Short pulse photoemission from a dispenser cathode under the 2nd, 3rd and 4th harmonics of a picosecond Nd: YAG laser

Abstract

We present new results concerning the photoemissive properties of a standard thermionic dispenser cathode, B type, a porous tungsten matrix impregnated with barium calcium aluminate, 5 BaO-3 CaO, 2 Al 2O 3, working in ultra-short pulses at a temperature far below the measurable thermoemission threshold thanks to a picosecond frequency doubled-tripled or quadrupled Nd: YAG laser delivering 35 ps pulses and a continuous electrical accelerating field applied to the cathode plane surface. Our experiments have shown that the maximum emitted charge in each current pulse corresponds to the maximum available superficial charge of the cathode and that the emitted charge is proportional to the illuminated surface area. The quantum yield values measured at low laser energy are about: η = 2.5 × 10 −5, η = 1 × 10 −4 and η = 3.5 × 10 −4 at the three photon energies h ̵ ω 3 = 2.34 eV, h ̵ ω 3 = 3.51 EV and h ̵ ω 4 = 4.68 eV respectively. Poisoning effectsdue to some residual gas molecules like O 2-H 2O existing in the vacuum cell modify the composition of the surface Ba-O monolayer and are at the origin of a decrease of the photoemitted current about 3.6% per hour. Heating slightly and permanently the cathode permits to limit these effects.