Plasma diffusion at low pressure

Sputtering of the coil electrodes and molecular dissociation in the plasma cause a gradual change of this lamp's atmosphere. Both its pressure and composition evolve over time and the emitted light color shifts to a pinkish hue. I think this color arises from hydrogen produced by the decomposition of hydrocarbon molecules (this tube has an oil pollution). Another interesting phenomenon is the increase in electron mean free path as a result of the decreased pressure, which causes the plasma to extend well beyond the electrode region. The effect of "grounding" the ovoid bulb with my fingers is particularly strong in that regime as charges can easily flow across the whole tube (see below).