Jochen's High Voltage Page

Plasma Jar

The "vacuum vessel" used in this experiment is a jam-jar - this is not recommended for imitation due to the high risk of implosion. The metal lid served as one electric connection, a second one was installed by drilling a small hole near the bottom and glueing in a screw with a piece of wire attached to form a sharp electrode. The jar was evacuated with a small hand pump (see picture). The pump was connected to the jar via a piece of rubber tubing and a copper pipe-end soldered to the lid.

Hand pump as it was used to create a vacuum in preserving jars. The achievable vacuum is around 60 Torr or 80 mBar.

Larger version

For the following two pictures, the sharp electrode was fed by a 20kHz, 20kV flyback, while the lid was not connected. While at normal atmospheric pressure, corona is barely visible, the pictures show that with decreasing pressure, the plasma region grows and fills a large fraction of the jar at the lowest possible pressure. The effect will probably be even better when higher frequencies (tesla coils) are used. Also, a noble gas filling (e.g. argon) should make much nicer plasma than air.

Corona on the sharp wire electrode at medium (left) and lowest possible (right) pressure.

Larger version

The series of pictures below shows how an arc (high current gas discharge) develops between between the lid and the wire electrode with increasing voltage. The voltage source is an obit (10kV, 50Hz) fed with variable voltage by means of a variac. The pressure is kept the same (lowest possible).

Formation of an arc. Voltage increases from left to right.

Larger version

Note how the gap between the plasma column (originating from the lid) and the wire tip becomes smaller, while at the same time the area on the wire covered by corona grows. This as well as the brighter plasma column is a sign of increasing current. Note also, that although the voltage source is AC (sinewave), the discharge is very asymmetric. The reason lies in the geometry of electrodes, one being point-like and the other being flat. This also results in a partial rectification of the current.

Vacuum vessels may implode. Imploding glas jars are very dangerous - lots of glass fragments flying around. You should at least wear safety goggles and gloves. Put the jar behind a clear plastic screen if possible.

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