Capacitively coupled Plücker tube: water vapor (with tap water residues)

This series shows the capacitively coupled Plücker tube presented there operating at 5-15 kV and ~30 kHz in water vapor at a pressure ranging from about 10 mbar (top) to less than 1 mbar (bottom). There are several interesting phenomena visible here, first at the highest pressure where the discharge is more filamentary than in dry air (see there). This is caused by the electronegative nature of water vapor, whose molecule traps free electrons to form negative ions in the plasma. This process causes the discharge to contract in order to limit charge losses.

The second, and perhaps most important phenomenon, is the appearance of a strong sodium emission at a few millibars of pressure. This phenomenon was already presented there and is caused by the release of elemental sodium from the tube wall by a reduction reaction. This release is driven by positive hydrogen ions produced in the discharge from the dissociation of water molecules, and accelerated in the radial direction by the wall's negative charge caused by the accumulation of highly mobile electrons there. This reaction presents an optimum in pressure because there is a tradeoff between the production rate of H+ ions in the plasma and its flux density to the wall. A too high pressure will cause a too low diffusion rate, while not enough of this ion will be produced at a too low pressure.

A very interesting aspect of this phenomenon is that it requires impurities from tap water to work. A tube with a clean glass wall and a fill of pure (distilled) water won't show any sodium emission. I think this is evidence that the sodium compound which is most effectively reduced by H+ ion, resulting in the release of that alkali in the discharge, is not the oxide or silicate found in the glass material, but more likely a carbonate compound coming from tap water (which is the source of the water vapor here). Another interesting aspect of this "sodium lamp" operation is that the discharge's chemical activity prevents the tube from becoming dark as a result of the presence of metallic sodium. That element reacts with the water plasma to form a chemically inert compound (most likely an oxide) before it returns back to the glass wall.

Finally, the third and last interesting phenomenon is the color change that occurs towards the lowest pressure level. The discharge color turns pinkish as a result of an optical emission becoming dominated by atomic hydrogen. The increasing mean electron energy at lower pressures causes a higher dissociation rate of H2 and H2O molecules which result in the H-alpha line at 656.5 nm becoming particularly strong. To be clear, the net density of species is lower than at higher pressures, but the density ratio of atomic hydrogen over water and molecular hydrogen increases at lower pressure, hence the change in the discharge's spectral characteristics.