Fun with vacuum and high voltage

Here's a quick and dirty experiment I did using some glass tubing, copper wires, a rotary-vane pump, and an oil burner HV transformer to show my nephews what happens when you apply high voltages at various air pressures. Here, I estimate that the pressure was below the millibar level, with the tube full of contaminants (oil residues mainly) resulting in the whitish light emission of carbon-based molecules - a discharge in pure air would be dominated by nitrogen and would show a blue color at the electrodes and a reddish one in between.

The picture below is a close-up view of the coil electrode, showing a hollow cathode effect (see there for more details):



Sputtering and chemical reactions certainly took a toll on that electrode. These mechanisms are responsible for gettering molecular gases, and had the tube been sealed, this would have resulted in a gradual decrease in pressure:



Interestingly, this tube shows the characteristics of a DC discharge, with a Faraday dark space visible on the left, despite the fact that I applied an AC high voltage at 25 kHz. This is due to the electrode asymmetry, with the coiled one acting as a cathode due to its size and configuration. As a result, this discharge tube work as a rectifier, albeit an imperfect one, hence why the Faraday dark space is not entirely dark. The fain discharge visible on the right is caused by current leakage to the vacuum pump, which acts as a (distant) ground electrode.