Excited Argon 1s5 Production in Microhollow Cathode Discharges

Diode-pumped rare gas lasers (DPRGL) have been in development for their potential to become high energy lasers with excellent beam quality that is typical of gas lasers. DPRGL require metastable densities on the order of 1013 cm-3 at pressures around one atmosphere for efficient operation. Argon 1s5 number densities have been measured in microhollow cathode discharges (MHCD) using tunable diode laser absorption spectroscopy. The MHCD had copper electrodes with gaps of 127 and 254 µm and hole diameters from 100-400 µm. Absorbance was measured at pressures of 37 Torr up to 400 Torr, where absorbance could no longer be detected. Fluorescence images were taken both perpendicular to the plate and viewing the hole end on to observe plasma uniformity and determine the absorbance path length. The images revealed the plasma formation was inconsistent and often not in line with the hole, making path length determinations difficult. Assuming a path length equal to the total plate thickness of either 198 or 325 µm resulted in the maximum possible Ar 1s5 number densities, ranging from 5.5x1012 to 4.0x1013 cm-3. The actual number densities could be three times lower depending on the actual path length. There were no consistent trends with hole diameter, pressure, or gap distance. Relative 2p state populations were measured by optical emission spectroscopy and used in a five-level laser model to estimate the electron density, electron temperature and the predicted gain. The relative populations indicate that the plasma was not in local thermodynamic equilibrium. The average electron density and temperature predicted by the model was 1:7 x 1014 cm-3 and 1.2 eV respectively. Despite number densities typically used in DPRGL the model predicted that gain would not be achieved due to the high laser threshold that is a result of the short gain length