Preparation and performance of transmission-mode GaAs photocathodes as sources for cold dc electron beams

Photoemission from GaAs cathodes with negative electron affinity (NEA) is a pplied for producing electron beams with very low longitudinal and transver se velocity spread. GaAs transmission-mode cathodes were activated with Cs and either O-2 or NF3 in an extremely high vacuum setup (base pressure belo w 10(-12) mbar). Quantum efficiencies of 20%-25% (at 670 nm) and long dark lifetimes (about 1000 h) could be achieved for both types of activation in a reproducible way. Using a method based on the adiabatic transverse expans ion of an electron beam in a spatially decreasing magnetic field, the mean transverse energy (MTE) of the photoemitted electrons was measured systemat ically, recording longitudinal energy distribution curves. Both the MTE and the longitudinal energy spread strongly depend on the value of NEA and the position of the extracted distribution relative to the bulk conduction ban d minimum. Electrons with energies above the conduction band minimum are th ermalized with the lattice temperature of the cathode, while electrons with energies below this level show a non-Maxwellian distribution with enhanced transverse energies. Thus, when extracting all electrons in a current limi ted emission mode, the MTE increases with the absolute value of NEA and rea ches values up to approximate to 100 meV. By cutting off the low energy ele ctrons with an external potential barrier, the longitudinal as well as tran sverse energy spread of the extracted electron ensemble are reduced. The MT E could be reduced down to about 28 meV at room temperature and to about 14 meV at liquid nitrogen temperature. The behavior of the MTE was found equi valent for (Cs, O) and fdr (Cs,F) activation layers on the same cathode. Co nclusions about energy loss and scattering in the emission of photoelectron s from NEA GaAs cathodes are discussed. (C) 2000 American Institute of Phys ics. [S0021-8979(00)00721-0].