Energy-resolved neutral profile measurements on MAST upgrade using passive Balmer-alpha spectroscopy

Disentangling the complex interplay of neutral fueling and transport considerations in the edge plasma pedestal region can be informed by direct measurements of the energy-resolved local neutral densities from passive Dα spectroscopy. In this work, we employed the MAST upgrade charge exchange spectroscopy system passive background viewing fan to assess the feasibility of resolving both the higher energy thermal neutral component due to charge exchange reactions with thermal ions and the cold component due to edge neutral recycling. The spectral coverage, resolution, and dynamic range of the CCD adequately capture the entire Dα line profile. The outer midplane tangential viewing fan of 19 lines of sight provides good spatial resolution of the edge plasma region. To infer the 1D radial profiles of the thermal and cold neutral densities, we used a modified version of the FIDASIM Monte Carlo neutral modeling package to develop a forward model of the line integrated spectroscopic signals. In combination with a numerical optimization scheme, the model enables a direct comparison of the spectrally resolved Dα profiles to the experiment data for a given range of assumptions on the cold neutral densities in the scrape-off layer. Quantitatively good agreement is obtained in recovering the 1D cold and thermal density profiles in the edge pedestal region as well as the spectral Dα line shape details, with typically observed cold and thermal neutral densities at the outer midplane separatrix of ∼3–5 × 1015 and 5–7 × 1014 m−3, respectively, for high-field side fueled plasmas. Inboard of the pedestal region the model consistently underestimates the measured cold component line-integrated intensity, suggesting that further characterization of the viewing geometry, reflections impact and/or FIDASIM model assumptions, is needed.