High-Z impurity transport and control in advanced tokamak scenarios on DIII-D

We report the first experimental evidence of active high-Z impurity expulsion in DIII-D high-performance (β_N>3.5) hybrid plasmas with low rotation and small ELMs. Detailed analysis shows that core and pedestal impurity transport is dominated by strong outward neoclassical convection, where ion temperature screening (∇Ti) dominates the main ion density gradient-driven pinch (∇ni), combined with enhanced turbulent diffusion, producing flat/hollow impurity profiles. This provides a critical solution for tungsten first-wall compatibility in DIII-D. In contrast, NBI-driven high-density, high-β_P scenarios exhibit impurity accumulation coinciding with: (1) density ITB formation, (2) increased toroidal rotation from enhanced torque, and (3) plasma performance degradation. TGLF/NEO modeling identifies the accumulation mechanism as strong neoclassical inward convection from high density gradients, further amplified by rotation-induced poloidal asymmetries in impurity density.



Acknowledgements: Supported in part by the US Department of Energy under DE-FC02-04ER54698, DE-SC0010685, DE-AC52-07NA27344, DE-AC02-09CH11466 and DE-SC0020287.