Multi-diagnostic characterization of neutrals and particle transport in the near X-point confined region of DIII-D

Experimental evidence suggests the neutral density in tokamak plasmas is at least 2D. We present a multi-diagnostic characterization of neutrals in the confined region of DIII-D leveraging spectral D_α measurements at the midplane and near the X-point. Synthetic diagnostics in the DEGAS2 code¹ are used to compare simulations with a set of diagnostic measurements. The charge exchange neutral spectroscopy diagnostic² (CENS) measures D_α emission from the edge (ψ_n ≈ 0.7) to the X-point. Meanwhile, main-ion charge exchange recombination spectroscopy³ (MICER) measures D_α emission across the midplane. Spectral measurements of the D_α radiance reveal emission from energetic neutrals in the confined region as opposed to ‘cold’ neutrals in the SOL. Traditional filter-based diagnostics, including LLAMA, are used to further constrain the neutral density. With the appropriate boundary source of neutrals, DEGAS2 can closely reproduce the CENS, passive-MICER, and filter-based measurements. Once constrained, DEGAS2 is used to calculate the neutral density and particle source. The neutral density is peaked near the X-point with values between 10¹⁵ -10¹⁶ m^-3.

¹D. Stotler, C. Karney. Contrib. Plasma Phys. 34 (1994)

²S. R. Haskey et al. Rev. Sci. Instrum. 95, (2024)

³S. R. Haskey et al. Rev. Sci. Instrum. 93, (2022)