Plücker tube 2.0

Today I had some fun with glass, vacuum, and high voltage (I'll post more about it later, I've some nice stuff to share :). Turns out molecular plasmas are way more stable when there are no metal electrodes in contact with the discharge. Here the gas fill consists of humid air at about a couple millibars of pressure - I ran a pure water vapor discharge in this tube before filling it with air, re-pumping it and sealing it off (water is very persistant because it easily sticks to surfaces due to the polar nature of its molecule). Operation is at 5-15 kV and ~30 kHz, with power coupled to a pair of capacitive electrodes formed by aluminum taped tube sections at the extremities. I've run this tube for over an hour already and unlike in electroded tubes, the plasma does not show any sign of changes (the discharge is quite intense too! the picture above was taken with the room lights on).

Considering how Geissler and Plücker tubes were designed (i.e., with inner metal electrodes), their driving current had to be kept low to prevent a too fast gas cleanup, especially with the oxygen and water vapor fills, which are particularly reactive. In hindsight, those tubes should have been driven capacitively with external electrodes and using a beefed-up Ruhmkorff induction coil as a power source... more intense and stable discharges could have been obtained this way (but the high voltage and high frequency may cause some practical issues though). Now I'm wondering how Moore's (carbon dioxide and nitrogen) tubes would have performed with such method of operation.