High current density dispenser cathode surface activation phenomena in long life vacuum electron devices

Summary form only given, as follows. Operational life of most vacuum electron devices is dependent on the function of the electron source used. In most fast wave and slow wave devices dispenser cathodes, which are made of porous tungsten impregnated with barium calcium aluminate, are used as the electron source. These are often overcoated with osmium ruthenium to lower the surface work function. The mechanisms involved in these electron sources are rather complex. When heated the impregnant reacts with the tungsten in the pores releasing barium which must travel through the pores by Knudsen flow to the surface of the cathode. Once there the barium uniformly covers the surface, by surface migration, up to a monolayer. This coverage and the metal substrate composition determine the surface work function. A very necessary process called activation must occur before the barium uniformly covers the surface and remains there for a sufficient time to provide the lowered work function. The key to this is the formation of a dipole of barium over oxygen over the metal substrate from which the electrons are pulled. The activation process is essentially a chemical rather than an electrical process. What the process must accomplish is the removal of any species that would either compete with oxygen for the dipole site or react with oxygen at the site and have the by-product evaporate.