Capacitively coupled Plücker tube

Here's a global view of the capacitively-coupled discharge tube shown there in operation, while still coupled to the pumping setup via the exhaust tubulation on the left. The external electrodes consist of 3 cm-long tube sections covered with aluminum tape, designed to maximize the coupling capacitance with the inner plasma. This is a crucial point in the lamp design as the electrode losses and the amount of current coupled to the plasma both depend on that capacitance, which I estimate at around 66 pF for each electrode. Interestingly, this capacitance is not a constant and changes as the glass material heats up and as corona discharges at the edges of the metal tapes cover more of the outer glass surface at higher applied voltages.

I stretched the central section of the tube to narrow its diameter so as to increase the plasma luminosity. This effect is caused primarily by a higher mean electron energy in the discharge, which has to increase in the narrowed tube section so as to raise the ionization rate in the gas in order to compensate for the increased charge loss rate to the wall via ambipolar diffusion. The hotter electron gas is what causes the enhanced excitation rate of atoms and molecules in the plasma. This "trick" was first put into regular use by Julius Plücker in the late 1850s, whose tubes became a standard in spectroscopic works and experiments. Such tube design is particularly useful as the coupled current in the present design cannot be as high as in electroded tubes. Besides, it is also important to keep that discharge current low in molecular plasmas, especially in sealed tubes. It's a condition required to limit the gas's dissociation and cleanup rates in order to ensure a sufficiently long useful life of the lamp.

The main benefit of keeping the metal electrodes outside the discharge tube is that stable plasmas can be run in chemically-active gas and molecular atmospheres. Keeping the tube connected to the vacuum system enabled the fill nature and pressure to be varied in order to see their effect of the discharge's light emission. The characteristics of a dry air fill can be seen there, while the discharge operating in water vapor is presented there.