Design and characterization of a megawatt-class laser-driven semiconductor switch with application to high-gradient accelerator testing

Julian Picard


Tuesday, February 26, 2019



PSFC Student Seminars

Abstract: Laser-driven semiconductor switches (LDSS) are of great use in producing short RF pulses in the microwave to far-infrared range. Such short pulses have potential applications ranging from the investigation of material properties to testing of high-gradient accelerator concepts and DNP-NMR. LDSS-based systems operate by using a laser to induce temporary reflectance in a semiconductor wafer. The wafer subsequently reflects RF from a source for a short time period ranging from nanoseconds to milliseconds. Previous experiments have demonstrated LDSS operation up to the kilowatt power level. This work presents an LDSS employing silicon (Si) and gallium arsenide (GaAs) wafers that have been used to produce nanosecond-scale pulses from a 3 μs, 110 GHz gyrotron at the megawatt power level. Photoconductivity was induced in the wafers using a 532 nm Nd:YAG laser, which produces 6 ns, 230 mJ pulses. Irradiation of a single Si wafer by the laser produced 110 GHz RF pulses with 9 ns width and reflectance of >70%. Under the same conditions, a single GaAs wafer produced 24 ns 110 GHz RF pulses with >78% reflectance. For both semiconductor materials, a higher value of reflectance was observed with increasing 110 GHz beam intensity. In dual-wafer operation, which uses two active wafers, pulses of variable length down to 3 ns duration could be created at power levels up to 300 kW. The switch was successfully tested at incident 110 GHz RF power levels up to 600 kW. To complement experimental results, a 1-D model reaction-diffusion model is presented that agrees well with experimentally observed temporal pulse shapes obtained with a single Si wafer.