Effect of the cusp magnets system on the hybrid RF ion source in NIFS by 2D fluid model

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Abstract
A two-dimensional fluid model of the RF ion source in NIFS test stand was established using COMSOL Multiphysics to examine the influence of the cusp magnets on plasma transport and RF power coupling. Particular attention was paid to the magnetic field intensity of the cusp magnets on driver backplane and its impact on plasma behaviour in both the driver and the extraction region. Simulation results reveal that strong magnetic confinement from the upper cusp magnets suppresses cross-field plasma transport, leading to reduced electron density near the plasma grid and less efficient RF power deposition. To explore potential optimization, a modified configuration with two smaller magnets of half the original size was investigated. This adjustment significantly enhances plasma density, increasing the peak density in the driver by nearly a factor of two. However, the enhanced plasma density is accompanied by increased spatial gradients in the extraction region, indicating that plasma uniformity is not improved under the modified magnetic configuration. These results reveal a trade-off between plasma density enhancement and spatial uniformity, arising from the interplay between magnetic confinement and plasma transport. This study provides insight into the role of cusp magnetic fields in regulating plasma generation and spatial gradients, and highlights the importance of balancing plasma density and uniformity in the optimization of RF negative ion sources.
Funding Information
  • the Science Foundation of Institute of Plasma Physics, Chinese Academy of Sciences (DSJJ-2024-03)

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