Pre-breakdown phenomena between electrodes in vacuum: the role of accumulation points in the onset of microdischarges

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 experimental results and of theoretical models available in literature. This paper describes a specific HV test campaign which is planned to be implemented in the MITICA Vacuum Vessel using mock-up electrodes reproducing in detail the geometry of the Beam Source and Accelerator. The test set-up will incorporate all 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 before the installation of the real components. This approach is intended to reduce the risk related to the HV insulation at 1 MV and, if necessary, to allow to develop the most effective solutions. The tests will be performed in the MITICA Vessel (already available in the NBTF site) both in vacuum and in low-pressure gas. In the paper, the test strategy and requirements are first introduced. Then the sequence of test configurations is defined. Finally, some design solutions for the electrode realization are described and a preliminary plan together with a list of test equipment is given