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This work investigates turbulence in the core plasma of the optimised stellarator
Wendelstein 7-X. It focuses on experimental characterisation and
evaluation of the electrostatic micro-instabilities, which drive turbulent fluctuations,
and the saturation of turbulence by zonal flows. Expectations for
Wendelstein 7-X are formulated by reviewing theoretical work and with
the help of gyrokinetic simulations. The experimental analysis centres on
line-integrated density fluctuation measurements with the phase contrast
imagining diagnostic in electron cyclotron heated hydrogen discharges. An
absolute amplitude calibration was implemented, and a method for reliable
determination of dominant phase velocities in wavenumber-frequency
spectra of density fluctuations has been developed. Line-averaged density
fluctuation levels are observed to vary between magnetic configurations.
The wavenumber spectra exhibit a dual cascade structure, indicating fully
developed turbulence. The dominant instability driving turbulent density
fluctuations on transport relevant scales is identified as ion-temperaturegradient-
driven modes, which are mainly localised in the edge region of the
confined plasma. Despite the line-integrated nature of the measurement, the
localisation of density fluctuations is shown by comparing their dominant
phase velocity with the radial profile of the E × B rotation velocity due to
the ambipolar neoclassical electric field. Nonlinear gyrokinetic simulations
and a simplified plasma rotation model within a synthetic diagnostic confirm
the localisation. Oscillations of the dominant phase velocity indicate
the existence of zonal flows as a saturation mechanism of ion-temperaturegradient-
driven turbulence. A direct effect on turbulent density fluctuation
amplitudes and radial transport is observed.