Evaluation of the Electron Temperature of the Ablation Cloud for the Small-Size Hydrogen Pellet Using Paschen Series in Heliotron J

We simultaneously measured the electron temperature (Te) and electron density (ne) using a low-dispersion near-infrared spectrometer in a small-size pellet ablation cloud in Heliotron J, a medium-sized helical-axis heliotron device. We applied the intensity ratio of the Paschen-α, β, γ and to determine Te based on the collisional-radiative model, which was fairly consistent with the partial local thermodynamic equilibrium (LTE) in the upper principal quantum numbers of 4, 5, and 6. For a typical pellet injection discharge, Te and ne were determined to be 0.9 eV and 4 × 1021 m-3, respectively. Our derived generalized empirical calibration curve demonstrates a weak influence of Te on ne evaluation, particularly in the range of 0.4 - 2.0 eV. Subsequently, we determined the region where the LTE is achieved for the Paschen series.