Investigating Antenna Characteristics and Plasma Impedance in the Development of AMAROK - A Positive ion RF-ICP demonstration source for DIII-D NBI system

The AMAROK system is an RF inductively coupled plasma (ICP) positive ion source, built to study antenna & plasma performance for a prototypic DIII-D Neutral Beam Injection (NBI) source. Power coupling is explored between 2–4 MHz, focusing on two racetrack antenna designs: multi-turn single-strap ("Spring") & single-turn multi-strap ("Wolverine"). A key challenge in this regime is ensuring robust matching while avoiding self-resonant frequency (SRF), which can severely reduce antenna-plasma coupling. This work tackles the issue of SRF falling within the desired range. Experiments show SRF of a 6-turn Spring antenna is ~4.5 MHz, while Wolverine's SRF is near 10 MHz—showing different suitability across the 2-4MHz range. SRF drops with more turns, suggesting low-turn antennas suit this range better. An analytical model estimates total system load at 200 kW RF for 10s, showing antenna load dominates plasma load. Antenna impedance varies greatly when enclosed in a secondary vacuum chamber, stressing importance of including nearby conductors to define matching. AMAROK is under construction, with most hardware & diagnostics near completion. Future work will focus on assembly, plasma testing, dynamic matching, & validating SRF model with plasma data.