INVESTIGATIONS OF THE TRANSITION FROM GLOW TO ARC IN A TRANSVERSE-FLOW CO2-LASER DISCHARGE

The paper concerns experimental investigations of the glow to arc transition in a transverse-flow CO2 laser discharge in a pressure range of 30-100 hPa. Constriction development was recorded with a high-speed film camera and storage oscilloscope, and the maximum density of the electric power deposited in the glow discharge below the instability threshold was determined. Spatially resolved diagnostics of the discharge prior to the instability onset were carried out with the help of an electric probe and a boxCARS set-up. It was observed that constrictions start to develop in discharge regions characterized by high plasma density or density gradient, large pressure pulsations or high density of negative ions. They propagate towards the electrodes at velocities in the range of 100-800 m s-1. The electric field inside a constriction is of the order of 10(4) V m-1. Thus the constrictions investigated differ both from thermal arcs and streamers. The probable mechanism of their development involves avalanche ionization at the constriction head without thermalization in the filament. The influence of discharge and flow conditions on the instability threshold was investigated. The favourable effect of preionization and flow turbulence on the discharge stability was confirmed. Also the possibility of raising the instability threshold by using an uncooled cathode was ascertained. Based on the results of investigations, an adjustment of the cathode profile is proposed as a simple and effective means of ensuring a uniform current distribution over the discharge channel width, which is a prerequisite of a stable discharge operation and laser action at increased electric power densities.