Emission Fluctuations of Tungsten-Based Barium Dispenser Cathodes

Abstract

Emission fluctuations in five different types of tungsten-based barium dispenser cathodes have been measured as functions of both frequency and current density. Two of the cathodes were of the L variety and the remainder of the ``impregnated'' variety. The L cathode spectra show that the emission fluctuations are caused by fluctuations in the number of adsorbed atoms on the tungsten emitting surface. The amplitude of the fluctuations is governed by the average surface coverage and theoretical studies indicate that the shape of the spectra is governed by the diffusion processes by which the surface is replenished. The results from the impregnated cathodes indicate that the contribution of the impregnant in the pore ends to the total noise is only significant when the pore ends supply an appreciable fraction of the total emission from the cathode, the majority of the surface, which is barium activated tungsten, behaving as an L cathode surface.