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 Direct Current 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 Xrays 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 microdischarge phenomenon is studied with the purpose to sketch a possible physical interpretation of this mechanism. In particular, the time evolution, during the electrode conditioning, of the MD fine dynamics seems to be related with the increasing asymmetry between the MD current 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 a toy model: the different current contributions collected by the two electrodes are evaluated, in particular those generated by secondary electron emission. The rate between anode and cathode currents evaluated by the toy model is in good agreement with the experimental one.