Low Cost Spectrometer and Plasma Spectroscopy

This area is for discussions involving any fusion related radiation metrology issues. Neutrons are the key signature of fusion, but other radiations are of interest to the amateur fusioneer as well.
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Nicolas Krause
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Low Cost Spectrometer and Plasma Spectroscopy

Post by Nicolas Krause »

I know there's been lots of impressive sensing work done on the forum before, some members have built their own quadropole mass spectrometers, but I don't recall reading anything about light based plasma spectroscopy. I found this low cost ($750) spectrometer online and started reading the following paper as an introduction to the technique. It says its applicable to low-temperature plasmas and notes that its a great technique for identifying species in the plasma itself. I'm curious if it would be possible to use this device as proof positive of fusion by detecting Helium, rather than using the more traditional neutron based methods which are well understood. Any comments would be much appreciated!
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Richard Hull
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Re: Low Cost Spectrometer and Plasma Spectroscopy

Post by Richard Hull »

Very nice thought! I have told the story many times here when I carried my fusor II to northern Virginia to the lab of Dr. Robert Bussard. I was there due to Tom Ligon, Bussard's lab rat and engineering technician. They wanted to calibrate their brand new $2500 Ocean Optics spectrometer. It was ordered with a special slit and narrow bandwidth for specific testing of the H beta line. Their fusor was so big that the plasma did not give enough light to work their optical fiber interface. (two foot from conflat window. My fusor plasma was 3" from its viewing surface. They were specifically looking at high resolution doppler broadening used to detect ion velocities. It worked great and Doc Bussard, Tom and I worked on it and the setup for several hours. It is to be remembered we did no fusion in fusor II! We just pumped in D2 made it glow at about 8kv and looked at the single overwhelming D2 line..... I got a steak diner out of the deal, too.

I would think that an optical spectrometer tuned to one of the helium lines would do two things.

1. Identify that there was a helium 3 signature, proving fusion.
2. Tell of the species velocity due to doppler broadening.

I would identify a strong helium 3 line that is within the visible range. (anything in the deep UV might demand a quartz window view port with associated losses or complete failure to transmit UV within the optical fiber). It is most important to talk to the engineers at the company who make and sell the spectrometer. Tell them what you are doing and advise them of the specific target line you would like to home in on.

This entire effort might fall on its face if there are not enough 3He nuclei produced, At 1 million neutrons/sec, you would have no more than 1 million atoms of 3He among quadrillions of D2, D, H, O, plasma ions producing the bulk of the light! Most all fusion produced H, 2H, 3He and neutrons are at energies that 30kv or 50kv will not turn or circulate and are on a one way trip towards the wall the moment they are produced. Finally, you are producing ultra high speed ions during fusion. They will not produce light until they grab an electron that neutralize them! Will these fusion ions even go neutral before they slam into the wall of the fusor?? Many flies might lie within this ointment. RGA's are normally used as they do not rely on light and tiny fractional identities of gas species, (fusion debris), usually demand an optional electron multiplier be used with the RGA. We are back to I don't know. Too many unknowns across the board, but nice thought for sure.

I had no idea that such spectrometers had gone down so much in price. I know the slit width was critical in the case of the H beta line when we monkeyed with the Ocean Optics back in the late 1990's. Back then, the spectrometer was $2000 but the fine tuning at the factory and special slit bumped the price up to $2500.

Richard Hull
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Fusion is the energy of the future....and it always will be
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jerry Biehler
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Re: Low Cost Spectrometer and Plasma Spectroscopy

Post by Jerry Biehler »

I have an assortment of different spectrometers. I do have one of the ocean optics ones that connects to a PCI card, another little serial based one acton spectrapro 150 and the big one I just got working again, a Optronic Laboratories 740A/D that I just got running https://hackaday.io/project/175985-optr ... ectrometer

The CCD ones are nice when you want to see a whole spectrum. But the problem is you have kind of awful dynamic range and for weak lines like Richard was talking about you really need the cooled version. Otherwise the signal gets lost in the noise on a room temp CCD because you have to use such long exposures to see anything. You see setups like this used for endpoint detection in semiconductor manufacturing, they look at the plasma with sensors and detect when certain lines show up which indicates the process has finished.

Another way to do this and if you have a specific wavelength you are looking for us used a PMT coupled to a band pass filter of the line of interest. Verity makes a lot of process endpoint detection stuff that could prove useful.

Another option is use a monochromator or scanning monochromator to scan the emitted light from the chamber and give you a chart. That's basically how my OL740 work as well as the Acton. I also have one of these verity monochromators which are also used for endpoint detection. https://www.ebay.com/itm/5350-Verity-In ... SwR4FexZam

These have a micrometer that is calibrated to readout in the wavelength of the selection of the gratings. Internally there is a PMT for measuring plus the PMT amp and high voltage power supply. Feed it +/-15v and you get an analog signal of the intensity of whatever line you have it set at. These are very sensitive. They make a version that has a stepper motor built on as well and then you can just set it up to scan over the spectrum. Its not fast but for stuff like looking at plasma it really does not need to be.

Motorized version: https://www.ebay.com/itm/Verity-EP200Ms ... SwsQJfEEWf

Shahriar over at The Signal Path turned a manual one into a scanning one. https://youtu.be/veETVeEsaNM

You might want to keep an eye out for Acton Research units too.

So the USB units are great for when you want to see the whole spectrum simultaneously and working with stuff like characterizing light sources. But they are not so great for low light like that you will see in a fusor.
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