Conceptual design of plasma neutralizer based on the CRAFT negative ion source neutral beam injection test facility

In a neutral beam injection (NBI) system, the accelerated ions should be neutralized before being injected into the magnetically confined plasma. Plasma neutralizer is promised for the future negative ion-based NBI system due to its higher neutralization efficiency. A plasma neutralizer prototype for the MW-level negative ion beam is under development under the project of the Comprehensive Research Facility for Fusion Technology (CRAFT). A global plasma model and a three-dimensional (3D) charged particle tracing model were used to evaluate the neutralization performance of the conceptual design of the plasma neutralizer prototype. The global plasma model indicated that, based on the CRAFT dual-driver RF negative ion source, the ionization degree of the beam-driven plasma was less than 1%, and the neutralization efficiency of 59% did not reveal its advantage. An external discharge power of 10 kW via filament arc discharge could obviously enhance the generated plasma density and increase the corresponding neutralization efficiency to 72%. Via the 3D charged particle tracing model, the energy loss and the uniformity of plasma distribution were studied by calculating the motion trajectories of electrons. The Halbach array of confining magnets showed the best performance in terms of the survival time and the distribution of the primary electrons.