Effect of magnetic field on voltage holding in the MITICA electrostatic accelerator

MITICA is the complete full-scale prototype of a 17 MW Heating Neutral Beam Injector for ITER. This experimental device, presently under construction in Padova, includes a negative Ion Source (H- or D-), and an electrostatic Accelerator (1 MV, 40 A, 3600 s). Voltage holding is recognized to be one of the most critical issues for the 1 MV accelerator operations, not only due to the complex multi-stage electrostatic accelerator structure, but also for the presence of magnetic field, which is necessary for deflecting the co-extracted and secondary electrons as early as possible, before they are accelerated. The required magnetic field is produced by a combination of several sources, such as permanent magnets and current-carrying conductors. In order to avoid breakdowns, the design of the accelerator shall guarantee that electrostatic configuration and the pressure distribution correspond to an operating point located on the left branch of the Paschen curve, i.e. where the H2 gas pressure multiplied by the largest electrode distance (p.d) is always below a certain limit, which is 0.1-0.3 Pa.m. Actually, the presence of the magnetic field might reduce this limit, moving the left branch of the Paschen curve towards the operating point thus affecting voltage holding even during the conditioning phase. In order to support the design of MITICA at low gas pressure and in the presence of magnetic field, an experimental campaign has been carried out at the High Voltage Padova Test Facility.