Plasma Striations in Krypton Gas
Dan VandenAkker & Ross Norman Summer Research 2006
Download the printer friendly Word Document version of this page. All the experiments on this page involved a radio-frequency oscillator circuit adapted from "Alkali Metal Vapor Spectral Lamps," W.E. Bell, A.L. Bloom, & J. Lynch, Rev. Sci. Instr. Vol. 32, pg 688 (1961). The diagram used to construct the cell is below.
Diagram of RF Circuit
Initial Experiment: create plasma in an enclosed tube of krypton gas.
This experiment involved the plasma discharge cell circuit discussed above, a 400V power supply, a bath of Liquid Nitrogen (LN2), and a glass tube of krypton gas. The power supply was connected to the plasma discharge cell circuit. The coils from the circuit were wrapped around the tube of krypton. The power supply was turned on, the lights were turned down, and the tube of krypton with the coils still wrapped around it was cooled in the LN2 bath. The LN2 bath caused the temperature inside the tube of krypton to drop significantly. Since temperature and pressure are directly
related, the pressure also decreased dramatically. Once the pressure was low enough, the electric field generated in the coils caused the ignition of the plasma discharge. As the tube was slowly removed from the LN2 bath and the pressure slowly increased, the large cloud of plasma that had formed between the coils turned into a large ball-like mass of plasma centered between the coils. Larger coil separations would permit multiple large masses all equally spaced between the coils. As the pressure increased, multiple smaller ball-like masses of plasma which would travel along the electric field lines between coils. Larger coil separations caused these traveled field lines to intersect in the center of the tube. At this intersection, the smaller masses would combine into multiple large masses as long as the magnetic field lines were intersecting. The next state as pressure increased seemed to be a chaos state where all plasma masses would temporarily disappear and a cloud of plasma would reappear. The final state as pressure increased was a string of very small masses of plasma which resembled string of pearls traveling along the convection currents inside the tube. This 'string of pearls' would branch out at the coils with larger coil separations. Smaller coil separations caused multiple strings of plasma which seamed to repel each other. Below are videos featuring the above discussed behaviors. Click here to see a short video with small coil seperations Click here to see a short video with medium coil seperations Click here to see a short video with large coil seperations
Second Experiment: recreate the striations found in the initial experiment in a
pressure controlled system This experiment involved the plasma discharge cell circuit discussed above, the same 400V power supply, a pressurized supply of krypton gas, a gas regulator, an open ended glass tube, a pump, a pressure gauge, and the parts necessary to create a vacuum tube. We used an old champagne bottle while waiting for custom made glass tubes because champagne bottles can withstand vacuum pressures. Below is a photo diagramming the final setup.
Diagram of Vacuum Setup With this setup, we were able to control the pressure inside the tube with the tube of krypton and the vacuum pump. After the vacuum chamber was air tight, we were able to decrease the pressure enough to create the original plasma cloud. Unfortunately, the vacuum setup didn't seem to be perfectly clean, and we therefore didn't have the most pure krypton possible. We found that pouring LN2 into a pool at the top of the champagne bottle would freeze some impurities to the glass and create a temporarily more pure krypton gas. We were able to produce the central mass of plasma as well as the smaller masses of plasma which ran along the magnetic field line. At high pressures, we were able reach a state that seamed to tease the idea of forming 'strings of pearls.' However, we were never able to produce the clear 'string of pearls' striations that were produced in the initial experiment.
Third Experiment: determine the plasma mass intensity's dependence upon
pressure and the plasma mass diameter's dependence upon pressure. This experiment involved the same setup as the second experiment. However, we also used a video camera with this experiment. The video camera was zoomed in tight on the area of the champagne bottle where the plasma was created and the pressure gauge. As we adjusted the pressure in the chamber, we recorded the plasma and pressure readings. We then pulled frames from our video footage and analyzed them with Scion Image software. By tracing a line over one plasma mass in Scion Image, we could obtain a graph of intensity versus pixels. This graph could be
analyzed for plasma mass intensity and diameter. Using a calibration, we were able to convert from pixels to cm. Below are figures of this analyzation process.
Screen Snapshot of frame analysis with Scion Image software
Scion Image plot with measurements qualitatively described
Excel graphs of our measurements Needless to say, our measurements were fairly speculative. Unfortunately, we were unable to determine a conclusive relationship between pressure and either diameter or intensity. Below are some of the frames that were pulled from the video footage. (The pressures are measured in 100psi)
Four frames of plasma masses at various pressures