Preparation of the vacuum insulation tests on the MITICA 1 MV electrostatic Accelerator

MITICA is the prototype of the ITER Heating Neutral Beam Injector (NBI), which is designed to generate a 40 A beam of Hydrogen/Deuterium negative ions, to accelerate the ions up to an energy of 1MeV and then to neutralize them, producing a neutral beam of about 16.6 MW power. The design of MITICA is as far as pos- sible identical to the design of the ITER HNB, except that in the ITER NBI, the neutral beam will be directed to the plasma in the Tokamak vessel via a duct, whereas in MITICA the neutral beam will be directed to a water-cooled calorimeter. The NBI system will consist of a Vacuum Vessel (AISI304L, overall dimensions 15 × 5 × 5 m3), where the negative ion Beam Source, which is the most complex component of the NBI system, will be installed, together with other components (i.e. gas-cell Neutralizer, residual Ion Dump and Cryo-Pumps). The negative ion Beam Source includes a Plasma Chamber (with a gas injection system and auxiliaries for the production of negative Hydrogen/Deuterium ions) and a multi-stage Electrostatic Accelerator, constituted by a series of 7 metallic grids each having 1280 apertures. The Plasma Chamber and the metallic grids of the multistage electrostatic Accelerator will be kept at different electric potentials ranging from about – 1MV (Plasma Chamber, Plasma Grid and Extraction Grid) to ground potential (Grounded Grid) in steps of 200 kV. The Vacuum Vessel will be grounded. The insulation between electrodes having different electric potential will be provided by gaps in vacuum (filled with low pressure Hydrogen during operation) and by alumina post insulators. The voltage holding capability of the MITICA Beam Source at 1 MV is a very challenging issue, which could not be fully addressed so far on the basis of the theoretical models and experimental results available in literature. This paper describes a specific HV test campaign to be implemented in the MITICA Vacuum Vessel using a mock-up of the Beam Source. The tests have been designed with the aim of incorporating the essential features both of the single-gap and of the multi-stage insulation, so as to obtain reliable data on voltage holding at 1 MV and, if necessary, to focus on the most effective solutions. The tests shall be performed in the MITICA Vessel (already available in the NBTF site) both in vacuum and in low-pressure gas, using a specific Test Power Supply, in parallel with the construction and assembly of the MITICA Beam Source. In the paper, the test objectives and requirements are first introduced. Then the strategy and test configurations are defined. Finally, some design solutions for the electrode realization are described and preliminary plan together with a list of test equipment is given.