Emission from miniature hollow cathodes

Abstract

Miniature hollow cathodes, internally coated with conventional oxides, have been studied. As visual inspection revealed, oxide deposits at the narrow orifice edge constricted the flow of electrons, and the initial emission from this edge was then spontaneously destroyed by saturation-caused deactivation, owing to positive-ion action. Large hexagonal crystal growth on the outer nickel body was also observed. While the external field causes the electrons to emerge from the cavity as a laminar flow, the conditions in the cavity change abruptly to a gas discharge with large field, and at this point ion bombardment damages the coating. The emission characteristics suggest a strong temperature dependence of the electron flow to an external collector. Although a current density of 10 amp/cm2 at 965°C was obtained where the proximity of the anode worked against early ionization breakdown, a load of 7 amp/cm2 at 920° C was maintained in other cases for 800 hours. Modified orifice arrangements showed that optimum emission is obtained with a sharp orifice edge and solid cavity walls, although no difference in emission was detected between a fully and a partly coated cavity. Grid-controlled operation revealed a decrease in transconductance, owing to field shielding and to the receding virtual cathode, while the focusing quality suffered mainly by the added orifice lens.In Section 3 an interpretation of the mechanisms of current flow to an internal and an external anode is attempted. Since the emission characteristics do not reveal the presence of the underlying emission laws in an explicit form, emission is related to an equivalent diode with dynamic perveance. Hollowness in the beam cross-section at higher currents is interpreted as the result of bunching near the orifice edge, localized current saturation, and of a weaker field along the axis.