IN-SITU SURFACE CLEANING OF PURE AND IMPLANTED TUNGSTEN PHOTOCATHODESBY PULSED-LASER IRRADIATION

This work relates to the study of high density ultrashort electron pul ses (> 10 kA/cm(2)) from pure and implanted metals. Before photocurren t measurements a cleaning and activation in situ of the photocathode i s required. The contamination of the photocathode surface is due to ex posure to air during transportation from the preparation cell to the p hotoemission cell. The surface cleaning effects of pure and implanted tungsten photocathodes was investigated by using a subpicosecond KrF e xcimer laser (lambda = 248 nm and tau = 450 fs) and the fifth harmonic of a picosecond mode locked Nd:YAG laser (lambda = 213 nm and lambda = 16 ps) irradiation of metal surfaces in vacuum (5 x 10(-8) mbar). Th is dry processing in vacuum is real time, in situ and monitored by mea suring continuously the surface photocurrent while a high voltage of 2 0 kV was applied between photocathode and anode to collect the emitted electrons. For so shea time the energy density of the laser pulse is found to be also an important parameter in cleaning process.