Effect of permanent magnets installed on the driver back plate on the plasma of a radio frequency ion source

Neutral beam injection (NBI) serves as an effective auxiliary heating method in tokamak fusion research. A based positive ion source with a single radio frequency (RF) driver has been developed at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). To reduce plasma loss on the inner surface of vacuum chamber, Permanent magnets installed on the back plate is essential. Two kinds configurations of cusp magnetic field (checkerboard type and symmetric type) have been carried out with the approach of finite element and experimental performance. The simulation results show that the plasma loss attributed by symmetric type is better than another case, and they both make a great impact on the plasma loss in the back plate. However, symmetric type easily leads to the plasma gradient drift on the vacuum chamber. It will cause to the non-uniformity of plasma on the extraction area, rendering it unsuitable for the extraction of plasma. Experimental results indicate that checkerboard type of cusp magnetic field has influence on reducing the plasma loss and confining of plasma. The tendency of plasma loss on the back plate, which can be verified by water flow calorimetry (WFC), decreased with permanent magnets. The heat load on the faraday screen and back plate decreased from 2.13 kJ/kW to 2.02 kJ/kW by magnets. Besides, the efficiency of RF ion source increased from 0.36 A/kW to 0.44 A/kW. It demonstrated that the probability of plasma loss on the back plate was lower, and more plasma was confined on the extraction area by cusp magnetic field. The results are helpful to optimize the structure and performance of RF ion source.