Impact of Faraday Shield on RF Inductive Coupling in the LUPIN Ion Source*

This study presents electromagnetic simulations of the Large, Uniform Plasma for Ionizing Neutrals (LUPIN) ion source, designed as an RF inductively coupled plasma ion source for future designs of the DIII-D Neutral Beam Injection (NBI) system. LUPIN operates with 20 kW of RF power at a 2 MHz driver frequency and incorporates an RF matching network. In high-power RF plasma sources, internal Faraday shields play a vital role in safeguarding the dielectric vessel or windows from erosion caused by capacitive coupling and the resulting energetic particle fluxes. The Faraday shield in LUPIN effectively minimizes interaction between the plasma and the chamber wall, preventing damage to system components, at the expense of some power losses. Its water-cooling mechanism efficiently dissipates heat, ensuring acceptable operating temperatures and enhancing the overall reliability and efficiency of the ion source.

LUPIN is designed to direct RF power into a quartz vessel measuring 20 cm in length with a 10 cm radius, generating hydrogen plasmas with electron densities of 10¹⁸ m^-3 for a 10-second duration. This setup provides flexibility to test designs, matching the power density of a larger (100 kW, 85 A) full-scale ion source. The simulations conducted using COMSOL illustrate the effects of the Faraday shield on power coupling both with and without plasma, investigating various plasma parameters to provide detailed insights into optimizing the ion source's performance.