Secondary electron emission properties of oxidized beryllium CFA cathodes

Heating an oxidized beryllium sample above 500°C for eight hours or more establishes a stable surface composition that consists of about 35% carbon in carbide form, and Be and O in nearly one-to-one atomic ratio for the remainder. The secondary electron yield of this surface has a maximum yield, δ_max, of 2.8±0.1 at the primary electron energy of 420±20 eV. The secondary electron yield decreases slowly with increasing sample temperature. The energy of the emitted electrons is analyzed using a retarding potential method with the primary electron energy E_p ranging from 10 to 1600 eV. For E_p>100 eV, most of the emitted electrons are the true secondary electrons (i.e., those electrons with energy less than 50 eV). The energy distribution of the true secondary electrons shows little change in functional form for E_p from 200 eV to 1600 eV, and for sample temperature from 20°C to 530°C. A small but steady change is observed in the narrowing of the peak width with increasing E_p, or increasing sample temperature. The current practice in processing the crossed-field amplifier (CFA) tube with an oxidized beryllium cathode includes a bakeout between 500°C and 550°C for several hours. The present study 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 not reduce the carbide content, but rather will reduce the oxygen content and consequently the secondary electron yield