Summary of 15 Years of HVDC Experiments and Tests in Vacuum at the High-Voltage Padova Test Facilities

This article summarizes 15 years of experimental activity at the High-Voltage Padova Test Facilities in support of the design and commissioning of the ITER neutral beam injector (NBI). High-voltage DC tests up to 800 kV were carried out in high and medium vacuum on electrode gaps ranging from a few millimeters to 150 mm. Systematic experiments on stainless steel electrodes in high vacuum validated a probabilistic model for predicting the maximum hold-off voltage of generic multielectrode systems and confirmed the presence of a significant total-voltage effect for gap lengths above a few centimeters. The spatial distribution of power dissipation during microdischarges in large vacuum gaps suggests charge accumulation points and mutual charged-particle exchange between electrodes to interpret the experimental observations. Time-resolved X-ray measurements indicate that, in a significant fraction of vacuum breakdown events, bursts of electrons impacting the anode act as precursors, with electron-stimulated desorption playing a key role in discharge development and voltage collapse. In medium vacuum, tests in Ar, H2, and N2 characterized the dependence of the hold-off voltage with pressure up to the left branch of the Paschen curve and clarified the influence of static magnetic fields on Townsend discharge. Long-duration tests under stationary electric fields up to 60 kV/mm revealed irreversible switch-on phenomena with microampere-level currents on short-gap configurations. Finally, acceptance tests on solid insulators, optical fibers, and spark gaps are reported, and the main lessons learned and open issues are discussed.