Effect of a thin insulation film on thermal bubble-triggered breakdown phenomena in liquid nitrogen

The effect of a thin insulation film on breakdown phenomena in liquid nitrogen in the presence of thermally induced bubbles is investigated with a cylinder-to-plane electrode. Bubbles were produced by a heater mounted in the cylinder, which is partially covered with an insulation film. The results show that the film's effect on the breakdown voltage is insignificant until a pore is formed on the film by a previous breakdown, but it becomes substantial after pore formation. The reduction rate in breakdown voltage after the pore formation depends on the gap length and heater power. The minimum breakdown voltage drops to the magnitude of the breakdown voltage in the gaseous phase at normal boiling temperature. The breakdown mechanism is discussed on the basis of bubble observation and numerical calculation of suspended-bubble motion.