Progress and results of the first high voltage vacuum insulation tests in MITICA

The Heating Neutral Beam injector (HNB) for ITER includes a Beam Source (BS) surrounded by a single vacuum gap, operating up to 1 MV total voltage. MITICA (Megavolt ITER Injector & Concept Advancement) is a full-scale prototype of the ITER Heating Neutral Beams (HNBs), devoted to test and optimize the design, under con- struction in Padova at the Neutral Beam Test Facility (NBTF) hosted by the Consorzio RFX. Recent experiments on HV bushing for MITICA and reduced-scale models at QST lab in Naka, Ibaraki, Japan, indicate that the breakdown probability of the HNB insulation might be critical. As a risk-mitigation measure for the HNB operation, a specific high-voltage test campaign has been undertaken in the MITICA test bed at NBTF. Full-size "mock-up electrodes", reproducing the outer geometry of the MITICA/ HNB accelerator are used, allowing swift implementation and test of different electrostatic configurations in realistic conditions. The main objectives are: (1) to verify the performance of the vacuum electrode insulation in the present design and: (2) to assess the minimal set of necessary modifications, possibly including the introduction of an inter- mediate electrostatic shield. All tests are foreseen during the time window before the installation of the real beam source in the MITICA vacuum vessel. The paper describes the test strategy and the ad-hoc development of diagnostics, including current sensors in vacuum, integrated video camera system and data acquisition network, devoted to identify the most critical regions where an insulation breakdown could occur inside the large vacuum vessel. Then, the results of the first test campaigns are presented and discussed, in terms of maximum withstand voltage of the insulation, in relation to electrode geometry, applied voltage rise time and vacuum conditioning procedure.