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 cathode surface and it influence on electron emission from cathode. The breakdown strength with coated cathode is found to indicate higher value than that with coated anode, which is also discussed in terms of the electron emission.