Demonstration of 1 MV insulation for the vacuum insulated beam source in the ITER NB system

For the realization of the ITER neutral beam (NB) system, a measure to achieve the acceleration voltage of 1 MV for the vacuum insulated beam source (VIBS) has been developed. In addition, the design basis for 1 MV vacuum insulation has also been developed by integrating previous empirical scaling of the voltage holding capability for plane and coaxial electrodes and new scaling for corner region where locally-concentrated electric field is generated. The scaling clarified that a product of electric field and voltage obtained by the electric field analysis in the beam source vessel was much higher than the allowable level. Therefore, in order to improve the voltage holding capability, electrostatic shields having intermediate potential have been proposed. Consequently, it was found that the VIBS needs to be surrounded by more than three intermediate shields instead of a single gap insulation of 1 MV. Reliability of the design basis of the developed intermediated shields has been experimentally verified by improving the 1.3 m single gap insulation in the voltage holding test. As a result, the sustainable voltage has been significantly improved from 0.7 MV with the single gap to 1 MV with the shields up to the power supply limit. This result ensures the realization of the 1 MV VIBS in the ITER NB system.