Quantum efficiency measurements of photocathode candidates for back-lighted thyratrons

Light-activated pseudospark switches, also called back-lighted thyratrons (BLTs), are low pressure, high voltage (typ. 10-50 kV), high current (typ. 1-100 kA) glow-mode switches. It is of interest to develop BLTs with reliable and practical optical triggering systems for applications of compact pulsed power. This paper reports the results of research into photocathode materials for BLTs to enhance switching performance and provide optimal cathode conditions for optical triggering. Effective photocathode materials have many specific qualities, the most important being low work function, high quantum yield, and long lifetime at typical BLT operation pressures of 1.3-133 Pa (0.01-1 Torr). Photoemission measurements were conducted with 266 nm, 5 ns laser pulses in a pressure range from 4 × 10^-5- 13.3 Pa (3 × 10^-7 to 0.1 Torr) using helium as the background gas. Quantum efficiencies up to 1.5 × 10^-5, 1.4 × 10^-5, and 1.2 × 10^-5 were measured for magnesium, copper, and molybdenum samples, respectively. An increase in gas pressure 4 × 10^-5- 13.3 Pa (3 × 10^-7 to 0.1 Torr) corresponded to an increase in quantum efficiency (QE) of 13% for magnesium and copper; the same increase in pressure corresponded to a quantum efficiency decrease of 10% for molybdenum. Square root of quantum efficiency shows a linear dependence on the square root of the sample surface´s electric field due to the Schottky effect. 2D electrostatic simulation of the electric field distribution in a typical compact BLT shows that the field strengths are up to hundreds of kV/cm near the surfaces of the electrodes when a voltage potential of 30 kV is applied between the electrodes. This indicates that higher photoelectron yields can be expected when the tested photocathodes are implemented into BLTs.