Characterization of X-ray Events 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 centimetres gap, can achieve HV values up to 400 kV each. The conditioning process consists of 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; high energy X-rays (up to hundreds of keV) and a global increase of gas emission (in particular H2 and CO2 are detected by the Residual Gas Analyser) are measured in correspondence to the current events. Three new Xrays detectors have been recently installed: a small LYSO (4 × 4 × 20 mm3), a LaBr3 (1" x 3/4") and a thin YAP (1"x 2 mm) scintillating crystals, coupled to photomultipliers. They all are small sized scintillators, with very fast pulses (40-100 ns) in order to minimize pile-up effects during the high intensity discharges. However, the high Z and densities guarantee a full energy absorption of the X-rays (they can measure up to 500 keV), with a significant probability, in spite of the small sizes. Energy resolutions are less than 9%. In this contribution we present a characterization of the micro-discharge dynamics occurring during the conditioning phase, focusing on the new details uncovered via the new diagnostics.