Photoemission monitored cleaning of pure and implanted tungsten photocathodes by picosecond UV laser

New results about picosecond laser cleaning of pure and implanted tungsten photocathodes are reported. During the transport from the preparation cell to the experimentation cell, the exposure of photocathodes to air leads to the contamination and the formation of surface oxides. Before any photocurrent measurement, in situ cleaning of surfaces is required. The surface cleaning effects are investigated in high vacuum (5×10−8 Torr) by using a picosecond mode-locked Nd3+:YAG laser with fifth order harmonic generator (λ=213 nm, τ=16 ps, repetition rate 10 Hz) irradiation. This non-destructive processing allows real-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 influence of the laser energy density and the effects of surface recontamination by residual gas are investigated. A phenomenological model is developed to describe the laser cleaning. Fitting the experimental data of photoemitted charges with calculated values given by this model is able to estimate the contamination rate and the work function at the surface during the cleaning.