Onset voltage for corona pulses in gaseous Ar under high pressure and in liquid Ar

HV, applied to a point cathode (radius of curvature ~1 to ~8 μm) in a nonpolar liquid, brings about a regular current pulse regime above a threshold value V_s. This regime is similar to the Trichel pulse regime for corona in air. We present results of an investigation of this threshold voltage V_s in argon over a wide range of densities. In gaseous argon, the V_s(P) values increase with pressure. In the liquid, V_s is independent of external pressure. The results show that V_s is mainly a function of fluid density N and cathode tip radius τ_p. The theoretical analysis of the phenomenon is carried out according to the criterion ∫α(x)dx constant, where a(x) is the Townsend ionization coefficient. It is shown that if the dependence of V_s on N for low density gas is extrapolated into the region of liquid densities, it greatly exceeds the experimental values. The dependence of V_s on the fluid number density ~ is derived. Its variation is found to be non-monotonic at very high N values