RF simulation and improvements of an ion source test facility at HUST

Huazhong University of Science and Technology is developing a small RF based ion source for testing key techniques of the negative ion source. Numerical simulations of the source are performed with a commercial high frequency finite element analysis (FEA) software. The Faraday screen (FS) is included in the 3D model to reproduce the realistic electromagnetic fields in the driver. The plasma is approximated as a homogeneous electrically conducting medium in the discharge zone. Physical images of the electromagnetic (EM) fields in the driver with/without FS or plasma are presented and compared: (1) The FS suppresses effectively the axial electric field in the discharge zone, therefore diminishes the capacitive coupling between the antenna and plasma. (2) The existence of plasma intensifies the induced azimuthal electric field in the discharge zone and further enhances the inductive coupling between the antenna and plasma. The equivalent antenna impedances loaded with plasma are evaluated by this FEA method as well as an analytical method based on the ICP transformer model. The results are used to design the matching network, which employs a transformer-insulated Γ-type topology. Two strategies of matching by adjusting capacitors and operating frequency are investigated and proved both feasible. To improve the RF coupling between the antenna and plasma, an optimized concept design of the RF driver which employs the ferromagnetic enhancement technique is presented and the simulation results prove its feasibility.