Secondary electron emission properties of oxidized beryllium CFA cathodes

Heating an oxidized beryllium sample above 500/spl deg/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, /spl delta//sub max/, of 2.8/spl plusmn/0.1 at the primary electron energy of 420/spl plusmn/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/sub p/ ranging from 10 to 1600 eV. For E/sub 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/sub p/ from 200 eV to 1600 eV, and for sample temperature from 20/spl deg/C to 530/spl deg/C. A small but steady change is observed in the narrowing of the peak width with increasing E/sub 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/spl deg/C and 550/spl deg/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.<
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