Physical study of a RF negative hydrogen ion source for neutral beam injection

In neutral beam injection (NBI) – the primary auxiliary heating method for tokamak plasmas – the negative hydrogen ion source (NHIS) functions as a critical front-end component governing neutral beam quality. The performance of NHIS remains a key challenge. This work presents a 3D fluid model developed for a double-driver NHIS to simulate and optimize surface-generated negative hydrogen ion density. A comparison of plasma parameters between the NHIS with Cs and without Cs demonstrates that surface generation yields negative ion density one order of magnitude greater than volume generation. However, the presence of the magnetic filter field induces asymmetry in negative ion density within the extraction region. To improve this asymmetry, two approaches are proposed: (1) increasing the power of one of the drivers and (2) adding a spacer plate to the expansion region. Both approaches significantly improve the symmetry of negative hydrogen ion. Finally, adding a magnetic shield to the back plate of the expansion region further optimizes negative hydrogen ion density, yielding a 69% increase downstream.