X-ray Micro-Discharges Fine Dynamics in a Vacuum High Voltage Experiment

The High Voltage Padova Test Facility (HVPTF) is an experimental device for investigating High Voltage (HV) insulation in vacuum, in support of the realization of MITICA, the prototype of a neutral beam injector for ITER. Inside a high vacuum chamber, two stainless steel electrodes, separated by a few centimetres gap, can achieve a voltage difference up to 800 kV. During the conditioning process of the electrodes, current micro-discharges (MD) and associated X-rays are observed, along with a global increase of gas emission (H2 and CO2 are detected by the Residual Gas Analyser). During the last five years, different X-rays detectors have been installed on HVPTF, with the aim to investigate the physical processes behind the conditioning. In this contribution, the fine dynamics of the micro-discharge phenomenon is studied with the purpose to sketch a possible physical interpretation of the phenomenon. In particular, the time evolution, during the electrode conditioning, of the MD fine dynamics is evaluated in relation with the increasing asymmetry between the MD peak values collected by the positive and the negative electrodes. The possible (increasing) contribution of the vacuum chamber, operating as a third electrode, is investigated, by means of COMSOL simulations.