Magnetic geometry effects on turbulent density fluctuations in Wendelstein 7-X

Line-integrated density fluctuation measurements from the core of the optimised stellarator Wendelstein 7-X are studied in the context of magnetic field geometry changes. The amplitude as well as the spectral distribution of fluctuations is affected by varying the magnetic field configuration. Weaker turbulent fluctuations are observed in configurations with higher rotational transform and lower magnetic mirror depth. Simulations with the gyrokinetic codes stella and GENE-3D are used to investigate the role of magnetic geometry for the experimental observations. The stabilising effect of the high rational transform configuration is reproduced by simulations and can be associated to stronger shaping of the magnetic field. This leads to an increased local shear effect, which limits the toroidal extent of ITG modes. For W7-X configurations with higher magnetic mirror depth, gyrokinetic theory and simulations predict weaker fluctuations, since they are closer to having the maximum-J property, which has a stabilising effect on TEM and ITG. The opposite trend is observed experimentally, which can therefore not be explained by geometry effects on turbulence alone.