Preliminary experimental research on RF coupling efficiency of CRAFT NNBI prototype ion source

A radio frequency inductively coupled plasma source was chose as the prototype ion source for CRFAT NNBI system. The RF coupling efficiency of this negative ion(H-) source is researched by experiment. The effect of the RF ion source's multi-cusp confinement magnet on the RF coupling efficiency is focused on. It is found that the RF ion source, equipped with a linear type cusp magnet array on the expansion chamber walls, exhibits a higher efficiency of 2% to 6.6% compared to the checkerboard type. Furthermore, the RF coupling efficiency improves by approximately 9% when a confinement magnet is used, as opposed to not having one. The variations in bias voltage have no impact on the RF coupling efficiency. Nonetheless, variations in the magnetic field within the driver region caused by changes in the PG current induce an impact on the plasma in that region. When the PG current was raised from 800 A to 2500 A, it led to a minor reduction in RF coupling efficiency of approximately 2%.