A paper published as a Letter in the IoP journal Nuclear Fusion this week by Dr. H. Frerichs studies for the first time the sensitivity of the Small Angle Slot (SAS) divertor at DIII-D on toroidal asymmetries. Such asymmetries can be a result of the imperfect alignment of the confining magnetic field with the toroidal opening of the slot divertor or be born from effects of the plasma equilibrium leading to 3-D effects on the plasma boundary.
Scientist Dr. H. Frerichs in the 3D-PSI group has studied the resulting impact on the neutral compression and heat flux characteristics in the SAS for DIII-D in detail using the EMC3-EIRENE plasma fluid and kinetic neutral transport code. These simulations 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 eld line-to-target angle facilitates the buildup of neutral density, thereby increasing radiative and neutrals-related (atoms and molecules) losses in the divertor. Small changes in the strike point location can be expected to have a large impact on divertor 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 divertor 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.
This work is funded under DoE Early Career grant DE-SC0013911
— News: 02-20-2018