Migration and escape of barium atoms in a thermionic cathode and their contribution to noise

Summary form only given. An understanding of the noise mechanisms inherent in thermionic cathodes is necessary insofar as flicker noise from the cathode can degrade amplifier performance. The noise is generally attributed to the fluctuations of the barium sites ("islands") on the cathode surface, from which the electron current is mostly drawn. Cathode lifetime likewise is dependent upon the migration of barium atoms to the surface of the cathode, and their subsequent sputtering off by ion bombardment and evaporation due to high cathodic temperature. To investigate the noise processes associated with these phenomena, we have developed a model of barium migration to the surface and its subsequent removal by bulk and monolayer evaporation and ion sputtering. The diffusion constant, which governs the migration, is determined from the product of the (thermally dependent) probability for transition multiplied by the probability that the target site is vacant. Similarly, the removal rates are governed by tube environmental parameters by barium bulk and monolayer evaporation rates, and by ion back-bombardment on the cathode.