STATISTICAL ANALYSIS OF CURRENT AND X-RAYS SIGNALS FOR A VACUUM HIGH VOLTAGE HOLDING EXPERIMENT

The High Voltage Padova Test Facility (HVPTF) is an experimental device for investigating HV insulation in vacuum, in support of the realization of MITICA, the prototype of a neutral beam injector for ITER. The facility investigates the physical phenomena underlying voltage holding in vacuum, such as the mechanisms causing breakdowns and the electrode conditioning process, along with testing technical solutions to increase the breakdown threshold. Inside a high vacuum chamber, two stainless steel electrodes, separated by a few centimeters gap, can achieve HV values up to 400 kV each. The conditioning process (typically) consists in the gradual increase of the breakdown voltage in time, until the system achieves a saturation value. Between two consecutive breakdown events, current micro-discharges involving the electrodes are observed in correspondence of high energy X-rays. A global increase of gas emission is measured too, in particular H2 and CO2. In this contribution we present a statistical analysis of micro-discharges events during the conditioning phase, in terms of frequency, amplitude and their occurrence with respect to the applied voltage. The highly time resolved X-ray diagnostics (single events are detected) and the precise timing with the current signal allow a deep characterization of the events occurring during high voltage conditioning.