Numerical exploration of non-axisymmetric divertor closure in the small angle slot (SAS) divertor at DIII-D

Frerichs, H.; Schmitz, O.; Covele, B.; Feng, Y.; Guo, H. Y.; Hill, D.

doi:10.1088/1741-4326/aaaf99

Title: Numerical exploration of non-axisymmetric divertor closure in the small angle slot (SAS) divertor at DIII-D

Journal Article · 28 February 2018 · Nuclear Fusion

DOI: https://doi.org/10.1088/1741-4326/aaaf99 · OSTI ID:1423897

Frerichs, H. ^[1]; Schmitz, O. ^[1]; Covele, B. ^[2]; Feng, Y. ^[3]; Guo, H. Y. ^[2]; Hill, D. ^[2]

Univ. of Wisconsin, Madison, WI (United States). Dept. of Engineering Physics
General Atomics, San Diego, CA (United States)
Max Planck Inst. for Plasma Physics, Heidelberg (Germany)

Numerical simulations of toroidal asymmetries in a tightly baffled small angle slot (SAS) divertor on the DIII-D tokamak show that toroidal asymmetries in divertor closure result in (non-axisymmetric) local onset of detachment within a density window of 10-15% on top of the nominal threshold separatrix density. The SAS divertor is explored at DIII-D for improving access to cold, dissipative/detached divertor conditions. The narrow width of the slot divertor coupled with a small magnetic field line-to-target angle facilitates the buildup of neutral density, thereby increasing radiative and neutrals-related (atoms and molecules) losses in the divertor. Therefore, small changes in the strike point location can be expected to have a large impact on diverter conditions. The combination of misaligned slot structure and non-axisymmetric perturbations to the magnetic field configuration causes the strike point to move along the divertor target plate, possibly leaving the diverter slot at some locations. The latter extreme case essentially introduces an opening in the divertor slot from where recycling neutrals can easily escape, and thereby degrade the performance of the slot divertor. Such a strike point dislocation is approximated by a finite gap in the divertor baffle for which three dimensional edge plasma and neutral gas simulations are performed with the EMC3-EIRENE code.

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