Size effect and statistical characteristics of dc and pulsed breakdown of liquid helium

Breakdown time lag in liquid helium is measured over a wide range of electrode sizes and pulsed electric field strengths. The breakdown time lag and dc breakdown strength are statistically analyzed by using the Weibull distribution function and weak link theory. It is found that the time lag depends on both electrical stress and the electrode surface area stressed above a critical level. It is supposed that breakdown triggering electrons are generated by field emission phenomena at small protrusion tips on the cathode surface. In higher external electric fields, a less sharper protrusion emits initial electrons with a shorter time lag and may become responsible for liquid breakdown. A theoretical equation is proposed to predict the electrode size and electrical stress dependency of the breakdown time lag, based on Fowler and Nordheim theory. It is shown that the equation is consistent with the Weibull distribution function under multiple stress of electric field and stressing time.