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

The achievement of burning plasma in ITER will require at least 33 MW of additional heating transferred to the plasma core by means of two 16.5 MW - 1 MeV injectors (N-NBI - Negative ion based - Neutral Beam Injector) of H or D atoms, with an extracted negative ion current of 40 A (i.e. 40 MW of extracted beam). A third Injector is foreseen as a future upgrade, to increase the power delivery to the ITER plasma up to 50 MW The N-NBI record so far obtained belongs to the Injector of JT60-U, in which have been extracted 17.4 A of negative current, accelerated up to 0.41 MeV. It is evident that the performances required for the ITER N-NBI exceed by far the present capability: for this reason, the Neutral Beam Test Facility - NBTF is under construction at the Consorzio RFX - Padova (I) premises, aimed at realizing the full scale prototype of the ITER N-NBI. Among other fundamental achievements (e.g. extracted current uniformity, efficient ion neutralization, long pulse duration), the success of the experimentation of the NBTF requires the achievement of a stable and easily obtained 1 MV voltage holding while negative ions are extracted and accelerated. The electrostatic configuration of the ITER and NBTF N-NBIs is extremely complex. It includes 5 main electrodes at high potential: five accelerating grids and a negative ion source, operating respectively at -0.2, -0.4, -0.6, -0.8, -1 MV with respect to the containing vessel. All electrodes have large surface (up to 10 m2 for the ion source, which constitutes the cathode); gap lengths are in the order of 100 mm between accelerating grids and a minimum of 980 mm for the -1 MV-to-ground gap. In such a peculiar situation, the full voltage attainment and holding is a key issue, being affected by Total Voltage Effect (TVE), Area Effect, residual gas pressure and electrode surface conditioning. This contribution aims to give an overview of the R&D so far carried out, in the Fusion Research Laboratories in particular the QST (formerly JAEA) lab in Naka, Ibaraki (Japan) , in the IRFM - CEA lab in Cadarache (France) and of the Italian Consorzio RFX lab in Padova. After a discussion of the present state-of-the-art about the voltage holding in the NBI accelerators, a description of the near future plan to achieve reliable voltage holding in the ITER N-NBI is given. The experiments will be performed in the labs of QST, RFX and, in a second stage, at the NBTF Facility in Padova. Finally, an example of spin-off of these researches is also given.