Magnetic Field Effect 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 owing to the complex multistage electrostatic accelerator structure, but also for the presence of magnetic field, which is necessary for deflecting the coextracted 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. To avoid gas breakdown between electrodes, the design of the accelerator shall guarantee that the electrostatic field configuration and the pressure distribution correspond to operating points located far enough on the left side of the well-known Paschen breakdown curve. For this reason, MITICA has been designed so that the (H₂ or D₂)gas pressure multiplied by the distance between electrode (p · d) shall not exceed 0.1-0.3 Pa · m. However, indications have been found in literature that the presence of magnetic field might shift part of the left branch of the Paschen curve more to the left, thus reducing the above-defined limit and possibly affecting the voltage holding criteria to be used in MITICA design. 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; during this campaign, a field distribution similar to that expected in MITICA has been realized. The magnetic field has been produced using permanent magnets located inside the electrodes or outside the vacuum tank. This paper describes the test assembly, procedure adopted, and the experimental findings; the results - ave been also successfully compared with simulations of the breakdown process. In certain magnetic field configurations, a clear effect has been recognized, indicating a nonnegligible shift to the left of the lower part of the Paschen Curve.