The color of IEC plasma

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jpnms
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The color of IEC plasma

Post by jpnms »

In DEMO fusors, the color of glow or jet mode plasma is bluish white in fully low pressure but when the pressure is comparatively high , it turns into purple.

What do you think the reason is?
Jack Puntawong
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Re: The color of IEC plasma

Post by Jack Puntawong »

I can think of two possible explanation:

1. Less gas molecule, less photon emitted. So, from the perspective of the human eye, the gas might look bluish white.

2. Less pressure, greater mean-free-path, higher energy in each molecule. The energy level of the gas molecule shifted from purple, to UV or X-ray. So there's even less visible light emitted.

From my experience, I couldn't really see any visible plasma after 15kV. The bluish light you were talking about might be from the thermionic emission from the grid though.

Jack Puntawong
NicKolas Garrett
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Re: The color of IEC plasma

Post by NicKolas Garrett »

This seems like a very plausible answer :-) especially about the higher degree of average electron excitement.
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Richard Hull
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Re: The color of IEC plasma

Post by Richard Hull »

The color produced is a combination of spectral frequencies based on the dominant species of gas that has been excited and is produced as the atoms recombine. The light is generated as the excited atoms recombine. Energy is absorbed to make the molecule lose an electron and is emitted as light as free electrons recombine with the ion.

For pure deuterium, the dominant line/color is red. Reading a bit about ionized gases would explain much of this.

I find it difficult to believe folks are attempting to do fusion and refuse to do the simplest of searches on the basics of plasma and ionization. This is a constant pet peeve here with those who want to help those who will not try and help themselves. Carl Willis has complained about this to the point of just giving up on many newbies.

http://en.wikipedia.org/wiki/Ionized-air_glow

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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Andrew Robinson
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Re: The color of IEC plasma

Post by Andrew Robinson »

Are you really all that surprised though Richard? That's the nature of today's world. It seems that in our highly connected world with all this information at our finger tips, people are getting lazier and lazier. Seems no one is willing to work for anything anymore.
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Adam Szendrey
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Re: The color of IEC plasma

Post by Adam Szendrey »

Although I generally agree that these days people ask before they look all too often, in this case the question was not "what causes the glow" and why it has one specific color. The question was about the change of the emission spectra as pressure drops. The color changes dramatically from the well known blueish purple to very pale blue. The answer (even if it can be deduced with a bit more research) is a bit less trivial than the classic glow when air is excited at a fixed pressure in a certain range. The process behind the glow it self at any given state is the same of course.

A "colder" more bluish color indicates that the average photon energy is higher. This is most probably due to the increase of the mean free path of particles in the chamber as pressure drops. Particles can travel a greater distance between collisions giving them more time to accelerate ,thus they can gain more energy which results in more energetic collisions, higher excitation, and that means higher photon energies when recombination occurs.
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Re: The color of IEC plasma

Post by John Futter »

I will chime in here

as the pressure decreasses the colour should get weaker not change colour
most all amateur efforts are coloured by contaminents/ leakage in this case nitrogen (blue) oxygen whitish blue and hydrogen (pink scarlet)and not counting diff pump products etc
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Adam Szendrey
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Re: The color of IEC plasma

Post by Adam Szendrey »

There is a very definite color change actually. The glow also gets weaker but the general increase in apparent color temperature is quite observable. I see this with each pumpdown. Your current (from your HV supply) also changes as pressure drops. First it increases then decreases and this will also have an effect on plasma temperature (because most demo power supplies are not stabilized, their voltage varies as their current changes).

Leaks and constant contaminants don't explain the pressure dependence of the color in my opinion. They ( contaminants) explain a general shift in the spectra, regardless of pressure. If there is a leak I don't think gases leak in "separately" (more of some gasses less of others, changing the gas ratios of air) unless it's an extremely small leak, so that shouldn't change the color if the color is constant regardless of pressure, as you say (there is already air in the chamber, so if air leaks in, gas ratios don't change, we are talking about demo fusors after all). Although pump rates CAN be different for the various gases in air, which can result in a spectra change based on gas ratio changes. I don't remember in what vacuum range this becomes detectable (with our eyes as a glow color shift). Not even sure this affects all pump types. Turbos do have different pump rates for different gases for example. There can be variable "contaminants" during pumpdown I guess, but I kind of feel they are not to blame for the color shift. For demo purposes I don't use a diffusion pump, and the line from the rotary pump is relatively long. I'm sure the rubber ring under the chamber cylinder outgases a bit but we are not talking about deep vacuum here...we are in the micron range.

Take a look at my pumpdown / vent videos. There is a very obvious color change (of course pretty much all photos/videos of fusors and generally "plasma glow" show false colors). The colors are not far from what I observed with my naked eye tho, and it shows the shift very well.

https://www.youtube.com/watch?v=gZzKyN0iee0
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Richard Hull
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Re: The color of IEC plasma

Post by Richard Hull »

Shifts in color or any color change in an ionized atmosphere tells that either the main gas enviroment dominant species is or has been changing or that more energy is being dumped in. What color is perceived is rather unimportant to fusion or even demo systems, though the hydrogen line is typically red or with contaminants pinkish when fusion is going on.

With the many brands of mechanical pump oils, diff pump oils, soluables in the oil that have accumulated over time, leaks, water vapor, etc., all leaping in and out of the picture throughout a pump down, a whole gang of colors and tints are possible. As mechanical pumps heat and boilers begin to work, all manner of volatiles cook off.

It is highly likely no two demo of fusison systems, as they glow clean, will absolutely match perfectly in what is seen, though generalizations can and have been made. Always, though, much ado about nothing as before admitting the real fusion goodie, you better not have any color at any voltage, just a pitch black chamber.

For demo systems, what you see is what you have and what you've got, regardless of color. You have a plasma composed of an ionized misch-mash of whatever gaseous species exist in your system.

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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Adam Szendrey
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Re: The color of IEC plasma

Post by Adam Szendrey »

Just out of curiosity it'd be interesting to see how the glow behaves if you went a system with dry nitrogen (hence you won't have water vapor or various gases in there, or to be exact, much less), to near atmosphere, without opening the chamber, then pumping down again . I observed that the glow color and the change in color during the pumpdown doesn't change significantly between a cold forepump and a hot one. Of course as Richard said this isn't really related to fusion, but in my opinion it's an interesting investigation into the dynamics of a vacuum/plasma environment.
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Re: The color of IEC plasma

Post by Dan Tibbets »

I agree that the glow discharge seen in a fusor is primarily due to spetroscopically defined colors. The glow is essentially the same as a neon or flourescent light. Depending on the fill gas- neon, argon, helium, etc. the color will be different. The visible light comes dominately from electrons dropping from excited orbits to lower or ground state orbits. Hydeogen alpha and beta wavelength lights are obvous choices with a hydrogen gas. Beta is deep red and alpha is yellow(?) Depending on the aviable energy , dominate state of most of the hydrogen ions, atoms and moleculaes there may be some percieved color gradation in the color as the contribution of these two basic hydrogen spectral lines vary in intensity. Molecular spectral lines tend to be more in the infra red region, and the most energetic atomic spectral lines tend to be more into the ultraviolet. Once fully ionized , I do not thinkyou see spectral lines(?). Just black body radiation based on temperature (and braking radiation which tends to be in the X-ray region). Because the gas/ plasma is rarified, you do not have any significant perceptible black body radiation. I think it would be completly washed out by the spectral emmisions.

This wordy ramble is concentrating mostly on only a pure hydrogen gas. Once other gasses and contaminates are added inthe picture becomes even more complex. As the vacuum drops the gas in the chamber can indeed vary. Mostly from gasses and solids incorperated in the walls and wires. The plasma/electrons and just normal outgassing will introduce copper, iron, nickel, sodium, water, nitrogen, oxygen, carbon,argon, ete., etc. in varing amounts. These all have their spectroscopic emmision lines that can contribute to the mix that gives a final percieved color.

A few years ago, there was a claim that a yellow-green spectral line detected by a contributor was due to a helium spectral line and thus a indicator of fusion. Discussion reviled though that any such helium was way too tine to be detected. The line was probably from sodium line emission derived from sodium sputtered off of the glass by the electron beams/ plasma bombardment. This illistrates that the net perceived color is a complex mixture of various element spectral lines, and this can change as the chamber is pumped down and plasma cleaned. The pressure changes- but so does the gas composition. In chambers that have been conditioned by prolonged pump down and plasma cleaning, the color may most closely approach a pure emmision line color dominance of say hydrogen alpha and beta combined ( I assume dominantly hydrogen beta if a deep red color). It should be noted that the best fusion comes from these conditioned machines which also implies the most pure fill gas conditions.

Dan Tibbets
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