Pre-breakdown phenomena between electrodes in vacuum: the role of accumulation points in the onset of microdischarges

The reliability of HV insulation across a single gap, in vacuum and low-pressure gas, is known to be a critical issue to be addressed in the development and realization of the electrostatic accelerator for the ITER Neutral Beam Injector (NBI).This paper describes recent experimental results obtained at the High Voltage Padova Test Facility (HVPTF) with the aim of developing an adequate high-voltage insulation system for the MITICA accelerator, the prototype for the ITER NBI.The experiments were carried out on a double-polarity configuration, using sphere-to-sphere electrodes, separated by 30 mm gap. The results showed that the onset of microdischarges is not solely dependent on the cathodic electric field, but also depends on the applied voltage, which determines the energy of the emitted electrons hitting the anode. Furthermore, this phenomenon seems to be dependent on a mutual exchange of charged particles between specific accumulation points on both electrode surfaces.The outcome of these experiments is reported and analyzed with both analytical and numerical models.