Conditioning Effect of Dielectric Breakdown in Liquid Helium

In a non-uniform field with a needle-plane electrode system, the breakdown voltage V b of a positive needle decreases as the number of breakdown events N increases, but that of a negative needle increases slightly with N. In a uniform field, the breakdown voltage decreases with increasing N. However, when the polarity of the voltage pulse is reversed after many breakdown events, V b increases suddenly and then again decreases slightly with increasing N, tending to saturate. These conditioning effects can be explained by the increase of the electron injection from the cathode due to the formation of a rough surface by the previous breakdown events.