PHOTOEMISSION MONITORED CLEANING OF PURE AND IMPLANTED TUNGSTEN PHOTOCATHODES BY PICOSECOND UV LASER

New results about picosecond laser cleaning of pure and implanted tung sten photocathodes are reported, During the transport from the prepara tion cell to the experimentation cell, the exposure of photocathodes t o air leads to the contamination and the formation of surface oxides, Before any photocurrent measurement, in situ cleaning of surfaces is r equired. The surface cleaning effects are investigated in high vacuum (5 x 10(-8) Torr) by using a picosecond mode-locked Nd3+:YAG laser wit h fifth order harmonic generator (lambda = 213 nm, tau = 16 ps, repeti tion rate 10 Hz) irradiation. This non-destructive processing allows r eal-time monitoring by measuring continuously the surface photocurrent , During cleaning, the photocurrent increases versus time and tends to reach a plateau, depending on the pressure in the vacuum cell. The in fluence of the laser energy density and the effects of surface reconta mination by residual gas are investigated. A phenomenological model is developed to describe the laser cleaning. Fitting the experimental da ta of photoemitted charges with calculated values given by this model is able to estimate the contamination rate and the work function at th e surface during the cleaning.