Description
The stellarator Wendelstein 7-X is equipped with several Doppler backscattering (DBS) systems localized at the outer midplane at two different toroidal positions. Key subject is the measurement of the perpendicular bulk plasma rotation and the backscattered power, which depends nonlinearly on the rms density fluctuation level. The synced operation of multiple DBS systems allows characterizing radial correlation lengths and zonal flow oscillations. The DBS measurement provides radial electric field profiles, which are compared with neoclassical predictions in distinct plasma scenarios in mixed electron-ion root regimes with strong central electron heating and pure ion-root conditions. Deviations between the measured and predicted perpendicular velocity indicate a contribution of the turbulent phase velocity and allow to disentangle the underlying micro-instabilities driven by temperature and density gradients.
The presentation focuses on a detailed comparison of the neoclassical radial electric field and DBS measurements for scenarios in which the linear growth rates of microinstabilities differ significantly, e.g. weak vs. strong density gradients and varying collisionality. The measurements are obtained at multiple fluctuation wavenumbers that can be directly compared with linear predictions. In addition, the interplay between low-frequency zonal flows, thought to regulate turbulence by nonlinear transfer of energy among different scales, and the fluctuation amplitude is characterized.