Silicon photocathodes with array of tips in a photo-injector

Photo-injectors for linear accelerators and free electron laser need a high brightness photocathode with a dark current lower than 10 mA. Silicon with array of tips shows a photofield emission current with a continuous argon laser with a quantum yield of 1%. The ratio η between photofield and field emission (dark) current is 20 near the threshold of field emission. With a pulsed Nd-YAG laser (λ = 1.02 μm pulses of 35 ps duration) we observe a pure photoemission charge (70 pC) with harmonic 4 (ℏ ω4 = 4.68 eV) and a photofield charge above the threshold which is 700 pC with harmonic 2 (ℏ ω2 = 2.34 eV). We calculate for the photo-injector CANDELA the dark current following the method of Loew and Wang, starting from the experimental Fowler-Nordheim curve and we find a dark current which is 1 μA at 3 GHz at Emax = 30 MV/m.