Validation of Helium Line-Ratio Spectroscopy for Electron Temperature and Density Measurements in the Fueling Region of C-2W

High-fidelity measurements of plasma parameters in the fueling region of the C-2W field-reversed configuration (FRC) device [1] are essential for assessing neutral fueling efficiency, stability characteristics, and tandem confinement properties. The open-field-line region near the mirror plugs is characterized by low temperatures, steep density gradients, and restricted diagnostic access. Helium line ratio spectroscopy offers a valuable non-invasive method to infer local electron densities and temperatures in this critical region. While the technique is well established in other fusion devices, its use in the tandem fueling region of C-2W provides new insight into the edge plasma behavior of mirror-embedded FRCs. Ratios of visible He-I lines at 667.8 nm, 706.5 nm, and 728.1 nm were acquired using a survey spectrometer with millisecond time resolution and interpreted using the collisional-radiative model of Munoz-Burgos et al [2]. The resulting spectra yield preliminary, radially resolved estimates of electron density and temperature consistent with expectations for the fueling region. By validating helium line-ratio spectroscopy for edge plasma characterization in C-2W, these results establish a foundation for future studies.