Breakdown processes in large-area cold-cathode vacuum triodes

Preionization of large aperture high pressure discharge pumped gas lasers which requires uniform 250 to 500 keV 10 to 100 mA cm(-2) electron beams of typically 1 to 10 x 10(3) cm(2) in the area were examined. Impedance collapse due to cathode plasma expansion renders diode electron guns and is inefficient in this application. The introduction of a control grid limits gun current to low, controllable values until plasma closure of the cathode grid gap occurs. Field enhancement at the grid can cause cathode grid or grid anode arcs which result in current runaway. Impedance collapse was studied by measuring element I-V characteristics in a 20 x 200 cm cold cathode triode electron gun. Impedance collapse of closure of the cathode grid gap and (2) grid anode arcs. Plasma closure rate was -2 x 10(6) cm sec(-1) independent of configuration or interelectrode potential. It is found that arc thresholds depend exclusively on local electric field strength at the grid which is a function of grid bias and grid geometry. Streamers and arcs are occasionally observed in the cathode grid region.