Fusor Forums

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

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

Nice bit of experiment. Keep us posted on this.

Richard Hull

Here's some matlab code

%co2 laser plasma cutoff density
clear all
clc

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
f=c/lambda;
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.

Andrew

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.

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)

Andrew

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.

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:
http://www.rtftechnologies.org/physics/ ... ometer.htm

Video of beam pulsing:
http://www.youtube.com/watch?v=SxVddPWnAQs

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
http://youtu.be/gki9ab-8_7A

Cage system for mounting tuning mirror to direct beam through fusor more images at:
http://www.rtftechnologies.org/physics/ ... ometer.htm

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

Beam expander assembly to reduce beam to size of optical detector
Test setup, soldering iron as heat source collimated through germanium lens Shadow viewed on detector

Direct imaging of expanded 10.6um beam from CO2 laser with thermal imager.

Synrad H48-1 with a 9x beam expander that has an internal adjustable iris to prevent reflection of the over expanded beam off the internal walls of the lens tube that cause a halo around the primary beam. Imaged with a thermal eye 2500AS

Before iris was added After iris was added Video of adjusting iris
http://youtu.be/5j9_Yv9_eOM

Germanium rear coupler mirror 99.5% reflective, 0.5% transmissive, used as a 200X power attenuator Coupler mounted on front of thermal imager beam expander Laser setup Laser modulation profile, 6% duty cycle, 1khz square wave, for further reduction in laser power Bench test imaging of shadow in beam cast by soldering iron tip Captured image of shadow Video of moving metal tip through imaging beam
http://youtu.be/x_UALp30oOA

Imaging setup running with fusor with no plasma.

Laser setup with fusor Beam aligned with core input window Thermal imager viewing beam at output window Shadow from central grid Shadow from central grid Videos of viewing the grid while repositioning the thermal imager
http://youtu.be/ZD-UnhpNOC4
http://youtu.be/N12B16Lp1gw

More pictures at
http://www.rtftechnologies.org/physics/ ... ometer.htm

Preliminary testing of the laser system with plasma.


Setup of fusor with quad ion injector system, vacuum system, and CO2 laser probe Quad injector power supply, 5kV, 3ma per channel for anode layer ion sources Ion injector power cables Starmode with ion injectors Grid with coolant flow off grid with coolant flow on Laser density probe of plasma Overall the laser probe observed nothing, which was expected at this point since the grid power supply is very under powered (5ma max) and can not bring the plasma density above cutoff at 10.6um. A capacitive pulsed system for the grid, and a interferometer setup for the laser are in progress.

The ion injectors work perfectly, however since they can flood the the entire chamber with plasma, they tend to over power the grid power supply when even one is running at maximum output. One interesting observation, by varying the ion injector power individually, the center of the star can be pushed around the inside of the center grid.

Andrew, This is some great data collection effort on your part. I am sure most everyone follows this with great interest. As this progresses and when you feel you might be able to issue a final summary report of your work, please do so. Some of your data already given is most intriguing.

Richard Hull

Plasma at 5mTorr Videos of thermal imaging fusor interior, some at 320x240 resolution, turning on and off grid coolant flow
http://www.youtube.com/watch?v=8suOQDY9668
http://www.youtube.com/watch?v=lxMEJhGmrmA
http://www.youtube.com/watch?v=UUihlkrJeQw
http://www.youtube.com/watch?v=8DfPozHL37A

Interferometer beam splitter mount Grid viewed through interferometer beam splitter Interferometer reference beam arm Interferometer reference beam arm Turning mirrors All updates so far
http://www.rtftechnologies.org/physics/ ... ometer.htm

Construction of the grid mounted ion collector to measure the distribution of ions hitting the grid surface based on temperature rise of each segment is complete.

Copper disk with one segment machined Machining down center thickness to 0.2mm Machining ion collector
https://www.youtube.com/watch?v=ZUyIb7z ... e=youtu.be

Ion collector segments Ion collector mounted on grid Ion collector mounted on grid Ion collector mounted on grid, grid installed in reactor

Report on findings so far

Impeccable work Andrew. I am following your efforts with great interest.

A 1 inch (25.4 mm) Zinc Selenide window costs ~$300 USD, and the thermal camera comes up around $500.

Is there a way to do thermal imaging of the fusor grid (to determine plasma/grid temperatures) for under $300 - $400?

Or maybe someone can point me in a different direction.
We are aiming to vary the grid geometry in the fusor, and take measurements to compare the effects of the different geometries. Aside from input voltage, current, and chamber pressure, if possible a measurement of the plasma temperature would be great if for a reasonable price.

You can likely get away with a smaller window.

Hi Ben,

Seek thermal makes a usb thermal camera for a little over $100
https://www.ebay.com/itm/Seek-Thermal-C ... :rk:2:pf:0

For the window look for ZnSe laser cutter lenses on ebay, you can get them pretty cheap. II-VI also makes 2" diameter lenses for laser cutters; these can easily be mounted in a rotatable 2.75" CF ring:
https://www.ebay.com/itm/II-VI-Infrared ... rk:43:pf:0

A measurement of the plasma temperature would require a langmuir probe of thomson scattering system.

That gives me some direction (and makes me grateful for Ebay once again).

With the ZnSe glass on the viewport, if I attach the thermal camera on the outside, I should be able to see the inner grid? I'm curious how the idea to use ZnSe glass came about. Is it reducing the luminosity of the light, or changing the wavelength?

Thanks for the help.

The Zinc Selenide (ZnSe) lens will replace the glass viewport and seal to the conflat flange with a viton o-ring. ZnSe is transparent in the IR wavelengths, so it will let the thermal camera see into the fusor.

You can also use a germanium viewport like I am currently using on the latest upgrade:
viewtopic.php?f=6&t=10294&start=130

Kind of late to this party.
It's my understanding that thermal-infrared cameras and exotic optical materials, like Andrew has been talking about, are for target temperatures in the sub-red-hot range. Like the narrow liquid-cooled tubes in Andrew's grid.

Ben, have you noticed that many fusioneers use grid wire made from refractory metals, like tungsten or molybdenum?
That's 'cause they have enough high voltage power that a stainless steel grid would melt. Implies operating temperatures so incandescent that you'd need a pretty dark filter to safely look at with your eyes. Luminous enough to easily get the brightness temperature with ordinary visible-light cameras and optical materials.

You could make a science-fairish optical pyrometer, by arranging for camera to simultaneously view the fusor grid and a reference piece of the same kind of wire. Little mirrors might be useful. Reference wire is heated with ordinary electric current to make its brightness, thus its temperature, match the incandescent grid. The rocket science in imaging thermometers is their ability to focus and detect radiant energy, at wavelengths where glass is opaque and silicon is transparent or insensitive.

Here's a factoid from incandescent lamp research 100 years ago -- maybe I learned it from a CRC handbook. A typical tungsten filament's luminance is a bit lower than blackbody at the same temperature, because the emissivity is less than 1.0. Its color temperature is higher than the actual temperature, because the emissivity is spectrally tilted in the blue direction. I bet if the incandescent wire were hollow, with a tiny hole offering a view to the interior, that spot in a closeup image would fairly represent the blackbody color and luminance.