Speaker
Description
Passive spectral measurements of Balmer-๐ผ are used to constrain DEGAS2 neutral transport simulations that show 20% of the neutral flux to the tiles is above 100eV, and 5% is above 1000eV at the outer strike point in typical DIII-D H-modes with pedestal top temperatures near 1000eV. Multistep charge exchange between ions and recycled neutrals transfers energy and momentum between the two populations and can greatly increase the neutral energy and mean free path. While this allows fueling deeper in the confined plasma, it also leads to higher energy neutrals that drive first-wall erosion and impurity generation. Due to the Doppler shift of the line emission, passive spectral measurements of Balmer-๐ผ using the charge exchange neutral spectroscopy (CENS) system [1] above the lower x-point on DIII-D along with the midplane main ion CER (MICER) system can clearly distinguish emission from these higher energy neutrals despite bright edge emission. Neutrals with energies in excess of the pedestal top ion temperature are often seen, and these energies can exceed 1keV on DIII-D H-mode plasmas. The spectral measurements provide strong experimental constraints and are used in conjunction with filter based measurements from filterscopes [2] and LLAMA [3] to constrain DEGAS2 neutral transport simulations. The spectral measurements are particularly important for quantifying the higher energy neutrals and their impacts on fueling and the first wall. DEGAS2 is monte carlo collisional radiative code that performs calculations in 3D cartesian geometry and includes a comprehensive set of neutral-plasma and hydrogenic molecule-plasma interactions. The experimentally constrained simulations reproduce the 32 separate spectra from CENS and passive MICER systems with a high degree of accuracy. Ionization source rate information along with the spatial and velocity distribution of neutrals in the confined plasma and those impacting the wall of the device are extracted from the simulations. Details of the measurements, simulation workflow and distribution of the neutrals impacting the DIII-D carbon wall will be presented, including initial assessments of their contribution to main-chamber sputtering and impurity production.
[1] S. R. Haskey, Rev. Sci. Instrum. 95, (2024)
[2] J. Herfindal et al. Rev. Sci. Instrum. 95 (2024)
[3] L. Horvath et al. Rev. Sci. Instrum. 95 (2024)