Implications of suppressed anomalous impurity transport in W7-X for stellarator reactors

In view of feasible reactor operation regimes, the transport of impurities must be governed by a trade-off between low impurity confinement of helium ash and small inward transport of edge-sourced impurities. Therefore, identifying, understanding, and predicting reactor scenarios of compatible impurity transport is crucial on the path to self-sustained fusion burn.

The Wendelstein 7-X (W7-X) stellarator was optimized for low neoclassical transport in the 1/ν-regime, which led to energy [1], particle [2], and impurity [3, 4] transport being governed by anomalous processes under standard operating conditions. In high-performance scenarios with tokamak-like confinement, anomalous transport is reduced [5], and impurity transport is governed by neoclassical processes [6, 7]. The neoclassical convection is inward-directed and causes the impurity density profiles to strongly peak. Consistent with neoclassical predictions, the peaking strength scales linearly with impurity charge. The transition into the neoclassically dominated regime is found to be governed by a critical a/L_ne of unity. The threshold value is universal across magnetic configurations and experimental scenarios, causing impurity accumulation to appear across a significant range of the W7-X parameter space.

In W7-X high-performance plasmas, high triple products and centrally peaked impurity profiles have always co-occurred. We show that the neoclassical inward impurity transport significantly hinders an efficient removal of helium ash, while leading to unacceptable concentrations of higher charge impurities. This stresses the importance of tailoring impurity transport in a stellarator-type reactor, where potential routes to success have already been identified in core electron root confinement regimes [8] or active density control schemes that enable steering the level of anomalous impurity transport through external actuators.

[1] Wappl et al 2025 PPCF 67 075025

[2] Bannmann et al 2024 NF 64 106015

[3] Geiger et al 2019 NF 59 046009

[4] Romba et al 2023 PPCF 65 075011

[5] Bannmann et al PRL, in preparation

[6] Romba et al 2023 NF 63 076023

[7] Romba et al 2025 PPCF 67 065016

[8] Beidler et al 2024 NF 64 126030