Study on physics mechanism of vacuum discharge toward electrostatic accelerator of the energy higher than 1 MeV

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Using a MeV accelerainr being developed for neutral beam injector ofITER, experiments on vacuum insulation have been carried out to establish physics and engineering basis toward general purpose electrostatic accelerator of the beam energy more than 1MeV.● A large bore FRP (glass fiber reinforced epoxy) insulator was tested its applicability toward the accelerator of more than 1MeV. Prevention of surface flashover was demonstrated by reducing the stress at cathode side triple junction (interface of vacuum metal and dielectric insulator) below 1kV/mm.● Data base on vacuum insulation was established for a range of vacuum gap width in real accelerator geometries.(1) Achievable voltage as a function of gap length seems to follow to Clump theory(2) Achievable voltages in the real accelerator geometries are about a half of that achieved with a quasi-ideal Rogowsky electrode, even the vacuum gap length were the same in both cases.(3) The deterioration of voltage holding in the real accelerator geometries are probably due to local electric field concentration in the accelerators, such as corner of the grid support or junction between grids and grid support structures.● Extrapolation of achievable voltage in the accelerator at various gas pressure suggests possibility of vacuum and glow discharge prevention even the voltage was 2MV by maintaining the pressure at 0.02Pa, though necessary vacuum gap length would be as long as 2m.