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Some experiments avoid this, but whether a reactor-scale stellarator can is unknown.","posed_since":"","precise":"Neoclassical theory predicts inward impurity convection in the ion-root regime (negative radial electric field) of non-quasisymmetric stellarators, while W7-X and LHD find impurity flushing or an impurity hole in some regimes. Determine whether an optimized reactor stellarator can keep core impurity concentration below the radiative limit, via turbulent transport, temperature screening or quasisymmetry. An answer is a validated neoclassical plus turbulent impurity transport prediction for reactor conditions.","problem_ref":null,"references":"","settled_by":"Impurity transport models validated on W7-X and LHD applied to reactor designs, confirmed by impurity injection experiments.","status_note":"","title":"Avoiding impurity accumulation in reactor 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