Pulsed HV vacuum breakdown of polished, powder coated, and E-beam treated large area stainless steel electrodes with 0.5 to 7 mm gaps

An investigation of the HV vacuum breakdown between polished. powder coated, and e-beam treated 304L and 316L stainless steel electrodes is described. Tests were performed with 160 ns, 1-cos(omega t), and 260 ns flat-top voltage pulses of up to 500 kV. The high voltage hold-off for the 160 ns pulse was similar to 130 kV/mm for 2 mm gaps for 80-mm diameter polished stainless steel electrodes, and 15% lower for 120-mm polished and e-beam treated electrodes. The longer 260 ns pulse gave 15% lower hold-off for 80-mm electrodes. These electrodes showed voltage hold-off that scaled as the square root of the gap between 0.5 and 7 mm. This total voltage effect has been interpreted in the past as due to accelerated particles. We analyze our data in terms of this mechanism and show that only nanoparticles of molecular size could be responsible. We also discuss how ions or background gas could affect the breakdown thresholds but existing models do not predict square root dependence. We test how extremely fine powers affect hold-off and show that contaminated surfaces have relatively constant reduced breakdown E-fields that intersect the clean-electrode voltage-dependent breakdown at critical gaps defined by the type and quantity of contamination. The hold-off was similar to 55 and 65 kV/mm with copper powder on the cathode and anode for 2 to 6.5 mm gaps, respectively, and similar to 95 and 75 kV/mm for tale powder on the cathode and anode for gaps <3.5 and 6.5 mm. Optical diagnostics show no difference in the light emission from clean and contaminated electrode breakdown arcs.