Plasma Striations in Krypton Gas
Dan VandenAkker & Ross Norman
Summer Research 2006
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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