Decay of emission from an oxide-coated cathode due to adsorption of matter liberated from the anode

The paper examines critically the process of decay of emission current of an oxide-coated cathode when poisoning agents liberated from the anode by electron bombardment are adsorbed on the emitting surface. Two types of adsorption process, mobile and immobile, are considered. The final equations for the decay process which appear in integral form in the two types of adsorption are solved numerically by estimating the probable values of the constants involved. An approximate analytical solution of the Integral is also given for the mobile type of adsorption. It is found that the decay resulting from mobile adsorption is generally of short duration and is likely to be spontaneously recoverable, but that resulting from immobile adsorption is of long duration and is probably permanent. For the former case the time constant of decay is about 10 ¿3sec, provided that the poisoning agent is oxygen at a pressure of the order of 10¿7 Hg and temperature of the order of 1,000°K, and that the adsorption process is confined to a fraction (about 0 1) of the actual cathode area. The hypothesis that a decay process resulting from mobile adsorption is responsible for the differences between the pulsed and the d.c. behaviour of an oxide-coated cathode is examined in general terms and is found to deserve consideration.