Characterizing impurity sourcing and transport in the high temperature boundary of DIII-D Wide Pedestal QH-mode plasmas

• Published in: Nuclear fusion 2024-12
• Main Authors: Ma, Xinxing, Abrams, Tyler, Ernst, Darin R., Wang, H. Q., Bortolon, Alessandro, Chen, Xi, Odstrcil, Tomas, Osborne, Thomas H., Scotti, Filippo, Thomas, Dan M., Truong, Dinh, Watkins, Jonathan
• Format: Article
• Language: English
• ISSN: 0029-5515
• DOI: 10.1088/1741-4326/ad9f83

Abstract Wide Pedestal QH-mode (WPQH) plasmas in the DIII-D tokamak show a sheath limited SOL, where electron density and temperature remain nearly constant along field lines from the midplane to the divertor. Consequently, parallel gradients are weak. The first Langmuir probe measurements in this regime point to a high sheath temperature with target electron temperature T e up to ~150eV, leading to high carbon self-sputtering. Midplane carbon densities from SOLPS-ITER modeling of these plasmas in a double-null configuration fall far below experimental measurements unless full drifts are activated. In the drift-dependent SOLPS-ITER modeling, anomalous poloidally uniform, radially varying transport is adjusted to match measured radial electron temperature and density profiles in the pedestal and SOL region. The resulting carbon density (C6+) matches measured carbon densities 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 flow of carbon as it moves upstream. Simulations with reversed toroidal field (ion B×∇B drift away from the X-point) 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 distribution. In double-null configurations which are usually used in WPQH plasmas, these drift effects tend to counterbalance, so that the carbon density and Z eff are reduced at the outer mid-plane only modestly, by around 7% in the modeling. Further predictive modeling indicates that the carbon density can be significantly reduced by an order of magnitude by implementing a single-null shape with reversed toroidal field.