Improved negative glow lamp design

The instability in the operation of the air glow lamp presented earlier is caused primarily by plasma-electrode interactions. So, the solution to that problem consists in sheathing the electrodes with a thin glass tube so as to prevent cathodic sputtering and chemical reactions with active species produced in the discharge. The resulting lamp assembly is shown above, before pumping and sealing. To protect the electrodes I used thin glass capillaries which I sealed individually while keeping them at around 400-500 C. The resulting electrodes thus consist of a central metallic conductor surrounded by a closed air volume at a few hundred millibars of pressure.

Such design prevents the capillary electrodes from "blowing up" when I seal them in the main glass tube. Moreover, the low-pressure air volume contained inside the capillaries become ionized when high voltage is applied across the lamp terminals. This helps the electrode's operation as this ionized gas transfers the central conductor's potential right to the inner capillary surface. This thus increases the electrode capacitance, which ensures an effective power coupling to the discharge in the main lamp volume.

It is obvious that the glow lamp is now a capacitively-coupled discharge tube. As a result it cannot be run on DC power, a potential of alternative polarity is needed for a current to flow between the electrodes. In the next pictures I drove a plasma in air (see there) and water vapor (see there) using a small battery-operated HV-AC inverter circuit. This new lamp concept is found to work well with reactive molecular fills, but it certainly cannot be run from the mains like standard neon glow lamps.