The fundamental nature of turbulent density fluctuations in standard Wendelstein 7-X (W7-X) stellarator discharges is investigated experimentally via phase contrast imaging (PCI) in combination with gyrokinetic simulations with the code GENE. We find that density fluctuations are ion-temperature-gradient-driven and radially localised in the outer half of the plasma. It is shown that the line-integrated PCI measurements cover the right range of wavenumbers and a favourable toroidal and poloidal location to capture some of the strongest density fluctuations in W7-X. Due to the radial localisation of fluctuations, measured wavenumber–frequency spectra exhibit a dominant phase velocity, which can be related to the $\boldsymbol {E\times B}$ rotation velocity at the radial position of a well in the neoclassical radial electric field. The match is robust against variations of heating power and line-integrated density, which is partly due to the localisation of fluctuations and partly due to effects of the radial gradient in the $\boldsymbol {E\times B}$ velocity profile on the wavenumber–frequency spectrum. The latter effect is studied with a newly built synthetic PCI diagnostic and global gyrokinetic simulations with GENE-3D.

In this Letter, we demonstrate by comparison of computer simulations and experimental results that excitation of turbulence-driven electromagnetic secondary instabilities can serve as a paradigm to explain the observed activity of otherwise stable modes in fusion devices. In particular, we discuss the excitation of Alfvén eigenmodes in electron cyclotron heated plasmas in the Wendelstein 7-X (W7-X) stellarator. In more than 700 experimental programs of W7-X Operational Phase 1.2b (from which 25 were selected and discussed in this paper), poloidal magnetic fluctuation measurements with a clear Alfvénic nature are seen to correlate with turbulence density fluctuations. In response, we perform nonlinear, global, gyrokinetic, electromagnetic simulations using profiles consistent with the experimental conditions. The results agree remarkably well with the measurements and reveal excitation of zonal flow activity and long wavelength Alfvénic modes by ion temperature gradient turbulence. Integer ratios between the primary and secondary growth rates suggest an excitation mechanism similar to the forced-driven excitation of zonal flow by Alfvén eigenmodes.

Analysis of the L–H and H–L transition power thresholds (P_th) and pedestal parameters are presented for the mega ampere spherical tokamak (MAST). The dependencies of P_th on the average, core plasma electron density, X-point height, and plasma current are described. Increasing X-point distance from the divertor floor over 10–12 cm is found to increase P_th by a factor of three, while X-point heights greater than this have no further influence. The X-point height dependence of P_th is also observed to be sensitive to the plasma current. An I_p decrease from 0.77 MA to 0.65 MA, is observed to lower P_th by a factor of three across the X-point height scan and increases the maximum X-point height at which P_th stops increasing by 3 cm. Finally, a comparison of the experimental results with the predictions by the finite beta drift wave model is made, which provides a reasonable condition for the transition into and out of the H-mode.

We present a synthetic phase contrast imaging (PCI) diagnostic for studying turbulence at the Wendelstein 7-X (W7-X) stellarator. We first describe the implemented instrument response model, which captures diffraction effects, detector noise, and the long-wavelength cutoff due to the phase plate of the PCI system. To verify the instrument response model, we show that it is capable of reproducing the PCI signal generated by the sound wave speaker used for calibration at W7-X. Next, we discuss the calculation of synthetic PCI signals based on the global, nonlinear gyrokinetic codes GENE-3D and EUTERPE, including results from some of the first stellarator simulations of this type with kinetic electrons (KEs) in GENE-3D. While the simulations used in this work lack a neoclassical radial electric field, which is crucial for reproducing experimental PCI signals, they do indicate that the dominant rotation direction and velocities of the turbulent fluctuations can be inferred from the wave number-frequency spectra of the PCI signals, as expected. The synthetic PCI wave number spectra are further shown to be similar to those of the line-integrated fluctuating electron density, with distinct differences between adiabatic and KE simulations, explainable by previously published turbulence models. For example, the wave number spectra of all adiabatic electron simulations analyzed here follow a power law with an exponent close to −5 for sufficiently large wave numbers. This indicates that universal features of electron density turbulence at W7-X may be studied using the PCI system.

We assess the turbulent particle transport being responsible for the limitation of the confinement and, thus, the overall performance of the neoclassically optimized stellarator Wendelstein 7-X. The radial particle transport is experimentally inferred from the evaluation of impurity injection into turbulence reduced and enhanced plasma scenarios revealing a completely different confinement behavior. The impact of the density gradient on the turbulent ion transport is theoretically estimated using large-scale non-linear gyro-kinetic simulations enabling, for the first time in Wendelstein 7-X, a quantitative comparison to the experimentally assessed impurity transport properties. We demonstrate that impurity transport in most of the Wendelstein 7-X discharges, up to now impossible to cover only with neoclassical estimations, is dominated by turbulence and can be modelled via gyro-kinetic simulations.

Line-integrated density fluctuation measurements from the core of the optimised stellarator Wendelstein 7-X (W7-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 with stronger shaping of the magnetic field. This leads to increased finite Larmor radius and local shear effects, which limit the toroidal extent of ion temperature gradient (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 trapped-electron mode and ITG. The opposite trend is observed experimentally, which can therefore not be explained by geometry effects on turbulence alone.