Role of Electron Stimulated Desorption in the initiation of HVDC vacuum arc

The present paper describes and analyzes the recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF), the laboratory aimed at supporting the development of the prototype for the ITER Neutral Beam Injector. Evidences of Electron Stimulated Desorption of gas released from anodic surfaces have been observed during the onset of high voltage Direct Current vacuum discharges between stainless steel electrodes, separated by vacuum gaps of a few millimeters and with voltages of about 200 kV. The count of x-ray photons, produced by the accelerated electrons impinging the anode surface and observed as breakdown precursor in the 40% of the cases, was analyzed together with the images taken by an ultra-high speed camera during the onset of breakdown events. Electron stimulated desorption of neutral gas from the anodic surface appears to be a candidate for the voltage breakdown. The dynamic of the observed phenomena has been compared with the results obtained by a numerical model. The latter assumes a Townsend like discharge which develops following the emission of gas from the anode surface. The comparison shows good agreement between the experimental data and numerical simulations.