Andreas Langenberg

We present a more detailed numerical analysis of the global GENE-3D simulations of Bähner et al (the W7-X Team) 2026 Nucl. Fusion 66 016007, focusing on turbulent heat and particle transport and the role of the equilibrium radial electric field across mirror ratio and rotational transform ( ι ) configuration scans in Wendelstein 7-X (W7-X). Ion and electron heat fluxes show only mild sensitivity to both configuration parameters. The particle flux, however, exhibits a pronounced dependence on the mirror ratio, with the low-mirror configuration driving a strong inward particle pinch and the high-mirror configuration producing a net outward flux. Density fluctuation levels at fixed plasma profiles do not reproduce the experimentally observed configuration dependence in the mirror scan, a discrepancy that can be understood in terms of the simulations not satisfying power balance. A synthetic phase contrast imaging (PCI) diagnostic applied to the simulations reproduces the experimental ordering of the wavenumber and frequency spectra in the ι scan but not in the mirror scan, consistent with the same interpretation. The equilibrium radial electric field modifies turbulent transport, with the largest effect on the electron heat flux in regions of strong E × B shear. A controlled parametric study, in which the E r profile is systematically varied in amplitude and radial location, confirms that transport suppression follows the classical shear decorrelation criterion, being strongest where the shear rate exceeds the linear growth rate over the widest radial extent.

The x-ray imaging crystal spectrometer (XICS) tomographic inversion code for Wendelstein 7-X has been modified to consider the effects of parallel flows and has been applied to analyze measurements taken during recent experimental campaigns. Previous analysis neglected the effects of parallel flows due to the primarily perpendicular geometry of the sightlines and the small magnitude predicted by neoclassical theory. To reconsider these effects, the incompressibility condition for plasma flows is used to calculate the parallel Pfirsch-Schlüter flow component for the equilibrium configuration. By incorporating this condition along with the geometry of the sightlines - i.e., the fractional contributions of perpendicular and parallel flows -, an updated expression for the measured flow is used for the profile inversion. Application of this modified inversion code to data from W7-X shows that the magnitude of the radial electric field and the flux surface averaged perpendicular flow are reduced by approximately a factor of 2 and brought into better agreement with neoclassical predictions and charge exchange recombination spectroscopy measurements.

Advances in time and space resolved measurements of highly charged states of tungsten (W) through x-ray imaging spectroscopy have enabled investigation of impurity transport in the Wendelstein 7-X (W7-X) stellarator. The high-resolution x-ray imaging spectrometer (HR-XIS) system on W7-X utilizes the Bragg diffraction properties of a set of multiple crystals to measure a range of impurity emission lines within sections of the 1–7 Å wavelength range, including transitions of W. A new indium antimonide crystal has been installed on the HR-XIS system to allow viewing of the 5.5–6.2 Å region focusing on emissions of W. Consequently, many bright W emission lines from W 40 + to W 47 + were observed in this wavelength range, both in plasmas with injected W and in those with only intrinsic W impurity sources, showing the high sensitivity of the diagnostic. Three W 46 + emissions correspond in wavelength and intensity with calculated photon emissivity coefficients and can be exploited for W transport and concentration applications in plasmas with T e ≳ 2.1 keV. An estimate of the core W density behavior in two separate turbulence-reduced ‘high-performance’ (HP) discharges on W7-X is done using the 5.6893 Å W 46 + line. The n W behavior in HP scenarios can be explained by previous experimental results and neoclassical predictions.

The Wendelstein 7-X stellarator has completed two consecutive experimental campaigns (OP 2.2 and OP 2.3) under a new operational strategy enabling more than one year of uninterrupted device availability. This approach, supported by exceptionally high subsystem reliability, allowed sustained high- efficiency plasma operations with up to 80–100 discharges per day across a broad range of magnetic configurations. Several key technical upgrades—most notably the first operation of a 1.5 MW class steady-state gyrotron, a new steady-state pellet injector, and advanced real-time feedback control systems significantly enhanced heating, fueling, and plasma control capabilities. Together, these improvements enabled major advances in long-pulse performance, high-β operation, and confinement optimization. Long-pulse discharges achieved 1.8 GJ of injected energy under fully detached divertor conditions, while reduced-field scenarios facilitated record volume-averaged β values approaching 3%. High- performance plasmas with centrally peaked density profiles, created via NBI or sustained pellet fueling, demonstrated strongly reduced turbulent transport and stellarator-record fusion triple products. Complementary studies of power exhaust and divertor heat loads revealed the role of scrape-off-layer drift physics in shaping strike-line patterns under attached conditions. Together, the results from OP 2.2 and OP 2.3 significantly expand the operational space of W7-X and strengthen its role as a leading platform for steady-state stellarator research and reactor-relevant plasma scenarios.

Steep density gradients generally lead to improved plasma performance in the neoclassically optimized stellarator W7-X. This is evident in the global energy confinement time as well as in the ion temperature and can be explained by a strong reduction of the ion temperature gradient turbulence. Such conditions can experimentally be realized by several methods: injection of cryogenic hydrogen pellets, appropriate combination of neutral beam and electron cyclotron resonance heating and, in some cases, with low power of electron cyclotron resonance heating after preconditioning of the first wall. The duration of the improved phases is determined by the ability to sustain the steep density gradient, by technical limitations of the involved systems and, eventually, by the plasma stability. This paper gives an overview of relevant experimental results and presents example discharges where the improved confinement conditions could be extended to multiple seconds: up to 4 s using neutral beam injection and from 14 to 40 s with steady state pellet injection. In these plasmas the turbulent thermal diffusivity is reduced by a factor of 3 to 4 in a broad radial range, which allows high ion temperatures of up to 3 keV at the densities of about 1.5 • 10 ²⁰ m ^-3 .

Two density fluctuation imaging systems, phase contrast imaging (PCI) and gas puff imaging (GPI) measure spatially resolved density fluctuations with high time resolution throughout the core plasma (PCI) and in the scrape-off layer (GPI) of the Wendelstein 7-X (W7-X) stellarator. Both systems combined give a comprehensive overview of overall fluctuation levels, spectral properties such as their distribution in frequency and wavenumber space as well as their spatial distribution. These tools are used to assess changes in density turbulence in three representative discharges that transition into stable divertor detachment by different strategies (impurity seeding, density ramping and power starvation). Several general trends are identified when the radiated power fraction is systematically increased: In the plasma edge, the line emission observed by GPI shifts radially inward with a drop in electron temperature, and normalized intensity fluctuation profiles follow this inward shift. Skewness and kurtosis of these edge fluctuations are reduced, indicating a reduction of large intermittent transport events, and poloidal phase velocities decrease in magnitude. These observations are consistent with a reduced power input into the plasma edge and a general reduction of turbulent activity. Core density fluctuation levels remain nearly constant in the impurity seeding scenario, indicating that detachment does not significantly impact turbulence there. However, a strong reduction in the dominant outboard fluctuation phase velocity is observed that deviates from the previous interpretation of neoclassical radial electric field changes, showing that the core plasma is not completely unaffected. In the density ramp and power starvation scenarios, undesirable and irregular large-scale events arise clearly in both diagnostic systems as the radiative fraction is increased. Impurity seeding therefore seems to be a promising strategy on W7-X to achieve detachment without significantly altering core turbulence, especially when targeting a specific operating point in core density and heating power.

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.

This paper presents the validation of the GENE-KNOSOS-Tango framework for recovering both the steady-state plasma profiles in the considered radial domain and selected turbulence trends in a stellarator. This framework couples the gyrokinetic turbulence code GENE, the neoclassical transport code KNOSOS, and the transport solver Tango in a multi-timescale simulation feedback loop. Ion-scale kinetic-electron and electron-scale adiabatic-ion flux-tube simulations were performed to evolve the density and temperature profiles for four OP1.2b W7-X scenarios. The simulated density and temperature profiles showed good agreement with the experimental data using a reasonable set of boundary conditions. Equally important was the reproduction of observed trends for several turbulence properties, such as density fluctuations and turbulent heat diffusivities. Key effects were also touched upon, such as electron-scale turbulence and the neoclassical radial electric field shear. The validation of the GENE-KNOSOS-Tango framework enables credible predictions of physical phenomena in stellarators and reactor performance based on a given set of edge parameters.

We apply the global gyrokinetic code GENE-3D to perform kinetic-ballooning-mode (KBM) turbulence simulations in the Wendelstein 7-X (W7-X) stellarator using experimentally relevant parameters. In contrast to the flux-tube approach, we observe saturation of the turbulent fluctuations at finite amplitude. Analyzing the turbulent characteristics, we identify the presence of weakly driven KBM turbulence hybridizing with ion-temperature-gradient and trapped-electron-mode turbulence in the scenario under consideration, showing that it is possible that KBM activity can be driven in W7-X, opening up the path for studying this type of turbulence, which is highly relevant for future fusion power plants.