The suppression of anomalous impurity transport above a critical normalized density gradient scale length in Wendelstein 7-X

Abstract In purely neutral-beam-heated scenarios in Wendelstein 7-X (W7-X), central peaking of impurity density profiles is observed. These are discharges typically considered as the main path towards high energy confinement in W7-X. Understanding the undesirable impurity peaking in this kind of discharge is thus paramount. In previous work, the transport in the impurity channel in this kind of discharge was found to be dominated by neoclassical and classical transport, implying a suppression of the anomalous transport component (Romba et al 2023 Nucl. Fusion 63 076023). This paper investigates dependencies of the impurity peaking on potential drivers of changes in the impurity transport channel. Based on a vast data set, peaking of impurity density profiles is found to be stronger for higher-charge species, scaling approximately linearly with impurity charge for low Z impurities. This scaling of the peaking strength is reproduced in 1D transport simulations, which are found to match with predictions of neoclassical and classical transport inside half radius. With the match present over a wide range in plasma profiles and impurity charge, a suppression of the anomalous transport component for species helium, carbon, and oxygen is inferred. This suppression of anomalous transport is not found to strongly depend on the magnetic field configuration, rendering it a more general W7-X feature. A critical normalized electron density gradient scale length <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>a</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:msub> <mml:mi>L</mml:mi> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> <mml:mo>≈</mml:mo> <mml:mn>1.0</mml:mn> </mml:mrow> </mml:math> is identified as a parameter indicative to the change in anomalous impurity transport. For these particular experimental scenarios, the appearance of this critical scale length is found to be coupled to a high plasma density. The critical value of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:mi>a</mml:mi> <mml:mrow> <mml:mo>/</mml:mo> </mml:mrow> <mml:msub> <mml:mi>L</mml:mi> <mml:mrow> <mml:msub> <mml:mi>n</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">e</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> is found to only be applicable in regions of peaked impurity density profiles, indicating a radial dependency.