Plasma Diagnostics in Fusors

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Mark J Siegel
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Plasma Diagnostics in Fusors

Post by Mark J Siegel »

Hi All,
I've been intermittently lurking for many years and was finally prompted to make an account after reading about Liam's work and Richard & Franks's musings on the mechanisms at play within fusors. Since the new user introduction is closed for 60 days, I went back to school a few years ago on the pandemic grant. After finishing one more class, I will graduate from the University of MI this fall with a degree in engineering physics. This past spring, I took a class on plasma diagnostics techniques. By far, it's the best class I've ever taken, and it's piqued my interest in understanding fusors better, like Liam. The class focused on the fundamentals of diagnostics, DC & RF Langmuir Probes, Spectroscopy, Microwave Interferometry, Schlieren Imaging, etc, in quasi-equilibrium and inductively coupled plasmas.

I'll pick up a rotary vane pump this week, and assuming it works as advertised, I will be up and running with a "demo fusor" shortly thereafter.

I have no idea what diagnostic techniques one could use in the non-equilibrium plasma in a fusor, but the one I can afford is a Langmuir Probe. I don't think the thin-sheath approximations we used in class would apply in a fusor.

There is a ton of information on non-equilibrium diagnostics. Is there a more pointed search term or technique I should focus on? I can get access to paywalled papers through the university as well.

I did ask my professor about working on this in his lab, and he didn't seem interested. It's far outside his area of expertise, ion thrusters. However, if I had some data to show him this fall, he might be more amenable to giving me a corner of one of his labs to work in (and more expensive toys to play with).
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Dennis P Brown
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Re: Plasma Diagnostics in Fusors

Post by Dennis P Brown »

The issue with fusors is they are excited by a physical cathode and common grounding shell anode. This has few if any analogs in real world plasma devices used in fusion energy. So a fusor can be a fun toy if one is interested in neutrons, but I don't see any value added for someone interested in plasma diagnostics related to many high end applications. Frankly, your prof with Ion Thruster's sounds more likely to use devices and methods useful for your degree. If you are interested in energy based fusion then again, fusors have little to essentially nothing to offer from a plasma point of view.

That all said, if you want a toy to do plasma studies and are interested in neutrons (and x-rays ...) then a fusor could offer you a challenge. But not one with much use in your area of study.
Mark J Siegel
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Re: Plasma Diagnostics in Fusors

Post by Mark J Siegel »

It's personal interest with the fusor.

The trend towards small cross vacuum chambers producing high Neutron numbers is somewhat counterintuitive. I'd like to know how the plasma characteristics (temperatures, density, charge, perhaps index of refraction, etc) in a fusor are distributed in space. I don't know that much has been done with that, let alone reproduced by other labs.

Regarding school, I went into school hoping to get a job in the fusion industry. Got disillusioned with fusion, slight shift towards semiconductors (optoelectronics). Recent class reminded me why I went to school in the first place. Fusors are something I can investigate without big funding or being forced to live in Boston or Seattle/Tacoma.

Final lab's inductively Coupled Plasma
https://youtube.com/shorts/mGMK4C6qcLc? ... j3ICR3-gYg
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Dennis P Brown
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Re: Plasma Diagnostics in Fusors

Post by Dennis P Brown »

Well, after getting your pump, a four-way vacuum cross system is a good 'chamber' to consider for fusion; a KF type flange system is good. You will need to look into vacuum components (hoses, connectors and couplings - for gauges, power, and probes) and vacuum measurement devices. This will enable you to get a vacuum system up, running and properly instrumented.

Then a neon sign transformer as a supply for a demo device that will create a nice air plasma. These are fairly safe but read up on what units to buy.

If you decide to do fusion later, then a power supply will be required (must be negative bias, at least provide 200 watts at or above 20 kV.) Remember, this supply is lethal. Then think about getting deuterium gas (a number of methods.) Finally, a neutron detector system.
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Richard Hull
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Re: Plasma Diagnostics in Fusors

Post by Richard Hull »

Musing over fusion in a fusor is a nice place to hang one's hat, but as Dennis notes, it is virtually a fool's errand. The increased yield in fusion in tiny devices is certainly due to mean free path issues being obviated. Fusion under ideal conditions is always a roll of the quantum dice in a "fair game". In a fusor, the dice are loaded to almost come up snack eyes every time. Thus, we gladly pay the billion to one net loss price to throw the dice in a fusor as we have an unlimited source of cash in the game, (Energy from our wall outlet).

I have an old analogy quoted many times here. We are mining for fusion as if going after coal with only a good old GI steel helmet on our heads and getting our coal, (fusion), by running at the coal face using our heads as a battering ram and flaking off a few scraps of coal from the mine's floor for what we accept as a win.

A plasmic bedlam can do fusion and get those hi-value neutrons for experiment. Nothing glamorous, just doing fusion at a tremendous loss, even in smaller, higher yield fusors. One order of magnitude improvement from 10e-9 to 10e-8 makes no real difference in our electric bill. It does boost our fusion and, thereby, neutron count. Like I say, I have a special deal with my power company.......They will keep pouring all the power I need into the fusor as long as I pay for it. (pennies a month, at most!).....A kilowatt in, a microwatt of fusion out.

Doing fusion at home is easy!

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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
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Liam David
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Re: Plasma Diagnostics in Fusors

Post by Liam David »

It sounds like you have a pretty good idea of what's available diagnostics-wise. Plasma diagnostics accessible on the amateur level are Langmuir probes (electron temperature, density, distribution function), UV-Vis-IR spectroscopy (composition, Doppler shift), x-ray spectroscopy (composition, electron temperature, density), and perhaps energy analyzers (electron, ion temperatures, distribution functions).

Microwave interferometry or reflectometry rely on expensive hardware and fusor densities are typically much too low for microwave frequencies. Vibration and other noise becomes very pronounced at higher frequencies. Schlieren imaging would suffer from the same problem: the plasma density is far too low to appreciably change the refractive index at optical frequencies. To see why, consider N=sqrt(1-wp^2/w^2), where N is the refractive index for EM waves in a cold plasma, wp is the plasma frequency, and w is the imaging frequency.

Thomson scattering is often used for electron temperature measurements, but has a terrible efficiency O(e-9) and is not accessible to the amateur. Similar challenges exist for LIF and TALIF.

Returning to the accessible ones, I would certainly start with Langmuir probes merely due to the cost. I agree that the thin sheath approximation does not hold and I would suggest looking into (if you haven't already) matrix and Child-Langmuir sheaths, orbital-motion-limited (OML) theory, and extracting electron distribution functions from probe characteristics.

Standard UV-Vis-IR spectrometers don't give the necessary resolution for Doppler broadening measurements. If you can get your hands on a monochromator, you're golden.

X-ray spectroscopy can in principle be used to estimate the electron temperature and density, as well as plasma purity (Z^2), but you'd likely have a hard time separating the plasma bremsstrahlung from the bremsstrahlung emitted by electron beams impacting the chamber.

I think there's a lot left to explore and explain in fusors. Even though they'll never produce a modicum of power, it's still a worthwhile pursuit and I'm glad that others here are interested in the diagnostics side of things. I'm interested in seeing what you get up to!
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Joe Gayo
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Re: Plasma Diagnostics in Fusors

Post by Joe Gayo »

To amplify Liam's response, check out the prior research by UW (they are now inactive in IEC research) - https://iec.neep.wisc.edu/

Check out https://doi.org/10.1007/978-1-4614-9338-9 for a good summary of work done on IEC devices. More recently the University of Sydney has performed some interesting research (Joseph Khachan, divergent ion motion using doppler spectroscopy and Richard Bowden-Reid did some interesting work on hollow cathodes).

IEC is not a pathway to net energy Fusion, but that's not why we are here. Hopefully, we are all not here to do the same thing over and over again and not look just a little bit further.
Mark J Siegel
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Re: Plasma Diagnostics in Fusors

Post by Mark J Siegel »

Dennis P Brown wrote: Mon May 13, 2024 4:20 pm Well, after getting your pump, a four-way vacuum cross system is a good 'chamber' to consider for fusion; a KF type flange system is good. You will need to look into vacuum components (hoses, connectors and couplings - for gauges, power, and probes) and vacuum measurement devices. This will enable you to get a vacuum system up, running and properly instrumented.

Then a neon sign transformer as a supply for a demo device that will create a nice air plasma. These are fairly safe but read up on what units to buy.

If you decide to do fusion later, then a power supply will be required (must be negative bias, at least provide 200 watts at or above 20 kV.) Remember, this supply is lethal. Then think about getting deuterium gas (a number of methods.) Finally, a neutron detector system.
I have a Spellman -30kV 10ma supply, although I need a cable, spherical cf 6" spherical chamber, with cf 2.75 ports, tc gauge, 2× 30kV isolators, 2× viewports, kf foreline port and hardware. :)

The rotary vane I'm supposed to be getting is a small Edward's 1. Small turbo would be nice, but man are they expensive. Some kind of cathode gauge would be nice, but I'm not sure it's necessary unless the intention is to go deep vacuum and back fill with D2 or other gas.
Mark J Siegel
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Re: Plasma Diagnostics in Fusors

Post by Mark J Siegel »

Joe Gayo wrote: Tue May 14, 2024 10:01 am To amplify Liam's response, check out the prior research by UW (they are now inactive in IEC research) - https://iec.neep.wisc.edu/

Check out https://doi.org/10.1007/978-1-4614-9338-9 for a good summary of work done on IEC devices. More recently the University of Sydney has performed some interesting research (Joseph Khachan, divergent ion motion using doppler spectroscopy and Richard Bowden-Reid did some interesting work on hollow cathodes).

IEC is not a pathway to net energy Fusion, but that's not why we are here. Hopefully, we are all not here to do the same thing over and over again and not look just a little bit further.
Hi Joe, thank you that's exactly what I was looking for. I want to reproduce some of the past experiments as a start.

Richard, Liam, I want to reply when I can hop on a PC rather than phone. Thank you for the replies!
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Dennis P Brown
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Re: Plasma Diagnostics in Fusors

Post by Dennis P Brown »

A turbo isn't needed unless you - like you said - want deep vacuum. Even for a fusor if the two stage pump can reach 1 or 2 microns that's been shown to be fine. Ditto a high vac gauge isn't needed.
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Richard Hull
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Re: Plasma Diagnostics in Fusors

Post by Richard Hull »

First amateur fusion ever done on fusor.net was done by me with only a yellow jacket two stage HVAC pump. Scott Little did the second fusion also with an HVAC pump. I wasted a lot of D2 getting a decent percentage into the fusor, but it worked. Good thing I had a virtual infinite supply of D2 (50 liter tank).

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
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
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Dennis P Brown
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Re: Plasma Diagnostics in Fusors

Post by Dennis P Brown »

The idea behind a deep vacuum for a fusor is simple: one reduces contaminates before adding deuterium gas. This is important if one is doing plasma diagnostics since those contaminates can change the fusor's performance in a non-standard manner. If serious about accurate and repeatable measurements, a high vac system is an important requirement.

As Richard mentions, operating a fusor in either mode - with or w/o a turbo - can require a lot of deuterium gas unless a gate valve is used between the chamber and vac system. Such a control valve via adjustment can vastly limit deuterium flow out of the chamber. This enables holding a deuterium pressure w/o needing to flow a huge volume of that expensive gas.

Ironically, just the contaminate issues in the plasma alone held research in real fusion back for nearly forty years! Not that I'm pointing at those physicist and laughing ...but I am.
Mark J Siegel
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Re: Plasma Diagnostics in Fusors

Post by Mark J Siegel »

Richard Hull wrote: Mon May 13, 2024 5:32 pm Musing over fusion in a fusor is a nice place to hang one's hat, but as Dennis notes, it is virtually a fool's errand. The increased yield in fusion in tiny devices is certainly due to mean free path issues being obviated. Fusion under ideal conditions is always a roll of the quantum dice in a "fair game". In a fusor, the dice are loaded to almost come up snack eyes every time. Thus, we gladly pay the billion to one net loss price to throw the dice in a fusor as we have an unlimited source of cash in the game, (Energy from our wall outlet).

I have an old analogy quoted many times here. We are mining for fusion as if going after coal with only a good old GI steel helmet on our heads and getting our coal, (fusion), by running at the coal face using our heads as a battering ram and flaking off a few scraps of coal from the mine's floor for what we accept as a win.

A plasmic bedlam can do fusion and get those hi-value neutrons for experiment. Nothing glamorous, just doing fusion at a tremendous loss, even in smaller, higher yield fusors. One order of magnitude improvement from 10e-9 to 10e-8 makes no real difference in our electric bill. It does boost our fusion and, thereby, neutron count. Like I say, I have a special deal with my power company.......They will keep pouring all the power I need into the fusor as long as I pay for it. (pennies a month, at most!).....A kilowatt in, a microwatt of fusion out.

Doing fusion at home is easy!

Richard Hull
I'm under no (de)illusion that a fusor won't produce energy, although I was hopeful about Helion...until I wasn't. I haven't trawled through enough of the posts about results yet. If the MFP is the driving factor on fusors' rates, do small crosses and large spherical chambers have similar rates for comparable mean free paths? Or do they both show increases in rates when the MFP approaches the chamber diameter? I suspect there's a Knudsen number that's "best" for fusors and a maximum operating voltage given by the Paschen curve. In larger machines, this is probably limited by the feed-throughs, psu, and cross-section voltage. In smaller machines like vacuum crosses, there could be an optimized voltage/current PSU. I suspect the data to figure this out is already on this forum.


Glow discharge in a tube has regions of light and dark spaces. Does this occur in a fusor in spherical shells?
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Dennis P Brown
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Re: Plasma Diagnostics in Fusors

Post by Dennis P Brown »

In the forum we tend to avoid making block quotes here. Absolutely no real fusion effort (and the investment start ups certainly aren't real efforts) has ever produced net energy. As for the companies taking investor's money, they are mostly a joke.

As for the light and dark spaces in Cook's tubes that has more to do with pressure (tends to cover a large range) and in a fusor, we work at a fairly standard pressure that does not create those types of zones.
Last edited by Dennis P Brown on Wed May 15, 2024 1:22 pm, edited 2 times in total.
Mark J Siegel
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Re: Plasma Diagnostics in Fusors

Post by Mark J Siegel »

Liam David wrote: Mon May 13, 2024 11:17 pm It sounds like you have a pretty good idea of what's available diagnostics-wise. Plasma diagnostics accessible on the amateur level are Langmuir probes (electron temperature, density, distribution function), UV-Vis-IR spectroscopy (composition, Doppler shift), x-ray spectroscopy (composition, electron temperature, density), and perhaps energy analyzers (electron, ion temperatures, distribution functions).

Microwave interferometry or reflectometry rely on expensive hardware and fusor densities are typically much too low for microwave frequencies. Vibration and other noise becomes very pronounced at higher frequencies. Schlieren imaging would suffer from the same problem: the plasma density is far too low to appreciably change the refractive index at optical frequencies. To see why, consider N=sqrt(1-wp^2/w^2), where N is the refractive index for EM waves in a cold plasma, wp is the plasma frequency, and w is the imaging frequency.

Thomson scattering is often used for electron temperature measurements, but has a terrible efficiency O(e-9) and is not accessible to the amateur. Similar challenges exist for LIF and TALIF.

Returning to the accessible ones, I would certainly start with Langmuir probes merely due to the cost. I agree that the thin sheath approximation does not hold and I would suggest looking into (if you haven't already) matrix and Child-Langmuir sheaths, orbital-motion-limited (OML) theory, and extracting electron distribution functions from probe characteristics.

Standard UV-Vis-IR spectrometers don't give the necessary resolution for Doppler broadening measurements. If you can get your hands on a monochromator, you're golden.

X-ray spectroscopy can in principle be used to estimate the electron temperature and density, as well as plasma purity (Z^2), but you'd likely have a hard time separating the plasma bremsstrahlung from the bremsstrahlung emitted by electron beams impacting the chamber.

I think there's a lot left to explore and explain in fusors. Even though they'll never produce a modicum of power, it's still a worthwhile pursuit and I'm glad that others here are interested in the diagnostics side of things. I'm interested in seeing what you get up to!
We covered/did the experiments for Microwave Interferometery, Schlieren, RF (with OML)/DC (with Child Langmuir) Langmuir Probes (did these first, then used them to verify other techniques), Spectroscopy, and Microwave Perturbation. I think the microwave interferometer could be done in a home lab on a shoestring budget. We used a Gunn Diode, Crystal Diodes, an Oscilloscope, and some hardware on the microwave line like an HP X532A 10Ghz Microwave Frequency Meter (on ebay for $95) but I don't know what utility it'd have. Even if a functional system could be built, it just resolves the average parameters.

Unfortunately we didn't cover the UV/IR spectrometry - we just looked at some "mystery" plucker tubes in the visible spectrum.

The techniques that I have used all assumed a quasi-neutral plasma at relatively low temperature. I don't how this compares to what's going on inside a fusor. For example, would I have to bias a Langmuir probe to about the same voltage as the inner grid?

Its pretty hypothetical at the moment, without a probe to bias and much reading to do.
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