3D EM modeling of RF inductively coupled LUPIN ion source

A three-dimensional (3D) electromagnetic (EM) simulations have been performed using COMSOL Multiphysics to optimize RF power coupling in the Large, Uniform Plasma for Ionizing Neutrals (LUPIN) ion source. LUPIN, currently operating at North Carolina State University, is a test-scale inductively coupled positive ion source designed for the DIII-D Neutral Beam Injection (NBI) system upgrade. It features a quartz plasma chamber (20 cm in length, 10 cm radius) surrounded by a helical RF coil operating at 2 MHz with up to 20 kW of input power. The coil geometry—specifically the number of turns, turn spacing, and proximity to the internal Faraday shield (FS) significantly influences the EM field distribution and power deposition within the system. The model enables identification and quantification of inductive power (azimuthal) and capacitive power in the plasma (radial and axial), allowing detailed analysis of their contributions. Inductive power indicates efficient magnetic coupling to the plasma, while capacitive power is linked to sheath formation and surface interactions. Geometric design parameters, including coil-to-shield spacing and shield slit configurations, are investigated for their impact on coupling efficiency and localized power dissipation. The model calculates network resistance (R_net) and plasma resistance (R_plasma), enabling the estimation of how much power is delivered to the plasma versus dissipated in the coil, shield, or surrounding air. Simulations are conducted for configurations with and without the FS, revealing how 3D effects and slit geometries influence the RF fields.