Effects of vacuum pressures on the performance of a velvet cathode under repetitive high-current pulse discharges

In this paper, the effect of vacuum pressures on the performance of a velvet cathode under repetitive high-current pulse discharges was experimentally investigated. A series of tests were carried out on a vacuum diode, powered by a ˜300 kV, ˜5 ns, ˜100 Ω and 1-300 Hz pulse generator. Typical results of discharge waveforms, vacuum pressure rises, and SEM pictures for total 1000 shots are presented and head-to-head compared at 30 Hz, 100 Hz, and 300 Hz, respectively. With the pulse repetition rate (rep-rate) increasing, it was found that the equilibrium pressure during the pulse series from 30 Hz to 300 Hz increased hyper-linearly that led to diode pressures of ˜0.01 Pa-˜1 Pa. When the pressure exceeded 0.1 Pa, the beam current degradation was observed in terms of pulse-to-pulse instability increment and pulse width decline. Velvet curling and erosion also became apparent. The nature behind these results was discussed closely related to the gas ionization due to such high background pressures. Technical methods to improve the vacuum condition for a repetitive operated velvet cathode were also proposed.