Investigation of pulsed HV breakdown between treated stainless steel and aluminum electrodes with 2 to 8 mm gaps

The paper describes an investigation of the HV breakdown mechanism between stainless steel and aluminum electrodes. The stainless steel electrodes have been treated with pulsed e-beam surface melting (EBEST), hydrogen vacuum furnace firing (HVFF), mechanical polish, and given a chromium oxide coating. The aluminum electrodes with various surface roughness and anodized coatings are also studied. Electrodes are cleaned in a littered clean air environment to minimize effects of air-born micro-particles. Breakdown tests were performed with a 160-ns- FWHM, l-cos(wt), voltage pulse of up to a 500 kV. The tests show that the breakdown voltage drops approximately as the square root of the gap for metal surfaces with gaps between 2 and 8 mm. The hold-off is about 1 MV/cm for 2.5 mm gaps for EBEST and mechanically polished stainless steel and 20% smaller for polished and machined aluminum. The standard model to explain this dependence of breakdown requires minute particles to cross the gap and initiate breakdown by forming plasma 011 impact to an electrode. For the short pulses used here the particles would need to be nanoparticles with only few hundred atoms. Optical diagnostics of breakdown arcs is presented to shed light on the perplexing physics limiting HV hold-off in these tests.