The ZrO/W(100) Schottky cathode: Morphological modification and its effect on long term operation

Abstract

An investigation of the change in various geometric and emission parameters over the life cycle of nine ZrO/W(100) Schottky cathodes was performed. The initial shape for all emitters was “Stage 1,” i.e., a large square, low work function (100) facet intersected by four (112) and (110) facets. The ending shape for all emitters was “Stage 0,” i.e., a round central (100) facet with four side (110) planes. Change in geometric parameters such as emitter apex radius r, cone angle α, central (100) facet size f, and neck size L were measured at the beginning and after 10 000 to 18 000 h of cathode operation. For a particular end form (or Stage), f scaled linearly with emitter radius r. In addition, emission parameters such as the axial angular current density I′, total current It, work function ϕ, electric field F, and extraction voltage Ve were monitored continuously. Of the nine sources investigated four experienced collapsing net planes (ring collapse) on the central (100) crystal facet. The emitters with ring collapse events had the largest increase in r and a corresponding reduction in the field factor β = F/Ve. For the most part β, ϕ, and r remained unchanged for those emitters not having ring collapse events. The ending I′ values mostly decreased at a constant Ve, but less so for those emitters with no ring collapse. Experimental results showed that for I′ ≤ 0.3 mA/sr and T = 1800 K ring collapse occurs. In contrast, for I′ ≥ 0.3 mA/sr ring collapse does not occur. For all emitters investigated, the parameters exhibiting the largest increase over the course of the life test were L and It, whereas α was typically unchanged.