Anode hotspot temperature estimation in vacuum gaps under 50 Hz alternating excitations

Anode hotspot temperatures for vacuum gaps subjected to 50-Hz AC excitations are computed. The transient heat diffusion equation is solved using a finite difference method. The effects of nonlinear variation of thermal properties with temperature and phase change are studied using the ANSYS 4.4 finite element package. The peak temperatures are estimated by a seminumerical method. The results of a parametric study of the effects of electrode material, field intensification factor, and radius of the spot are presented. The results for stainless steel, copper, and aluminum anodes indicate that the temperature of hot spots can reach the melting point, so they are the main sources of microparticles. The asymmetry of the prebreakdown current waveform about its own peak, which is caused by thermal instability at the anode, is attributed to the nonlinear variation of thermal properties with temperature. The methods discussed can be used to estimate the size of microparticles originating from thermally unstable regions at the anode. >