Charge-state independent anomalous transport for a wide range of different impurity species observed at Wendelstein 7-X

Abstract

In this paper, the plasma volume averaged impurity confinement of selected charge states and impurity species has been characterized for the Stellarator Wendelstein 7-X (W7-X), covering a wide range of atomic charges ( Z = 12–44) and atomic masses ( M = 28–184). A comparison of the experimental findings to theoretical neoclassical and turbulent transport expectations suggests, aside from/in addition to the neoclassical transport, an additional significant anomalous transport mechanism, which is not inconsistent with the predictions of a turbulence dominated impurity transport and is in agreement with the experimental results from recent transport studies based on the direct measurements of impurity diffusion profiles, performed at W7-X.

Steady State ECRH Operation at the W7-X Stellarator

“…These long time constants would result in impurity accumulation if the sources were appropriately large. Experimentally, however, the values could not be confirmed [8]. Rather, that in the steady state ECRH plasmas the impurity confinement time was below 200 ms Fig.…”

Effects of collisions on impurity transport driven by electrostatic modes

“…Recent measurement in the stellarator Wendelstein 7-X indicate that the impurity transport is dominated by turbulent diffusion (Langenberg et al 2018;Geiger et al 2019). Diffusion coefficients two orders of magnitude larger than those calculated from collisional transport have been measured for iron impurities (Geiger et al 2019), and the impurity confinement time appears to be insensitive to the impurity charge number (Langenberg et al 2020) -in contradiction to predictions for collisional transport (Helander & Sigmar 2005). 2 S. Buller and P. Helander On the other hand, the experimental observations may be consistent with recent theoretical calculations of the transport due to electrostatic turbulence (Helander & Zocco 2018), which, for heavy species, give transport coefficients independent of the impurity charge and mass (Angioni et al 2016;Helander & Zocco 2018).…”

“…Recent measurement in the stellarator Wendelstein 7-X indicate that the impurity transport is dominated by turbulent diffusion (Langenberg et al 2018;Geiger et al 2019). Diffusion coefficients two orders of magnitude larger than those calculated from collisional transport have been measured for iron impurities (Geiger et al 2019), and the impurity confinement time appears to be insensitive to the impurity charge number (Langenberg et al 2020) -in contradiction to predictions for collisional transport (Helander & Sigmar 2005).…”

“…Z u ≡ 0.3v Ti /(ν ii a) refers to the charge number at which impurity-ion collisions are expected to have an order-unity effect on the flux due to parallel compressibility (3.17). The parameters are taken form the following scenarios: W7-X -LBO impurity study (Langenberg et al 2020); TJ-II -LBO impurity study (Zurro et al 2014); LHD -TESPEL impurity study (Tamura et al 2016). In Z u we used k a ∼ k ⊥ ρ i ∼ 0.3 to obtain one estimate for both (3.15) and (3.16).…”

Simultaneous Observation of Silicon and Boron Impurity Behaviors in the Core Region of a Mid-Density LHD Plasma

“…The study of the atomic number (Z) dependence of the impurity transport in magnetically confined fusion plasmas is necessary to understand whether turbulent or neoclassical transport is dominant in plasmas. In general, the turbulent-dominant impurity transport does not exhibit a Z-dependence [1]. On the other hand, the neoclassical impurity transport does exhibit such a Z-dependence.…”