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Thermal imaging of fusor grid / plasma

Posted: Thu Aug 08, 2013 5:35 am
by Andrew Seltzman
I recently bought a microbolometer based thermal camera (160x120, 100mK resolution in 7-14um) and added a ZnSe macro lens(10" fl, 6" away from core center) view port on my fusor, allowing imaging of the inside of the vacuum chamber in the visible and far infrared. Posted are some preliminary pictures of a heated grid (~85F) in the infrared, demonstrating the transparency of the viewport in the 7-14um wavelength. For these pictures the grid cooling system was turned off and all liquid coolant was removed from the grid. Pressure was probably in the mTorr range, and the plasma was on for 10-15sec.

The final plan with the system is to measure the spacial distribution of ion bombardment on the grid surface vs elevation angle along the grid and spatial distribution of plasma density at the center of the grid.

The coolant flow through the grid will maintain the grid surface at a constant temperature, a copper ion collector array will be mounted on the grid surface; 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. Knowing grid surface temperature and collector temperature at different angles, the variation in ion flux to the grid vs position may be determined.

Plasma density distribution will be determined by back-illuminating the plasma with a pulsed CO2 laser genlocked to the thermal camera's vsync pulse in the NTSC video signal. The signal will also trigger an LC pulse forming network to pulse the plasma to high enough density to exceed the density cutoff of the laser beam (if possible) or diffuse the beam as it travels through the plasma density gradient, which will allow some determination of density contours in the plasma.

ZnSe viewport
Viewport mounted
Thermal Imager (L3 2500AS)
Back glass viewport temperature
grid insulator temperature
Liquid cooled grid version 2
CO2 laser
Andrew Seltzman

Re: Thermal imaging of fusor grid / plasma

Posted: Thu Aug 08, 2013 12:22 pm
by Mike Beauford
Now that is a really slick idea you have there. What kind of plasma densities are you expecting in your fusor?

Also, can you please post your results?

Mike Beauford

Re: Thermal imaging of fusor grid / plasma

Posted: Thu Aug 08, 2013 1:30 pm
by Richard Hull
Nice bit of experiment. Keep us posted on this.

Richard Hull

Re: Thermal imaging of fusor grid / plasma

Posted: Thu Aug 08, 2013 10:37 pm
by Andrew Seltzman
Here's some matlab code

%co2 laser plasma cutoff density
clear all

e0=8.85E-12; %permitivity
e=1.6e-19; %electron charge
me=9.11e-31; %electron mass kg
lambda=10.6E-6; %co2 laser wavelength
c=3e8; %speed of light
w=2*pi*f; %angular freq

nc=e0*me*w^2/e^2; %plasma density 1/m^3
disp(['plasma density= ',num2str(nc,3),' m^-3'])
disp(['plasma density= ',num2str(nc/100^3,3),' cm^-3'])

plasma density= 9.96e+024 m^-3
plasma density= 9.96e+018 cm^-3

So pretty high for cutoff(arc/shock tube/z-pinch density region), Probably out of the capabilities of a fusor even being driven by a PFN, unless there is some multiple well formation at the focal point. If a can't see any effects with cutoff, I'll try to observe any refraction effects from the density gradient. If that doesn't work, I'll try to rig up the laser as an interferometer to determine density.


Re: Thermal imaging of fusor grid / plasma

Posted: Fri Aug 09, 2013 2:27 pm
by Mike Beauford
I originally thought when I first looked at your setup you were doing laser interferometry. I'm pretty sure you might be the first on fusor net to actually do it this way. The only one I can think of that might have done it in the past is Carl.

Re: Thermal imaging of fusor grid / plasma

Posted: Wed Aug 14, 2013 4:45 am
by Andrew Seltzman
Another interesting way of determining plasma density is to dope the deuterium plasma with a gas who's optical properties (strongly absorbs / reflects / transmits) at the 10.6um wavelength depend on its ionization state. Anyone know of such a gas(can be a single atomic species or a molecule, perhaps even a thermally evaporated metal atom beam)


Re: Thermal imaging of fusor grid / plasma

Posted: Wed Aug 14, 2013 7:01 am
by Jerry Biehler
You might look at CO2, I know it can be detected by IR, but thats at NIR, not sure what it does at thermal IR ranges.

Re: Thermal imaging of fusor grid / plasma

Posted: Sat May 24, 2014 7:06 am
by Andrew Seltzman
Preliminary tests of thermal imaging of CO2 laser beam, the first step to a spatially resolved measurement of plasma density at the focal point of the fusor.

Laser, controller, and thermal imager
Diverging lens
More pictures of setup: ... ometer.htm

Video of beam pulsing:

Re: Thermal imaging of fusor grid / plasma

Posted: Tue Jul 08, 2014 2:53 am
by Andrew Seltzman
New updates on the laser density probe

Laser faceplate adapter to thorlabs optics cage system
-0.5" FL diverging lens
Thermal imager viewing diverging beam on wall
6x Beam expander
Expanded beam viewed on thermal image plate
Video of parallel beam on wall through 6x beam expander

Cage system for mounting tuning mirror to direct beam through fusor
more images at: ... ometer.htm

Re: Thermal imaging of fusor grid / plasma

Posted: Sat Jul 12, 2014 9:13 am
by Andrew Seltzman
Beam expander magnification increased to 9x for more uniform power distribution (top of gaussian of the beam)
Turning mirror
Mirror mounted on beam expander
Beam projected by turning mirror
Core input lens
Core output lens with mounting bracket