DOI: 10.25549/usctheses-c3-209283
title: Magnesium-based photocathodes for triggering back-lighted thyratrons

description: This dissertation presents experimental studies of application of magnesium-based photocathodes to optically triggered pseudospark switches, called back-lighted thyratrons (BLTs).  Magnesium was chosen as a low work function metal photocathode for its potential to increase triggering performance of the switch with a higher photoemission performance than traditional BLT cathodes. Improvement in triggering performance of plasma switches is of interest for device development of compact pulsed power systems where the size of switching units can limit the overall size and the mobility of the system. Experiments were conducted on photoemission performance of photocathode candidates under BLT-relevant conditions; and delay and jitter performance of a BLT with photocathode candidates with changing helium pressure and switch voltages. ❧ A review of photocathode literature showed that Mg and Cu are the most promising candidates for increasing the photoemission during the triggering of BLTs. As a commonly used BLT cathode in the switch literature, Mo was chosen together with Mg and Cu to be tested under BLT-relevant pressure and field conditions. Quantum efficiency measurements of high-purity foils of Mg, Cu and Mo showed a superior performance of Mg and Cu over Mo. Mg had the highest quantum efficiency of 1.5 x 10^-5 among all three materials. ❧ After photoemission measurements in a test bed were concluded, testing of these cathodes for their switching performance was done in two stages. First, an unfocused UV laser beam (8.5 ×10^6 W/cm2) with a wavelength of 266 nm was used for delay measurements of a BLT with Mg, Cu and Mo-based cathodes. Mg-based cathodes showed at least a thirty-fold reduction in delay and jitter compared to Cu-based and at least an eighty-fold reduction in delay and jitter compared to Mo-based cathodes at any given helium pressure and switch voltage pair.  Subsequently, a partial focusing of the same light source was utilized (7.4 ×10^7 W/cm2) for delay measurements of a BLT with copper electrodes at constant switch voltage and changing helium pressure before and after integration of a Mg foil. These measurements showed an order of magnitude shorter delay and jitter throughout the pressure range when the high-purity Mg-foil was present at the switch cathode. ❧ Theoretical estimations of electron emission from the cathode during the triggering suggested that the main mechanism responsible for the observed change in delay and jitter was the increased photoemission due to the lower work function of the Mg cathode and that the effect of temperature on triggering is negligible. ❧ SEM images of the high-purity Mg foil integrated at the BLT cathode for 10^6 shots showed signs of melting around the bore hole. No degradation of the switch performance was observed for the duration of 10^6 shots. ❧ In conclusion, magnesium-based cathodes for BLTs showed an important potential for small triggering units for optical triggering, especially when the intensity of the optical source is limited. A future work involving plasma simulations is suggested for assessing potential of different cathode/optical source pairs for triggering BLTs.