DEMONSTRATION OF 1 MV VACUUM 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) have been developed. In addition, the design basis for 1 MV vacuum insulation has 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 empirical scaling clarified that a product of electric field and voltage in the beam source vessel was much higher the allowable level. Therefore, in order to improve the voltage holding capability, electrostatic shields having intermediate potential has been proposed. Consequently, it was found that the VIBS needs to be surrounded by more than three intermediate shields instead of single gap insulation to realize the voltage holding capability of 1 MV. Reliability of the design basis has been experimentally verified in the voltage holding test by using the high voltage bushing with the developed intermediated shields. As a result, the sustainable voltage has been significantly improved from 0.7 MV to 1 MV up to the power supply limit. This result ensures the realization of the 1 MV VIBS in the ITER NB system.27th IAEA Fusion Energy Conferenc