Description
Wendelstein 7-X (W7-X) is a neoclassically optimised stellarator in which turbulent transport plays a dominant role for overall confinement. Understanding how magnetic geometry affects turbulence in this type of device is essential to advancing the stellarator concept and successfully optimising stellarators for reduced turbulence. With its flexible magnetic geometry, W7-X provides a unique platform to investigate these dependencies experimentally. Previous studies have shown that density fluctuation measurements using phase contrast imaging (PCI) reveal clear trends when the rotational transform and mirror ratio are varied in W7-X [Bähner et al. 2026 NF 66 016007]. These trends were partially
reproduced by growth rates and density fluctuation amplitudes in numerical simulations using stella and GENE-3D. However, for a more detailed comparison including wavenumber spectra, it is necessary to establish a geometrical mapping between measured and simulated wavenumbers. Similar comparisons have been done for other fluctuation diagnostics [González-Jerez et al. 2024 NF 64 076029] but the mapping needs to be generalised to the more complex local magnetic geometry at the PCI measurement location. In this work, we describe the details of the geometrical mapping, how it varies between the different magnetic configurations of W7-X and show the application for a comparison of experimental wavenumber spectra to nonlinear results from stella. Beyond the considerations of magnetic geometry for measurement interpretation, we continue to investigate its impact on plasma turbulence using data from the latest operational phases of W7-X (autumn 2024 and spring 2025), which covers additional magnetic configurations. In particular, we extend the comparison of varying magnetic mirror to the negative mirror configuration, which reverses the maximum-J property and its associated impact on the interaction between trapped electrons and turbulent modes. Additionally, we compare the turbulence-relevant geometric quantities identified thus far, such as local magnetic shear at various radial and poloidal positions. This provides a less diagnostic-focused assessment of magnetic configurations with the goal of evaluating the relevance of these quantities to the overall performance.
Support for the MIT and SUNY-Cortland participation was provided by the US Department of Energy under Grant Number DE-SC0014229.