Progress and results of the High Voltage Vacuum Insulation Tests at the ITER Neutral Beam Test Facility

The Heating Neutral Beam injectors (HNBs) for ITER, required for plasma heating and current drive, are designed to deliver 16.5 MW of 1 MeV Deuterium beam or 870keV Hydrogen beam to the ITER plasma. The injectors rely on a vacuum insulated Beam Source (BS) where the insulation between the BS (acting as a -1MV cathode and having size of the order of tens of m2) and the vacuum vessel (at ground) is provided by a single long gap of about 1m. MITICA (Megavolt ITER Injector & Concept Advancement) is the full-scale prototype of the ITER Heating Neutral Beams (HNBs), devoted to test and optimize the HNB design, under construction in Padova at the Neutral Beam Test Facility (NBTF) hosted by the Consorzio RFX. Recent experiments on High Voltage Bushing for MITICA and reduced-scale models at QST lab in Naka, Ibaraki, Japan, indicate that the HNB insulation might be critical. As a risk mitigation strategy for MITICA/HNB operation, a dedicated high-voltage test campaign has been initiated at the MITICA test bed at NBTF at the end of 2023. In this campaign, full-scale "mock-up electrodes," reproducing almost faithfully the outer geometry of MITICA, are employed to test and optimize the electrostatic design of the MITICA/ITER HNBs as well as to identify suitable procedures for high voltage conditioning of the electrodes. This paper describes the progress in the experimental campaign, shows the main experimental results and provides some lessons learned regarding the operation of such large multipotential vacuum insulated electrostatic system.