Thermal imaging of fusor grid / plasma

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PatrickRothschild
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Thermal imaging of fusor grid / plasma

Post by PatrickRothschild »

Great Information sharing .. I am very happy to read this article .. thanks for giving us go through info. Fantastic post. I appreciate this post. This will consist of an array of small copper segments mounted to the grid with a standoff of known thermal conductivity, the collector to grid temperature difference will them be proportional to the heat flux flowing through the standoff.
John Futter
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Re: Thermal imaging of fusor grid / plasma

Post by John Futter »

Patrick
it is even easier than you make out all you have to know is the current flowing into the grid times the accel voltage = heat input
being in vacuum we are neglecting the neutron production as it is an impossibly small fraction of the power in and we are ignoring the conduction losses through the grid stalk (these are usually stainless a bad heat conductor

q = σ T4 A
q= power input
σ = 5.6703 10-8 Stefan Boltzmans constant
A=radiative area in sq meters
T4 = temp in kelvins raised to the forth power

edit to makeup for room temp T4 is modified to (T1^4 - t2^4) t1 = grid t2 = room temp
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Rich Feldman
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Re: Thermal imaging of fusor grid / plasma

Post by Rich Feldman »

I think you didn't factor in the emissivity of grid metal, which isn't even close to blackbody.
e.g. if e=0.4, absolute T needs to be about 25% higher to match the radiant power density of blackbody.
Let's not talk about what fraction of HV power is deposited on the current-sourcing surfaces, as opposed to the current-sinking inner grid.

Been meaning to follow up on my previous speculation about measuring the spectral emissivity of tungsten.
Discovered a marvelous and authoritative paper by Larrabee, at MIT in 1957.
https://pdfs.semanticscholar.org/06fa/f ... 9ec6c7.pdf

His monochromator and PMT were focused at a spot on a long straight tube of pure tungsten, with 0.125" OD and 0.001" wall thickness. Whose fabrication makes a good story by itself.
Blackbody reference, at nearly the same temperature, is a 0.013" round hole offering a view to the interior.
A detail I didn't think of is a zero-luminance reference, so scattered light reaching the detector can be subtracted out.
That's another 13 mil hole, that goes through both front and back sides of the luminous cylinder. The glass vacuum enclosure and filament placement are carefully designed to minimize reflected light in the all-the-way-through view.
emis.JPG
All models are wrong; some models are useful. -- George Box
John Futter
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Re: Thermal imaging of fusor grid / plasma

Post by John Futter »

Twas only the grid current I was concerned with as it is what heats the grid minus what is lost due to secondary electron emission no i did not factor in Emisivity of Tungsten now we can 0.471 ahead as a constant. Just need a handle of secondaries or suppress them to get an accurate count and you will get an arm wavey answer that is closer than an educated guess
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