Description
The use of solid hydrogen pellets in magnetically confined fusion plasmas has been proposed both as a promising fueling technique and as a tool to investigate the effect of impurities on plasma dynamics. Pellets are typically injected at high speed, depositing fuel in the denser inner plasma regions, where the evaporation rate is higher. In this work, we present a spectroscopic model for the characterization of the resulting ablation cloud by analyzing the Balmer emission in the 300–700 nm wavelength range. Atomic level populations were calculated assuming local thermodynamic equilibrium. An extensive dataset of line shapes for transitions up to n = 9 → 2 was computed using state-of-the-art Stark-broadening simulations. The spectral intensity was obtained by solving the radiation transport equation for a spherical plasma source under uniform conditions. Instrumental broadening was accounted for by convolving the synthetic spectra with a Gaussian function of appropriate width. The model successfully reproduces measured pellet ablation cloud spectra at the Large Helical Device across the entire Balmer region, including both the optically thick Balmer α line and the emission beyond the ionization limit. The inclusion of opacity effects enables an independent estimate of the pellet core radius during the vaporization process. Additionally, it is shown that the use of the Inglis–Teller approach, together with detailed Stark-broadened line profiles, successfully describes line merging and continuum lowering phenomena, whereas other models, such as the widely used Stewart–Pyatt formulation, fail to reproduce the experimental observations.