  MITICA (Megavolt ITER Injector & Concept Advancement), the full scale
prototype of ITER Heating Neutral Beam, is under realization at the Neutral
Beam Test Facility (Padova, Italy). It is designed to deliver 16.5 MW to ITER
plasma, obtained by accelerating negative Deuterium ions up to 1 MeV for a
total ion current of 40 A and then neutralized. MITICA Acceleration Power
Supply is composed of several non-standard equipment, beyond industrial
standard for insulation voltage level (-1 MVdc) and dimensions. Voltage
withstand tests (up to 1.265 MVdc) have been performed in five subsequent steps
(from 2018 to 2019), according to the installation progress, after connecting
equipment belonging to different procurements. During integrated commissioning,
started in 2021, two breakdowns occurred in a position of the HV plant not
still identified, so they could be occurred either in air or in SF6. To
identify the locations of possible weak insulation points, the existing
diagnostics for partial discharge detection (the precursor of breakdowns) as a
first step have been improved on air-insulated parts by consisting in a set of
instrumentation, like capacitive probes and off-the-shelf instruments for AC
application (acoustic and electromagnetic sensors). The paper deals with the
instruments qualification to assess their suitability for DC usage and then
with the investigation performed in MITICA, in particular: 1) sensitivity
assessment campaign, with artificially produced corona effect to identify the
minimum threshold of each diagnostics 2) voltage application to MITICA plant,
moving the instrumentation around equipment and increasing progressively the
voltage looking for corona phenomena to identify possible weak insulation
points.
