Capillary electroded tube - pressure sequence

Here's a sequence showing the aspect and color as a function of pressure of the electrical discharge burning in dry air in my experimental plasma tube made with capillary electrodes (see there). The tube was connected to the gas pumping setup via one of the capillaries and the pressure was gradually reduced from about a hundred millibars (top) to below the millibar level (bottom). The camera's shutter time was adjusted in each case in order to get a proper exposure (other parameters were kept constant).

Interestingly the right electrode operated as a cathode despite running the tube using an AC source (c.30 kHz 2.5-7.5 kV rms). I think this arose as a result of more sodium released from the base of that particular electrode, at the junction with the glass support. This was certainly caused by the electrode's shorter length and its operation in the abnormal glow regime, which forced a stronger current on the triple (glass-metal-gas) point of the electrode assembly. Sodium's characteristic yellow light emission is even visible around that cathode in some shots, especially at the lowest pressures. That electrode even became incandescent at intermediate pressure levels as a result of the combination of a strong electric current, an elevated cathodic electric field, and a high ion mobility in the gas. Such condition also causes heavy sputtering as the cathode is not hot enough to cause sufficient thermionic emission of electrons to lower the cathodic sheath electric field. Interestingly, the tube's UV output is particularly noticeable (cue the blue fluorescence of the underlying sheet of paper) at the extremes of the pressure range.