Space-charge effect on local electric breakdown of polyethylene at 77 K

Abstract

The local breakdown of polyethylene adjacent to the tip of a needle electrode was studied at liquid-nitrogen temperature. It was found from the results of the local-breakdown voltage of various waveforms that the injected space charge from the needle electrode reduced the intense electric field at the needle tip. The injection depth of the space charge in polyethylene was assumed to be about 10 μm from the characteristic traces of the local breakdown and the light-emitting region at the needle tip when ac voltage was applied. The electric field distribution adjacent to the needle tip was calculated by incorporating an injected space-charge density of 3.87×10−3 C/cm3 (2.42×1016 electrons/cm3) which made the electric field strength at the needle tip equal to zero. The injected space charge modified the electric field such that the electric field strength at the edge of its distribution attained the maximum value. The results of the calculation give a reasonable explanation for the characteristic traces of the local breakdown.