Guess who got a new vacuum pump for Christmas :)

That's right, now I can make plenty of nice discharge tubes for fun! Today I finally got to properly test my new pump (and some additional glassworking tools), and I made the tube shown here. This "lamp" features a glass tube containing two internal stainless-steel coil electrodes with side feedthroughs, and a relatively large ovoid bulb blown at its extremity. The intention is to see if the plasma can extend to that volume, away from the electrode space.

The gas fill is air at low pressure (around 5 mbar when I sealed the tube) with some oil impurities coming from the electrodes (I did not clean them properly). To energize the discharge I used a 15 kV HF power supply operating at 30 kHz. Initially the plasma was confined mostly inside the coil electrodes (see picture below), which effectively work as hollow cathodes given their geometry. Interestingly, molecular dissociation in the discharge and metallic sputtering caused the fill pressure to drop gradually, up to a point where I could finally pull the discharge into the ovoid bulb by touching it (top picture). This process is driven by the high-frequency operation of the tube, which allows a capacitive current to flow through the glass wall. Touching the ovoid bulb causes a capacitive coupling to the ground (I am at that potential while both electrode are at HV potential, but at opposite phase relative to each other), which diverts some current from the main plasma.

The gettering of the tube's atmosphere by the plasma and the electrode sputtering is a continuous process, so the lamp's gas fill composition and pressure change continuously. This post shows the lamp at a much later stage of its operation.