Neutral beam injection serves as the effective auxiliary heating method in tokamak fusion research. Positive ion source with single radio frequency driver has been developed at the Institute of Plasma Physics, Chinese Academy of Sciences. To reduce plasma loss on the inner surface of the vacuum chamber, permanent magnets installed on the back plate are essential. Two configurations of cusp magnetic field (checkerboard type and symmetric type) have been carried out with an approach of finite element and experimental performance. The simulation results showed that the distribution of electron for the checkerboard type (type I, type II, and type III) was more uniformity. Heat loading on the inner surface of faraday shield was the lowest with type III. Hence, the checkerboard type III was the useful method to protect the faraday shield. Electron flux would move toward the bottom of the chamber with magnets from the simulation results. It was beneficial to increase the degree of gas ionization on the extracted area and improve the extracted current. Experimental results indicated that the checkerboard type of cusp magnetic field has little influence on reducing the heat loading. The heat loading on the faraday shield and the back plate decreased from 132.97 to 130 kJ because the majority of heat loading was from eddy current. However, the efficiency of radio frequency (RF) ion source increased from 0.36 A/kW to 0.43 A/kW with magnets. The checkerboard type of the cusp magnetic field was useful to improve the extracted current. The results were helpful to optimize the structure of RF ion source.
Yang et al. (Sun,) studied this question.