Direct current glow discharges in xenon between a flat, 100 µm thin cathode and a ring shaped anode, separated by a distance of 250 µm, 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.

Microhollow cathode discharges are high-pressure, nonequilibrium gas discharges between a hollow cathode and a planar or hollow anode with electrode dimensions in the 100 μm range. The large concentration of high-energy electrons, in combination with the high-gas density favors excimer formation. Excimer emission was observed in xenon and argon, at wavelengths of 128 and 172 nm, respectively, and in argon fluoride and xenon chloride, at 193 and 308 nm. The radiant emittance of the excimer radiation was found to increase monotonically with pressure. However, due to the decrease in source size with pressure, the efficiency (ratio of excimer radiant power to input electrical power), has for xenon and argon fluoride a maximum at ∼400 Torr. The maximum efficiency is between 6% and 9% for xenon, and ∼2% for argon fluoride.

Microhollow cathode discharges (MHCDs) operated in rare gases are sources of intense excimer emission. Of particular interest is argon, because of its relatively low cost and the short wavelength (128 nm) of its excimer emission. The measured internal efficiency, obtained in static argon at atmospheric pressure, was found to be on the order of 1%. Flowing argon through a direct current (DC) MHCD at atmospheric pressure caused the argon excimer internal efficiency to increase to 6%, indicating that the low efficiency in static argon is mainly due to impurities. Applying 10 ns pulses to the DC plasma resulted in an increase in excimer power from 30 mW DC to 180 mW peak power, at an efficiency of 5-6%. The increase in excimer power correlates with an increase in the electron density. For DC operation, electron densities of 10 15 cm −3 were measured in atmospheric pressure argon micro-plasmas, which increased to values beyond 10 16 cm −3 for nanosecond pulsed operation. This increase in electron density and excimer power is due to pulsed electron heating, an effect that has allowed us to raise the mean electron energy from 1 eV, for DC operation, to 2.25 eV in the pulsed mode.

The excimer emission from direct current glow discharges between a planar cathode and a ring-shaped anode of 0.75 and 1.5 mm diameter, respectively, separated by a gap of 250 m, was studied in xenon and argon in a pressure range from 75 to 760 Torr. The thickness of the ''cathode boundary layer'' plasma, in the 100 m range, and a discharge sustaining voltage of approximately 200 V, indicates that the discharge is restricted to the cathode fall and the negative glow. The radiant excimer emittance at 172 nm increases with pressure and reaches a value of 4 W/cm 2 for atmospheric pressure operation in xenon. The maximum internal efficiency, however, decreases with pressure having highest values of 5% for 75 Torr operation. When the discharge current is reduced below a critical value, the discharge in xenon changes from an abnormal glow into a mode showing self-organization of the plasma. Also, the excimer spectrum changes from one with about equal contributions from the first and second continuum to one that is dominated by the second continuum emission. The xenon excimer emission intensity peaks at this discharge mode transition. In the case of argon, self-organization of the plasma was not seen, but the emission of the excimer radiation ͑128 nm͒ again shows a maximum at the transition from abnormal to normal glow. As was observed with xenon, the radiant emittance of argon increases with pressure, and the efficiency decreases. The maximum radiant emittance is 1.6 W/cm 2 for argon at 600 Torr. The maximum internal efficiency is 2.5% at 200 Torr. The positive slope of the current-voltage characteristics at maximum excimer emission in both cases indicates the possibility of generating intense, large area, flat excimer lamps.

Articles you may be interested in A global model of the self-pulsing regime of micro-hollow cathode discharges