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) Direct Current insulation in a vacuum, in support of the development of the prototype of a neutral beam injector for ITER, named MITICA. Inside a high vacuum environment, two stainless steel electrodes, spaced by a few centimeters gap, can achieve a voltage difference of up to 800 kV. During the electrode conditioning process, small current bursts, known as micro-discharges (MDs) occur, accompanied by the emission of X-rays, and of an overall increase in gas emission measurement. This study focuses on the fine dynamics of the MD phenomenon measured by the current signal and by the X-ray detectors, in an attempt to provide a possible physical interpretation of this mechanism. In particular, it appears that the MD fine dynamics are related to the growing asymmetry between the MD current values collected by the anode and cathode, observed during the electrode conditioning. The role of the vacuum chamber, acting as a third electrode, is investigated by an updated toy model, which evaluates the different current contributions collected from the two polarized electrodes, particularly those generated by secondary electron (SE) emission. The ratio of anode to cathode currents computed by the toy model closely matches the experimental one. Further experimental observations supporting the hypothesis of anode gas emission and expansion are discussed. Finally, a first study about the statistical distribution of the time intervals between successive MD suggests the occurrence of almost two different trigger mechanisms, evolving during the electrode conditioning process.