In situ surface cleaning of pure and implanted tungsten photocathodes by pulsed laser irradiation

This work relates to the study of high density ultrashort electron pulses (> 10 kA/cm(2)) from pure and implanted metals. Before photocurrent measurements a cleaning and activation in situ of the photocathode is required. The contamination of the photocathode surface is due to exposure to air during transportation from the preparation cell to the photoemission cell. The surface cleaning effects of pure and implanted tungsten photocathodes was investigated by using a subpicosecond KrF excimer 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). This dry processing in vacuum is real time, in situ and monitored by measuring continuously the surface photocurrent while a high voltage of 20 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.