Air glow lamp

Turns out it is possible to make an air-filled negative glow lamp. The operating voltage is of course quite higher than that of standard neon and argon glow lamps, so I'm using a small battery-operated inverter circuit to drive the lamp shown above. The fill pressure is a few millibars (left) and the cathode is the right electrode. It is covered with a negative glow which generates plenty of UV-A light, mostly nitrogen's second positive system band around 377 nm (C-B transitions), an optical emission which excites a particularly strong blue fluorescence from the sheet of paper underneath.

Unfortunately this lamp is subjected to a particularly strong sputtering of its cathode and to a fast gas cleanup caused by the dissociation of the molecular fill and the reaction of its components with the cathode and its sputtered material. The characteristics of the lamp are thus not stable and the visual aspect of its discharge changes over time until the plasma extinguishes. The evolution shown above occurs in the span of a few minutes when the lamp is driven at a few milliamperes (the camera shutter time was adjusted for a proper exposure).

The solution to this particular issue is show in the next image.