Dielectric breakdown in liquid helium

Breakdown characteristics in liq.He are studied and compared with those of other cryogenic liquids such as liq.Ar, N2. Breakdown strength of liq.He is much higher than that of liq.Ar but lower than that of liq.N2. The formative time lag of breakdown in liq.He is much shorter compared with the electron transit time between electrodes expected from low field electron mobility. Under non-uniform field condition, the breakdown strength of negative needle is always lower than that of positive needle and also even than that of parallel electrode system with uniform field. Breakdown strength is strongly dependent on the work function of cathode metal. Pure liq.He shows higher breakdown strength than impure liq.He. These facts indicate that electron injection from cathode plays most important role in the breakdown process in liq.He. (author)