SECONDARY-ELECTRON EMISSION PROPERTIES OF OXIDIZED BERYLLIUM CFA CATHODES

Heating an oxidized beryllium sample above 500 degrees C for eight hou rs or more establishes a stable surface composition that consists of a bout 35% carbon in carbide form, and Be and O in nearly one-to-one ato mic ratio for the remainder. The secondary electron yield of this surf ace has a maximum yield, 6(max), of 2.8 +/- 0.1 at the primary electro n energy of 420 +/- 20 eV. The secondary electron yield decreases slow ly with increasing sample temperature. The energy of the emitted elect rons is analyzed using a retarding potential method with the primary e lectron energy E(p) ranging from 10 to 1600 eV. For E(p) > 100 eV. Mos t of the emitted electrons are the true secondary electrons (i.e., tho se electrons with energy less than 50 eV). The energy distribution of the true secondary electrons shows little change in functional form fo r E, from 200 eV to 1600 eV, and for sample temperature from 20 degree s C to 530 degrees C. A small but steady change is observed in the nar rowing of the peak width with increasing E(p) or increasing sample tem perature. The current practice in processing the crossed-field amplifi er (CFA) tube with an oxidized beryllium cathode includes a bakeout be tween 500 degrees C and 550 degrees C for several hours, The present s tudy suggests that this heating is sufficient to convert the oxidized beryllium CFA cathode surface to the stable composition with the large secondary electron yield. Heating to a much higher temperature will n ot reduce the carbide content, but rather will reduce the oxygen conte nt and consequently the secondary electron yield.