Characterizing Impurity Sourcing and Transport in the High Temperature Boundary of DIII-D Wide Pedestal QH-Mode Plasmas

Wide Pedestal QH-mode plasms in the DIII-D tokamak show a sheath limited SOL, where electron density and temperature remain nearly constant along field lines. Consequently, parallel gradients are weak and drifts play an essential role in the boundary impurity transport. The first Langmuir probe measurements point to a high sheath temperature with target T_e up to 150eV, leading to high carbon self-sputtering. Midplane carbon densities from SOLPS-ITER modeling in a double-null configuration fall far below experimental measurements unless full drifts are activated. The drift-dependent modeling reproduces the carbon density profile just inside the separatrix. The C2+ density near the outer strike point is also consistent with spectroscopic imaging. With the ion B×▽B drift towards the X-point, carbon in the lower divertors is redistributed from the private flux region to the high field side and pushed upstream in the SOL by poloidal E×B drifts. The B×▽B drift dominates the radial transport of upstream carbon. Simulations with reversed toroidal field show a radically different behavior, where carbon accumulates on the low field side in lower divertors, and is pushed towards the high field side in the upper divertors, indicating a strong effect of particle drifts on impurity sourcing.