First results of turbulence investigations at the Wendelstein 7-X stellarator with the phase contrast imaging diagnostic

Wendelstein 7-X (W7-X) is currently the world's largest optimized stellarator with a plasma volume of 30 m³. As the reduction of neoclassical transport is one of the optimization criteria, turbulent transport mechanisms are believed to play a much more important role now. Numerical gyrokinetic simulations suggest a significantly different appearance of turbulence in stellarators than in tokamaks. However, a systematic experimental investigation of turbulence in optimized stellarators has not been done yet. To address this topic the phase contrast imaging (PCI) diagnostic was installed at W7-X and successfully put into operation in the recent experimental campaign OP 1.2a. The PCI diagnostic allows for non-invasive spatiotemporal measurements of electron density fluctuations. Ion temperature gradient turbulence and trapped electron modes can be resolved - in the hot core up to the colder edge. This talk presents an analysis of the obtained experimental data and a comparison to GENE simulation results. One key focus is the characterization of turbulence in different magnetic field configurations, which are expected to have a major influence on the growth of the instabilities and the resulting spatial localization.