Observation of impurity accumulation and its compatibility with high plasma performance in W7-X

Zhang, D; Buttenschön, B; Jablonski, S; Kubkowska, M; Ford, O; Alcusón, J A; Beidler, C D; Burhenn, R; Beurskens, M N A; Langenberg, A; Pablant, N; Reimold, F; Rahbarnia, K; Smith, H M; Wegner, Th; Wurden, G; Bozhenkov, S A; Feng, Y; Brunner, K J; Fuchert, G; Gao, Y; Geiger, J; Giannone, L; Höfel, U; Hirsch, M; Huang, Z; Knauer, J; Kremeyer, T; Krychowiak, M; Kwak, S; Laqua, H P; Laube, R; Neuner, U; Pasch, E; Pavone, A; von Stechow, A; Svensson, J; Thomsen, H

doi:10.1088/1361-6587/acf0e7

Cited by 3 publications

(1 citation statement)

References 65 publications

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“…Unfortunately, only two such plasmas were observed in the 2018 campaign and while these could be reproduced in the 2022/3 campaign, the required set of conditions for this scenario to occur are not yet known as the majority of experiments are conducted at higher ECRH power. Accumulation of impurities in the plasma core is observed due the reduction of impurity flux towards neoclassical levels [47], indicating a strong reduction in turbulent transport.…”

Section: Post-boronisation Low-power Scenariomentioning

confidence: 99%

Turbulence-reduced high-performance scenarios in Wendelstein 7-X

Ford,

Beurskens,

Bozhenkov

et al. 2024

Nucl. Fusion

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In the Wendelstein 7-X (W7-X) stellarator, turbulence is the dominant transport mechanism in most discharges. This leads to a 'clamping' of ion temperature over a wide range of heating power, predominantly flat density profiles where hollow profiles driven by neoclassical thermo-diffusion would be expected and by rapid impurity transport in injection experiments. Significantly reduced turbulent transport is observed in the presence of strong core density gradients found transiently after core pellet injection and irregularly after boronisation or boron pellet injection. Density peaking is also achieved in a controlled manner in purely neutral beam heated discharges where particle transport analysis reveals an abrupt reduction in the main-ion particle flux leading to significant density profile peaking not explained by the NBI particle source alone. The plasmas exhibit a heat diffusivity of around chi=0.25±0.1 m^2 s^-1 at mid radius, a factor of around 4 lower than ECRH dominated discharges. Despite the improved confinement, the achieved ion temperature is limited by broader heat deposition and the lower power-per-particle given the higher density. This is overcome with limited reintroduction of ECRH power, where the low heat diffusivity diffusivity is maintained, the density rise supressed and ion temperatures above the clamping limit are achieved.The applicability of these plasmas for a high performance scenario on transport relevant time scales is assessed, including initial predictions for planned heating upgrades of W7-X, based on a range of assumptions about particle transport.

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Section: Post-boronisation Low-power Scenariomentioning

confidence: 99%

Turbulence-reduced high-performance scenarios in Wendelstein 7-X

Ford,

Beurskens,

Bozhenkov

et al. 2024

Nucl. Fusion

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X-ray tomography system concept for stellarator TJ-II

Jabłoński,

Chernyshova,

Malinowski

et al. 2024

Physics of Plasmas

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A method based on the principle of x-ray tomography is a good way of assessing the parameters of the radiation emission distribution in the plasma. In this method, a set of radiation detectors is usually placed around a selected section of the plasma or in two groups of detectors arranged perpendicularly to each other in the top and side positions. Such an arrangement is intended to provide a sufficient amount of data to reconstruct the distribution of radiation emission intensities across the plasma cross section. For the plasma produced in the TJ-II stellarator, a two-dimensional (2D) imaging detector is proposed with the ability to discriminate photon energy in only one specifically selected position in the toroidal direction. The proposed diagnostics could come with advantages of a 2D toroidal camera, providing access to 3D information, and would complement standard poloidal tomography. It could be used either for better constraining of the 2D tomography in simple cases with axisymmetric SXR emissivity, for simulation validation of 3D effects in a synthetic diagnostic approach, or to attempt direct 3D tomography. The subject of this contribution is to present the idea of a 3D x-ray tomography system based on the use of a square matrix of GEM detector and the obtained preliminary results of the conducted simulations. The developed diagnostics is to contain 33 462 detection pixels associated with lines of sight. The analysis was carried out on the basis of the numerical simulations based on the GEM_MP.exe and Tomography.exe computer codes (both created at IPPLM) enabling the consideration of free-free, free-bound, and bound-bound x-ray radiation reaching each of the pixels of the GEM diagnostics (GEM_MP.exe code) and testing algorithms proposed for tomographic reconstruction of the plasma emissivity (Tomography.exe code). During calculations, real profiles of electron density and electron temperature, as well as real geometry of the measuring system were taken into account. Additionally, the influence of factors such as filters used in the detector and the efficiency of the used active medium on the quality of tomographic reconstruction was examined. The summary evaluates the proposed tomographic reconstruction method.

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Global gyrokinetic analysis of Wendelstein 7-X discharge: unveiling the importance of trapped-electron-mode and electron-temperature-gradient turbulence

Wilms,

Bañón Navarro,

Windisch

et al. 2024

Nucl. Fusion

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We present the first nonlinear, gyrokinetic, radially global simulation of a discharge of the Wendelstein 7-X-like stellarator (W7-X), including kinetic electrons, an equilibrium radial electric field, as well as electromagnetic and collisional effects. By comparison against flux-tube and full-flux-surface simulations, we assess the impact of the equilibrium ExB-flow and flow shear on the stabilisation of turbulence. In contrast to the existing literature, we further provide substantial evidence for the turbulent electron heat flux being driven by trapped-electron-mode (TEM) and electron-temperature-gradient (ETG) turbulence in the core of the plasma. The former manifests as a hybrid together with ion-temperature-gradient (ITG) turbulence and is primarily driven by the finite electron temperature gradient, which has largely been neglected in nonlinear stellarator simulations presented in the existing literature.

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Observation of impurity accumulation and its compatibility with high plasma performance in W7-X

Cited by 3 publications

References 65 publications

Turbulence-reduced high-performance scenarios in Wendelstein 7-X

Turbulence-reduced high-performance scenarios in Wendelstein 7-X

X-ray tomography system concept for stellarator TJ-II

Global gyrokinetic analysis of Wendelstein 7-X discharge: unveiling the importance of trapped-electron-mode and electron-temperature-gradient turbulence

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