The poisoning and reactivation kinetics of uncoated tungsten-based dispenser cathodes exposed to water vapor

Water vapor is typically a major component of the ambient in a demountable vacuum system. It can also be present in the ambient of a sealed off microwave tube. In each case the presence of water vapor will have a marked effect on the performance of a tungsten-based dispenser cathode. For example, when the cathode is not operating, water vapor poisons the cathode surface, leading to a significant reactivation delay if a fast turn-on of a few seconds is required. A background pressure of water vapor also affects the operation of these cathodes, preventing the full emission capability of the cathode from being realized. Measurements are discussed in this paper pertinent to the poisoning experienced by such cathodes when exposed to low pressures of water vapor at temperatures between 300 and 1600 K. The experiments were performed under conditions such that the poisoning flux at the cathode surface was accurately known. The effect of different poisoning fluxes is presented, together with the influence of different cathode impregnant types. As the poisoning temperature is increased from room temperature the poisoning due to a given exposure of water vapor becomes more severe. With the poisoning fluxes used here, no reduction in the poisoning was observed until the cathode was exposed at elevated temperatures that could be far higher than typical operating temperatures. It has been found that, for a given cathode type and poisoning flux, there is a specific temperature above which the cathode will operate in an unpoisoned state. As the poisoning flux is lowered or the cathode dispensing rate is increased, this temperature decreases. Surface analysis results are presented and the mechanisms of the poisoning and reactivation process discussed. The reactivation results are analyzed in terms of the preferred cathode types for operation in nonideal vacua and the partial pressure of water vapor that can be tolerated.