X-RAY MEASUREMENTS REVEAL THE FINE DYNAMICS OF CURRENT MICRO-DISCHARGES 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 an HV difference up to 800 kV During the conditioning process of the electrodes, current microdischarges and the presence of associated X-rays are observed, along with a global increase of gas emission (H2 and CO2 are detected by the Residual Gas Analyser). Different X-rays detectors have been installed during the last five years on HVPTF, with the aim to investigate the physical processes behind the conditioning by means of X-rays analyses. In this contribution, new information provided by the different devices are presented: in particular, the identification of a fine dynamics in the micro-discharge phenomenon is shown. Moreover, the growing contribution of the vacuum chamber during the electrodes conditioning, operating as a third electrode, is investigated. A possible physical interpretation of the micro-discharge dynamics is sketched. Finally, the new set of LYSO scintillators is described and preliminary results about the recent electro-polished electrodes campaign is introduced.