Nonuniform emission distribution on a scandate impregnated cathode

The effect of barium supply into a W–Sc2W3O12.-coating film on the emission distribution of a scandate cathode has been investigated. Scandium concentration on the cathode surface was increased by activation heating. This increase was coincident with the rise in emission current by heating. However, when the film was coated on the unimpregnated tungsten pellet, the scandium concentration did not increase. These results indicate that the barium supply into the film is necessary for liberating scandium atoms from the scandium tungstate in the film and for forming a monolayer of Ba–Sc–O on the surface. We measured the heating-induced change in the scandium content of the film. The result showed that the scandium content decreased only in the part of the film that covered the impregnated areas. No decrease was observed in the film that covered tungsten metal. The liberation of scandium is considered to take place only in the film to which barium atoms are mainly supplied. This nonuniform scandium liberation in the film will cause nonuniformity in the Ba–Sc–O monolayer on the cathode surface, which in turn causes emission nonuniformity. q1999 Elsevier Science B.V. All rights reserved