Dielectric Breakdown of Liquid Helium and Influence of Electrode Coating

Polarity and conditioning effects of breakdown in liquid helium and the influence of electrode coating by solid insulator are studied. The breakdown strength under positive needle condition decreases with repeating breakdown events. However, it increases under negative needle condition, which can be explained in terms of severe damage of the cathode surface and its influence on electron emission from the cathode. The breakdown strength with coated cathode is found to indicate higher values than that with the coated anode, which is also discussed in terms of the electron emission. The breakdown strength of insulating polymer film in liquid helium is lower than that in liquid nitrogen, but becomes comparable by covering of the sample and electrodes with 3-methylpentane, which can be explained by the environmental effect in liquid helium. The barrier effect in liquid helium is also studied.