an electrostatic confinement principle which use high voltage electricity to create a "potential well" inside the reactor. Imagine the deuteron circulating inside the chamber for a thousand times until it fuses with another deuteron to create He-3 and neutron. This is some of the picture from a member of fusor.net

http://www.rtftechnologies.org/physi...-test-runs.htm

As for plasma density and temperature measurement, I have heard of a Langmiur probe that measure the plasma directly. Though, I have no paper or theory behind the device to construct one. If you could provide me with some text for "educational use", that would be awesome!

Thanks a lot.

Jack Puntawong
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Old 05-16-2013, 12:13 PM #5
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Default Re: Measuring the temperature of plasma using laser ?

Jack,

Let me do some hunting around on te web. I'll PM you a message with my email, and you can send me an email from there. I'll be able to send anything I've found. Though it might be a couple weeks till I have access myself!

If you find a paper you would like to read, let me know. My university might have access to the journal.
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Old 05-17-2013, 06:47 AM #6
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Default Re: Measuring the temperature of plasma using laser ?

Fiat lux,

Thanks for the support! However, do you have any text (possibly an undergrad level) on optics and laser design you can recommend ? I want to look more deeply into this field because I'm really interest in the principle of laser amplification. This subject is quite to find on the internet. I don't really have a book that focuses on laser and optics alone so anything you recommend is great! I'll keep you guys update on this project. After I have finish building my power supply, I'll share some plasma pictures with you guys.

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