Experimental assessment of magnetic configuration effects on core turbulence in Wendelstein 7-X – methods, insights and limitations*

Optimized stellarator experiments such as Wendelstein 7-X (W7-X) demonstrate the successful reduction of neoclassical transport [Beidler et al. 2021]. In turn, the confinement  behavior of W7-X is dominated by turbulent transport [Klinger et al. 2019, Bozhenkov et al. 2020] and research on stellarator optimization now moves towards turbulence optimized configurations for future stellarator reactors. In light of this progress, we address the question of how magnetic geometry affects turbulence in W7-X, experimentally. Turbulent density fluctuations throughout the plasma core of W7-X are measured by the phase contrast imaging (PCI) diagnostic [Edlund et al. 2018, Huang et al. 2021]. A clear effect of the magnetic configuration on the PCI density fluctuation amplitude and wavenumber spectra is observed in a database approach covering a large range of plasma parameters, as well as in dedicated comparative experiments at various heating power and density levels. Dedicated gyrokinetic flux tube simulations reproduce the observed trend, but only if the experiment parameters and local geometry are matched as closely as possible. In contrast, overall confinement is not significantly affected despite the difference in density fluctuation amplitudes. The question is therefore raised how local observations (experimental and numerical) can be used effectively for the assessment of global transport in the inherently three-dimensional geometry of stellarators.