Characterizing Plasma Impedance with Power Sweeps in LUPIN – a Reduced Scale ICP Ion Source

The Large Uniform Plasma for Ionizing Neutrals (LUPIN) ion source is a reduced-scale, four turn, inductively coupled plasma (ICP) prototype designed to investigate plasma coupling at varying power and inform possible design upgrades for the DIII-D neutral beam injection (NBI) system. LUPIN will also inform the design of Wolverine, a multi-strap antenna configuration designed to reduce the voltage difference between coil turns. We found peak antenna voltages, V_peak, of 3.1 kV at 7.5 kW, and V_peak approximately doubled with reflected power Γ_reflection = 10% in the four-turn configuration, providing an experimental basis to compare against Wolverine. A circuit model for the four-turn coil was validated against V_peak and I_peak, at the ICP antenna across a 0.2 kW – 7.5 kW power sweep; additionally, plasma impedance, R_plasma and inductance, L_plasma were measured. A hairpin probe was used to measure the electron density (n_e) across 0.2 kW to 1.5 kW; experimentally measured n_e enabled the calculation of L_plasma and R_plasma for model comparison. Plasma breakdown was achieved at 100 mTorr in helium powered by 0.2-1.5 kW RF driven at 2 MHz; helium gas at 100 mTorr was selected for the initial impedance characterization due to ease of plasma breakdown. We found less than 5 % error between modeled and experimental V_peak and I_peak. The L_plasma experimental data agreed with the analytical model with less than 10% error between 0.2 – 0.5 kW, but the error increased to a factor of 2 at 1.5 kW– indicating the need for model adjustment.