Breakdown Processes in Large-Area Cold-Cathode Vacuum Triodes

Preionization of large-aperture high-pressure dischargepumped gas lasers requires uniform 250 to 500 keV 10 to 100 mA cm-2 electron beams of typically 1 to l0x103 cm2 in area. Impedance collapse due to cathode plasma expansion renders diode electron guns inefficient in this application. Introduction of a control grid limits gun current to low, controllable values until plasma closure of the cathode-grid gap occurs. However, field enhancement at the grid can cause cathode-grid or grid-anode arcs resulting in current runaway. An experimental study of impedance collapse was conducted by measuring element I-V characteristics in a 20x200 cm cold-cathode triode electron gun. Impedance collapse of the gun was dominated by two processes: (1) plasma closure of the cathodegrid gap and (2) grid-anode arcs. Plasma closure rate was ~2x106 cm s-1 independent of configuration or interelectorde potential. Arc thresholds were found to depend (after conditioning) exclusively on local electric field strength at the grid which is, in turn, a function of grid bias and, more importantly, grid geometry. Streamers and arcs were occasionally observed in the cathode-grid region, phenomena deserving of further investigation.