Archived - Pressure Cooker Plasma appearence with voltages and current

[Dan Tibbets]

Below is a composite picture of the glow discharge in my 'Pressure Cooker Fusor' at progressively lower vacuums. First picture is probably ~ 1000-2000 microns. Last few pictures were probably ~ 40-80 microns. ( I was able to get measured vacuums down to ~ 35 microns with a thermocouple gauge earlier- but the gauge subsequently failed, possibly due to EMP effects, so pressure is estimated based on appearance)
The first few pictures show glows from the shell (anode) due to the modest vacuum and contamination ( I left a valve to the unpowered pump open and oil backstreamed into the chamber- what a mess, will probably need several more iterations of cleaning). The images shows the plasma appearance and the measured milliAmps and Volts. The power supply was a 9 kV 30 mA NST rectified, without a capacitor as per R. Hulls schematic. The peak voltage for the unloaded transformer output was ~ 6200V DC at 120 input volts, and ~ 7200 V at 140 input volts. There was some ripple in the unfiltered output so my measured voltage was presumably less than peak voltage (5%, 20%?). The input voltage in all but the last three images was set at 60 volts on the Variac. All but the last two photos were taken at 1/30th sec, f8 and ISO of 600. The image with the highest measured voltage was with the Variac briefly set to 140 volts.
The last image was taken through a blue filter to isolate the 'spikes' more. Are these spikes representive of the 'Star Mode' (at relatively low voltages)? At these voltages I did not see a good transition through a 'Bugle' stage. At the intermediate vacuums it was evident if the voltage was turned down.
As the vacuum droped (over a period of ~ 2 hrs) the amps first climbed (voltage droped) breifly as the glow was established, then droped continually with improving vacuum.

A dark border region grew out from the electrodes with improving vacuum. When these dark zones were well out from the electrode (wires) but not yet touching was when I typically see a Bugle. For some reason with this setup I did not see it well. The visible poisser also did not last long. As the dark zones grew together was when the beams become more evident. Some of my grids show tighter beam focus- generally with smaller wires, smaller diameter grids, and especially a spiral grid. The developement of these spikes/ beams as the dark zones meet makes me think that the beams are indead focused, as opposed to some suggestions that they are the product of a subtractive process- ie the ions that are striking the grid are lost, so that the surviving ions(and consequent recombinations) show up more in the paths between the grids.

I'm assumeing my 'dark zones' are Aston Dark Spaces- see excerpt below. I downloaded this discriptive 7 page pdf from somewhere, but I cannot find the link after searches here, and with Google, so if someone knows the link I would appreciate it.


"Structure of Glow Discharge"

"Aston Dark Space – A thin region to the right of the cathode with a strong electric field. The electrons are accelerated through this space away from the cathode. This region has a negative space charge, meaning that stray initial electrons together with the secondary electrons from the cathode outnumber the ions in this region. The electrons are too low density and/or energy to excite the gas, so it appears dark."




Dan Tibbets

[Richard Hull]

Thanks Dan for this cool display of voltages and current appearances at different pressures. I archived this due to its instructive nature. Now, if we can just get the newbs to search it out, a lot of questions will be answered.

Exact visual appearances vary somewhat, machine to machine, but the general regime is universal.

Anyone that obtained my fusor videos will see the amazing "beam dance" in fusor II. This was a bell jar demo device. In the video, a single, thin, pencil beam exits the central grid area. Even though the voltage and current remain the same, this beam dances wildly over the entire outer grid, changing postions over just about every opening in the geodesic, never staying put for more than one second until, finally, the working pump gets the pressure low enough to where the beam fixes itself to one opening. Amazing stuff.

In any vacuum glow mode device, the visual appearance can exhibit bizarre behavior due to the smallest change in any of a large number of parameters.

Richard Hull

[Dan Tibbets]

Another example of plasma/glow appearance in the Pressure Cooker Fusor. This time with a loop anode and a grounded cathode. There did not seem to be nearly as much current buildup despite the huge effective cathode area. I assume this was because there was very little heating of the cathode (fusor shell and wire) so very little thermonic electron production.

Dan Tibbets