HV Holding in Vacuum, a Key Issue for the ITER Neutral Beam Injector

This overview summarizes the R&D activities aimed to secure for the ITER Neutral Beam Injectors a stable voltage holding, one of the most challenging issue for the NBI systems operation. The huge size of the accelerator and the highest voltage (1 MV) ever used in such systems make prominent some aspects that are, in other application, almost not influent or well under control, like the Area Effect, Total Voltage Effect, the highly complex electrostatic configuration and, last but not least, the cleaning degree of the electrodes surfaces. A clear explanation of the mechanism leading to the voltage breakdown in these conditions is far from being well established. But in the last years a great effort have been deployed by the NBI community to reliably achieve 1 MV voltage holding that is required for the NB injectors for ITER. Theoretical models and new tools have been derived to explain the mechanism of voltage conditioning and for the design of the complex electrodes geometries, that supersede the simple electric field optimization. The 1 MV across a single gap has not yet been achieved but, on the basis of the results so far obtained, it is now considered to be achievable.