Self-organization in cathode boundary layer microdischarges

Direct current glow discharges in xenon between a flat, 100 mum thin cathode and a ring shaped anode, separated by a distance of 250 mum, were found to be stable up to atmospheric pressure. The glow discharge structure in this electrode configuration reduces to only the cathode fall and negative glow, with the negative glow plasma serving to conduct the current radially to the circular anode. Photographs taken in the visible range of the spectrum and at the wavelength of excimer emission for xenon (172 nm) indicate the transition from a homogeneous plasma to a structured plasma when the current is reduced beyond a critical value that is dependent on pressure. The plasma pattern consists of filamentary structures arranged in concentric circles. The structures are most pronounced at pressures below 200 Torr and become less regular when the pressure is increased. The self-organization of such plasmas indicates the existence of two branches of the voltage-current density (V-J) characteristic with positive slope. For conventional glow discharges in the current range of interest (milliampere), the only discharge mode with a positive slope of the V-J characteristic is the abnormal glow mode. At a critical current density, the discharge transfers from the abnormal glow into an arc. However, by cooling the cathode, it seems to be possible to stabilize the discharge, even in the glow-to-arc transition range. This second stable region in the V-J characteristic of such 'cathode boundary layer discharges' would explain the existence of a plasma pattern with two distinct values of current density at the same discharge voltage.