Influence of magnetic filter field on the radiofrequency negative hydrogen ion source of neutral beam injector for China Fusion Engineering Test Reactor

In the design of negative hydrogen ion sources, a magnetic filter field of tens of Gauss at the expansion region is essential to reduce the electron temperature, which usually results in a magnetic field of around 10 Gauss in the driver region, destabilizing the discharge. The magnetic shield technique is proposed in this work to reduce the magnetic field in the driver region and improve the discharge characteristics. In this paper, a three-dimensional fluid model is developed within COMSOL to study the influence of the magnetic shield on the generation and transport of plasmas in the negative hydrogen ion source. It is found that when the magnetic shield material is applied at the interface of the expansion region and the driver region, the electron density can be effectively increased. For instance, the maximum of the electron density is 6.7x 10 ¹⁷ m⁻³ in the case without the magnetic shield, and the value increases to 9.4 x10 ¹⁷ m⁻³ when the magnetic shield is introduced