Electrical Discharges Gallery

Here are some photographs of various unusual or interesting glow discharges.  All

effects shown are easy to recreate, although various high voltage DC and RF power

supplies and a high-vacuum pump are required in most cases.  Pressures, where

given, were measured with a very accurate capacitance manometer.

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Microwave Oven Plasmoids

Free-floating arc discharges, given the name of "microwave oven ball lightning," are unlikely to be related to

the naturally occuring ball lightning.  Nevertheless, they are easy to produce in almost any microwave oven

and fascinating to observe.  The large older oven shown above, equipped with an 800-watt output magnetron,

has sustained plasmoids for up to eight seconds when operated in the vertical position shown.  The trick to

making the discharges is (1) placing a resonant conductive structure within the oven that develops a voltage

antinode; and (2) having an ionizing and heat-generating mechanism in close proximity to the high voltage

antinode to "seed" the discharge.  Here I have wrapped a copper quarter-wave stub with paper, and ignited

the paper.  The flames provide ionization and convection to help the arc float free of the copper stub.

Sustenance of the free arc is provided through resistive heating of the conducting gas by microwave currents.

It is my hypothesis that the diameter of the plasmoid is inversely proportional to the wavelength of the RF

excitation; and that the power required to sustain the discharge is proportional to the ionized volume (or

roughly the cube of the wavelength).  Thus a 10 GHz field could probably support a cloud of pea-sized arcs,

while a very old industrial 900 MHz microwave oven might be able to spawn the occasional monster with a

diameter 2.5 times larger than what is seen in the pictures.