Photoemission Study Of A Large Area Containing Scandium Hot Cathode

Photocathode is widely used in various fields. A laser-driven photo-injector was first demonstrated in 1985 in LANL, afterwards, theories and applications of phtocathode technologies had been developed quickly. DC and RF photocathode guns are already employed in RF accelerators. Application of photocathode to Linear Induction Accelerators (LIA) has not matured up to now for LIAs'special environment and requirements. Experiments have been carried to research this problem in this paper.Cold cathode and thermionic cathode experiments had been done in a 2 MeV injector test stand for linear induction accelerators. With improved experimental setup photoemission experiments are able to be conducted. One viewing window is made of quartz for the passage of UV laser. A 10 ns frequency quadrupled Nd:YAG laser (266 nm) was used to drive this injector. The laser pulse was synchronized with a 120 ns diode voltage pulse stably with the change of original timing system. A 100-mm-diameter scandate dispenser cathode was tested as a photocathode. The cathode worked at temperatures below or near the thermionic emission threshold while the vacuum was better than 5×10-5 Pa. The diode voltages varied from 1.4 MV to 1.8 MV and the diode gap is 14 cm. The cathode was illuminated by the laser at 70°incidence angle. The laser beam's radius was about 6 mm after expansion and the illuminated area on the cathode was from 2.2 cm2 to 3.3 cm2 (depending state of the laser).Experiments involve 4 stages:1. the synchronization between laser and diode voltage was realized and stable photoemissions were observed, emissions with plasma formation caused by the laser were also observed; 2. the synchronization and measurements of laser were improved, the effects of laser energy, diode voltage and cathode temperature were tested; 3. credible measurement of laser energy was achieved, factors which affect the photoemission were studied; 4. experiment waves were improved.The cathode worked ranging from 700℃-900℃(lower than the thermionic emission threshold) in most experiments, the normal peak laser power on the cathode was less than 3 MW/cm2. The maximum peak current collected at the anode was 100 A and the maximum average current density was 45 A/cm2. The maximum quantum efficiency measured at 1MW peak laser power was about 2.4×10-4.The illuminated cathode region would be heated by the incident laser. This temperature rise has been estimated using analytical method and ANSYS simulation. The maximum temperature rise in the region was about 53℃while the peak laser power density was 3 MW/cm2. Its effect is not significant in the experiments. The method developed here is also available for other laser conditions.The emitted currents of different laser power reveal that the space charge effect was clearer with higher laser power. According the results of cathode photoemission uniformity more current and higher quantum efficiency may be achieved with a larger laser spot. Experiments results of different cathode temperatures show that continuous heating was necessary to keep cathode's photoemission performance. This operation temperature is at least 200℃below the normal thermionic operation temperature. Lower operation temperature is good for increasing cathode's life and decreasing loads of assistant system.The results here will be a valuable reference for possible applications of photocathode technologies to linear induction accelerator in future.