Life-limiting mechanisms in Ba-oxide, Ba-dispenser and Ba-Scandate cathodes

Sufficient thermionic cathode life of several years in a vacuum electron tube environment at increased emission current density is one of the key parameters in advanced tube development. When improvements are investigated, accelerated life tests are applied in order to quantify their impact. Hence the investigation of cathode life limiting effects is the basis of accelerated life predictions. A quite general dependency of cathode life for different types is given by the dependence on operating temperature, due to the fact, that a lot of processes involved scale with temperature, such as chemical supply reactions, transport, diffusion, desorption and evaporation processes. Typically these processes are accelerated when the temperature is increased, and hence lifetime decreases. Other life limiting mechanisms are changes induced in the cathode when drawing current, which can be intrinsic changes, which are most pronounced in oxide cathodes and usually are irreversible, and/or surface changes by ion bombardment, where the ions are created by the extracted electrons via ionization of the residual gas. The removal of surface monolayers containing Ba by ion bombardment usually is reversible especially for Ba dispenser or I (impregnated) cathodes (by resupply of Ba), if the ion current is reduced for example, by lowering the residual gas pressure by continuous pumping. The third class of life limiting mechanisms is given by the presence of poisoness chemicals usually in form of gases in vacuum tubes, which can either affect chemical supply (dispensing) reactions or directly poison emission by adsorption of high work function surface monolayers on top of the emitting surface or f.i. oxidize base metal surfaces. This influence usually is the strongest in the activation and early life stage, where the amount of poisonous gases, e.g. containing oxygen, is highest, but is then continuously reduced by pumping. Usually a clean handling of the vacuum components and a good vacuum processing (baking and degassing, efficient pumping) is needed to avoid detrimental influences on cathode life.