Temperature Transients in Oxide-Coated Cathodes during Pulsed Operation

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Abstract
Experimental results presented demonstrate that transient changes in the thermionic emission of oxidecoated cathodes which occur during heavy pulse current drain depend upon the anode material and its past history. When an anode is used which has not been suitably processed a decay characterized by a 100‐μsec time constant occurs similar to that described by Sproull, and by Matheson and Nergaard. When a Ti anode which has been suitably processed is used the emission decay no longer occurs and contrariwise an emission enhancement is observed. An experimental study of the emission enhancement leads to the postulate that the effect is a consequence of a temperature increase at the emitting surface. A solution to the heat equation subject to boundary conditions appropriate to an oxide‐coated cathode during pulsed current drain is obtained taking into account the Joule heating (J2ρ), emission cooling (Jχ), and radiation at the emitting surface. The analysis shows that for active cathodes significant temperature changes can develop at the surface of the oxide coating during single pulses of microseconds duration. The temperature changes inferred from experimental results presented are in agreement with the predictions of the theory. A numerical solution appropriate for conditions during temperature limited current drain has been obtained for the nonlinear heat equation which shows that a thermal instability can develop in active cathodes passing high current densities. It is proposed that this thermal instability can account for the phenomenon of cathode sparking.

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