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

For the realization of the ITER neutral beam (NB) system, a method for achieving the acceleration voltage of 1 MV for the vacuum insulated beam source (VIBS) has been developed. Additionally, the design basis for the 1 MV vacuum insulation has also been developed by integrating previous empirical scaling of the voltage holding capability for plane and coaxial electrodes with new scaling for the corner region where the locally-concentrated electric field is generated. The scaling clarified that a product of electric field and voltage obtained by 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 shielding up to the power supply limit. This result ensures the realization of the 1 MV VIBS in the ITER NB system

Development of long-pulse high-power-density negative ion beams with a multi-aperture multi-grid accelerator

JT-60SAの加熱・電流駆動を担う中性粒子入射装置では、500 keV, 22Aの負イオンビームを、100秒間連続で加速する負イオン源・加速器の開発が最重要課題である。量研機構では、多孔多段の負イオン加速器を開発し、負イオンビームの高エネルギー化、長パルス化の技術開発を進めている。今回、加速中の負イオンビームの直接衝突を減らすと共に、衝突により発生する二次電子の量を抑制することを目的として、加速電極の孔径や厚みを最適化した加速電極を開発し、5段静電加速器に適用した。その結果、1MeVの負イオンビームのパルス幅を従来の0.4秒から60秒間に伸長することに成功し、JT-60SA用長パルス加速電極の原理実証に成功した。これらの開発研究の状況について報告する。29th edition of the Symposium on Fusion Technolog