The island divertor in Wendelstein 7-X employs a chain of stationary magnetic islands to separate the confined plasma from the divertor targets. In this configuration, the scrape-off layer (SOL) exhibits complex patterns of counter-propagating bi-normal E & times; B plasma flows with typical velocities of a few km s-1. Due to the long parallel connection length in the island divertor, such flows can well compete or dominate parallel transport on the open field lines to the divertor targets, which could significantly alter heat and particle flux patterns. Reversed field experiments clearly indicate an important role of drift flows for density distributions in the SOL and at the divertor. These effects cannot be reproduced by state-of-the-art models such as EMC3-EIRENE, where drift flow physics is not yet included. The mismatch between experiment and simulation poses a critical challenge in predicting heat loads for optimizing W7-X divertor operation at higher heating powers, and for the design of future stellarator reactors. In a separatrix plasma density scan in attached plasmas, a complex picture emerges: drift flow velocities decrease slightly towards higher densities, and divertor heat loads become more up-down symmetric. However, the density distributions in the SOL and at the divertor still diverge from expectations. The role of turbulent transport as another cross-field transport mechanism in this context is explored.

A novel approach for estimating turbulent transport coefficients in fusion devices is presented. The diagnostic method is established on the analysis of the conditional variance of one-point time series of density or temperature fluctuations. It is tested on data obtained from probe measurements in the edge of the tokamak ASDEX Upgrade and the stellarator Wendelstein 7-X, and on synthetic data from the gyrofluid transport model GEMR. The approach demonstrates a remarkable degree of accuracy, typically within a factor of two of the actual transport measured by more difficult means. It is a simple and accurate way of evaluating turbulent particle and heat transport coefficients that does not require measurements of the velocity fluctuations.

In this work, we present numerical simulation results of powder injection in W7-X using the EMC3-EIRENE and DIS codes. First, we model powder injection experiments performed in W7-X with the Probe Mounted Powder Injector. The simulation results qualitatively agree with visible imaging measurements. Secondly, we perform predictive simulations to guide the installation of an Impurity Powder Dropper in W7-X, allowing to choose in between several available non-vertical ports to maximize the amount of powder penetrating into the plasma, as well as the verticality of the port, to minimize sticking of the powders in the in-vessel stainless steel guiding tube. Port AEM41 is selected as the best candidate for IPD installation. The robustness of the simulation results has been verified for different plasma densities, powder materials and sizes, powder friction coefficient and changes in the plasma flow.

The turbulence characteristics of the scrape-off-layer (SOL) plasma in the W7-X stellarator are investigated using a gas-puff-imaging (GPI) diagnostic, newly installed and operated during the OP 2.1 campaign. The SOL plasma on W7-X features a set of island divertors for heat and particle exhaust and provides a unique environment for studying SOL turbulence and transport. This paper focuses on the O-point region of the magnetic island divertor SOL in the standard magnetic configuration. Fourier and cross-correlation analyses show that turbulence flows are predominantly in the poloidal direction (i.e., direction tangent to the last closed flux surface) with significantly weaker radial motion. This suggests dominant ExB convection and suppressed filamentary transport compared to those observed in the far scrape-off-layer region of tokamaks, as further supported by high-resolution skewness and kurtosis data that show the absence of intermittent, bursty filamentary events. Additionally, a relationship between the radial profile of the connection length and the sheared poloidal flow structure is reported, suggesting a possible linkage among magnetic topology, turbulence dynamics, and turbulence generation.

In the Wendelstein 7-X (W7-X) stellarator, a ball-pen probe (BPP) has been routinely employed to measure Ti in the island scrape-off layer (SOL) across large parts of the recent 2024/2025 campaigns. The high temporal resolution of 25 mu s allows us to resolve Ti fluctuations, their probability density functions, and modulation via low-frequency magnetohydrodynamics (MHD) modes. We present a unique comparison of the ion temperature measured using BPP with a more conventional retarding field analyzer (RFA). A good agreement between the two diagnostics is observed, when fast Ti measurements are reduced to the same temporal resolution (5 ms) using 'RFA-like' averaging. Using an RFA-like interpretation of the fast BPP data, we find a linear decrease in SOL Ti and Te with line-integrated density. The SOL ion-to-electron temperature ratio tau i,e ranges between tau i,e= 1 and 3 with smaller tau i,e values observed for higher densities (collisionalities) and at positions close to the LCFS. The upstream BPP Ti measurements are compared to the average C2+ Ti obtained by coherence imaging spectroscopy (CIS), in the divertor region, magnetically mapped to the BPP. The downstream Ti measured by CIS is about half the upstream Ti and exhibits a similar inverse scaling with line-integrated density. Finally, we benchmark the experimentally obtained SOL Ti and Te against the EMC3-Eirene modeling and find good agreement for high density plasmas, but significantly diverging results for low density plasmas.

After a long device enhancement phase, scientific operation resumed in 2022. The main new device components are the water cooling of all plasma facing components and the new water-cooled high heat flux divertor units. Water cooling allowed for the first long-pulse operation campaign. A maximum discharge length of 8 min was achieved with a total heating energy of 1.3 GJ. Safe divertor operation was demonstrated in attached and detached mode. Stable detachment is readily achieved in some magnetic configurations but requires impurity seeding in configurations with small magnetic pitch angle within the edge islands. Progress was made in the characterization of transport mechanisms across edge magnetic islands: Measurement of the potential distribution and flow pattern reveals that the islands are associated with a strong poloidal drift, which leads to rapid convection of energy and particles from the last closed flux surface into the scrape-off layer. Using the upgraded plasma heating systems, advanced heating scenarios were developed, which provide improved energy confinement comparable to the scenario, in which the record triple product for stellarators was achieved in the previous operation campaign. However, a magnetic configuration-dependent critical heating power limit of the electron cyclotron resonance heating was observed. Exceeding the respective power limit leads to a degradation of the confinement.

This work investigates the parallel coherence of plasma filaments through numerical simulations using the hot-ion two-fluid hermes-2 model within the BOUT++ framework. Realistic field lines in the scrape-off layer (SOL) of magnetic fusion devices, especially in stellarator configurations possess a highly varying curvature along the magnetic field line. A varying curvature creates a parallel E × B velocity gradient which might tear the filament apart. The main parameters controlling this process are the collisionality and the electron plasma beta. Simulations of realistic curvature variations along field lines in a circular ASDEX Upgrade-like tokamak and Wendelstein 7-X stellarator (W7-X) show the parallel displacement between different filament sections to correlate with the curvature. The rapidly varying W7-X curvature and the low average curvature drive reduce the propagation of the filament to only a few hundred meters per second. The effect of a finite ion temperature on filament propagation in a W7-X field line geometry is found to be a higher diamagnetic current resulting in stronger charge separation. This work supports simulations and experimental findings that filaments in W7-X are comparably slow due to the large major radius of the device. They do not perform ballistic motion and hence do not drive significant turbulence spreading in the SOL.

The effect of magnetic islands in the core region of Wendelstein 7-X (W7-X) on fast ion confinement is explored through simulations with the BEAMS3D code. A magnetic configuration where the n / m = 5 / 5 island chain is shifted to r / a similar to 0.7 allows the exploration of core island physics in W7-X. The control coil system on W7-X allows the tuning of the island size either increasing the island width or decreasing it. A coupling of the BEAMS3D code to the FIELDLINES code provides a versatile mechanism for incorporating magnetic islands and stochastic regions into the BEAMS3D code. Collisionless simulations suggest that the presence of core islands degrade the confinement of passing particles in the region of the island chain. Full neutral beam simulations of W7-X show a similar behavior with confinement decreasing as the island width is increased. Comparisons between a vacuum magnetic field and low beta HINT2 simulation are made showing similar fast ion behavior. Measurements of lost fast ions in W7-X confirm this trend with the control coil suppressed island configuration showing lower losses than that with no control coils applied. Simulations of fast ion wall loads are performed suggesting no drastic change in loss pattern and a slight reduction in losses with minimized islands.

Plasma filaments have been measured with alkali beam emission spectroscopy in the plasma edge, divertor island, and scrape-off layer of Wendelstein 7-X. Due to the high intensity of a 1–2 kHz plasma mode, a new, correlation based conditional averaging algorithm was used to search for filaments in the signals. With that method, effects of different magnetic configurations and density levels on filament properties are observed. In configurations where the islands are small and do not play an important role for the connection length topology, filaments behave similar to tokamaks. In contrast, in configurations with larger magnetic islands and more complex connection length profiles, filaments behave quite differently, for instance they may or may not appear in the inner side of the divertor island depending on the plasma parameters. Coupling between the filaments and lower frequency events are also showed. The role of filaments in the global and local particle transport is briefly discussed.

Modelling the scrape-off layer of a stellarator is challenging due to the complex magnetic 3D geometry. The here presented study analyses simulations of the scrape-off layer (SOL) of the stellarator Wendelstein 7-X (W7-X) using spatially varying diffusion coefficients for the magnetic standard configuration, extending our previous study (Bold et al 2022 Nucl. Fusion62 106011). Comparing the EMC3-Eirene simulations with experimental observations, an inconsistency between the strike-line width (SLW) and the upstream parameters was observed. While to match the experimental SLW a particle diffusion coefficient D approximate to 0.2 m(2) s(-1) is needed, D approximate to 1 m(2) s(-1) is needed to get experimental separatrix temperatures of 50 eV at the given experimental heating power. We asses the impact of physically motivated spatially varying transport coeffients. Agreement with experimental data can be improved, but various differences remain. We show that drifts are expected to help overcome the discrepancies and, thus, the development of SOL transport models including drifts is a necessary next step to study the SOL transport of the W7-X stellarator.

Boronisation was first used for wall conditioning in W7-X during the OP 1.2b operational period, which was characterized by the use of the fine-grain graphite Test Divertor Unit (TDU) and inertial cooling only. After this period, deposited layers were observed on all inner surfaces. Deposited layers were analyzed on 21 inner wall tiles using ion beam analysis methods, the deposited layers consisted mostly of boron with additional carbon and oxygen. During the operational period OP 2.1 with an actively water cooled divertor made of carbon fiber reinforced carbon, different materials were exposed during two individual boronisations using the multi-purpose manipulator. Deposited boronisation layers on the samples were analyzed using nuclear reaction analysis. The deposited layer thicknesses showed some variation depending on substrate material and surface roughness, but a systematic dependence on material and/or roughness was not observed. Under the typical boronisation conditions at W7-X, one A x h (Ampere times hour) of boronisation results in a boronisation layer with a thickness of about 30 +/- 15 x 1015 B-atoms/cm2 (about 3 +/- 1.5 nm) at the position of the multi-purpose manipulator. The oxygen gettering capacity of the layers is up to 0.5 - 0.9O/B.

A system for studying the spatio-temporal dynamics of fluctuations in the boundary of the W7-X plasma using the Gas-Puff Imaging (GPI) technique has been designed, constructed, installed, and operated. This GPI system addresses a number of challenges specific to long-pulse superconducting devices like W7-X, including the long distance between the plasma and the vacuum vessel wall, the long distance between the plasma and diagnostic ports, the range of last closed flux surface locations for different magnetic configurations in W7-X, and management of heat loads on the system's plasma-facing components. The system features a pair of "converging-diverging" nozzles for partially collimating the gas puffed locally $\approx$135 mm radially outboard of the plasma boundary, a pop-up turning mirror for viewing the gas puff emission from the side (also acting as a shutter for the re-entrant vacuum window), and a high-throughput optical system that collects visible emission resulting from the interaction between the puffed gas and the plasma and directs it along a water-cooled re-entrant tube directly onto the 8 x 16 pixel detector array of the fast camera. The DEGAS 2 neutrals code was used to simulate the H$_\alpha$ (656 nm) and the HeI (587 nm) line emission expected from well-characterized gas-puffs of H$_2$ and He and excited within typical edge plasma profiles in W7-X, thereby predicting line brightnesses used to reduce the risks associated with system sensitivity and placement of the field of view. Operation of GPI on W7-X shows excellent signal to noise ratios (>100) over the field of view for minimally perturbing gas puffs. The GPI system provides detailed measurements of the 2-dimensional (radial and poloidal) dynamics of plasma fluctuations in the W7-X edge, scrape-off layer, and in and around the magnetic islands that make up the island divertor configuration employed on W7-X.

We present the first validated synthetic diagnostic for fast ion loss detectors (FILDs) in the Wendelstein 7-X (W7-X) stellarator. This model has been developed on, and validated against experimental data from, a FILD provided by the National Institute for Fusion Science (NIFS-FILD), with potential future applicability to the existing Faraday Cup FILD (FC-FILD) on W7-X as well as the scintillating FILD (S-FILD) currently under development. A workflow combining Monte Carlo codes BEAMS3D and ASCOT5 is used to track fast ions produced by neutral beam injection from the moment of ionization until they are thermalized or lost from the last closed flux surface, and from there to a virtual plane which serves as a projection of the entrance aperture to the FILD. Simulations in ASCOT5 are analyzed via a geometric method to determine the probability of transmission through the FILD aperture and onto the detector as a function of normalized momentum, pitch angle, gyrophase, and position at the virtual plane. This probability is then applied to the simulated ions arriving from the plasma, producing a simulated signal from a computationally tractable number of simulated fast ions. Simulated signals are presented for two W7-X experiments with neutral beam injection and quantitatively compared with experimental measurements from the NIFS-FILD diagnostic. An estimate of the frequency of charge-exchange with neutral particles in the edge is performed, and it is found that this process may have a significant impact on the measured signals.

The formation of the radial electric field, E r in the scrape-off layer (SOL) has been experimentally studied for attached divertor conditions in stellarator W7-X. The main objective of this study is to test the validity in a complex three-dimensional (3D) island divertor of simple models, typically developed in tokamaks, relating E r in the SOL to the sheath potential drop gradient at the target. Additionally, we investigate the effect of the edge E r shear on the reduction of density fluctuation amplitude, a well-established phenomenon according to the existing bibliography. The main diagnostic for measurements in the SOL is a V-band Doppler reflectometer that can provide the measurement of the E r and density fluctuations with good spatial resolution. 3D measurements of divertor parameters have been carried out using infrared cameras, with the exponential decay length of the divertor heat flux ( λ q ) resulting in a suitable proxy for the model-relevant λ T , the exponential decay length of the temperature at the divertor. In the investigated attached regimes, it is shown for the first time that the formation of the E r in the SOL depends on parameters at the divertor, following a E r ∝ T e / λ q qualitatively similar to that found in a tokamak. Then, from the analyzed plasmas, the observed E r shear at the edge is linked to a moderate local reduction of the amplitude of density fluctuations.

Experimental evidence of parametric decay instability (PDI) is observed in the Wendelstein 7-X stellarator, when high-power microwave beams cross a stationary magnetic island at the plasma edge. Here, trapping and build-up of upper hybrid waves within a density bump (measured within the island by alkali beam emission spectroscopy) is responsible for the reduction of the instability power threshold below the maximum gyrotron power. In this paper, we provide the first experimental evidence of the connection between the trapping mechanism in the island density bump and excitation of PDI-related signals. We show correlations of periodic crashes in the PDI-related signals with quasi-continuous fluctuations at the plasma edge, which, additionally, cause a flattening of the density profile in the island. We demonstrate that flattening of the experimental density profiles can suppress the trapping mechanism and inhibit the low-threshold PDI. PDI on the edge island could alter the power deposition profile and reduce the efficiency of the electron cyclotron resonance heating system, simultaneously posing a serious threat to the optimal operation of microwave-based diagnostics and plasma-facing components.

A universal energy partition mechanism between ions and electrons has been confirmed to exist in the scrape-off layer of both the WEST tokamak and the W7-X stellarator. A peaked plasma density structure induced by an infinite magnetic connection length structure is observed to destroy this dependence and enhance the local ion and electron temperature ratio in the stellarator. A theoretical analysis reveals that the ratio of ion and electron parallel heat conduction is predominant in determining this universality, while electrons would further be cooled in the density-peaked region.

Hot spots and bright patterns on plasma-facing components (PFC) in visible light are observed in electron cyclotron resonance heated (ECRH) plasmas at low densities in the stellarator Wendelstein 7-X. The events are often located far outside of any convective plasma loads and led in some cases to damages of diagnostic components, where the interaction zones qualitatively agree with fast particle loss simulations. Reciprocating electric probes indicate that this phenomenon is related to a non-thermal electron population that can have a beam like character, being directed in one parallel direction. Further, the electrons can be trapped on rational flux surfaces and be used to map magnetic islands.

A single-reservoir particle balance for the main plasma species hydrogen has been established for W7-X. This has enabled the quantitative characterization of the particle sources in the standard island divertor configuration for the first time. Findings from attached scenarios with two different island sizes with a boronized wall and turbo molecular pumping are presented. Fueling efficiencies, particle flows and source locations were measured and used to infer the total particle confinement time τ p . Perturbative gas injection experiments served to measure the effective particle confinement time τ p * . Combining both confinement times provides access to the global recycling coefficient R ¯ . Hydrogen particle inventories have been addressed and the knowledge of particle sources and sinks reveals the core fueling distribution and provides insight into the capability of the magnetic islands to control exhaust features. Measurements of hydrogen fueling efficiencies were sensitive to the precise fueling location and measured between 12% and 31% with the recycling fueling at the strike line modeled at only 6%, due to much higher densities. 15% of the total 5.2 × 10 22 a/s recycling flow ionizes far away from the recycling surfaces in the main chamber. It was shown that 60% of recycled particles ionize above the horizontal and 18% above the vertical divertor target, while the remainder of the recycling flow ionizes above the baffle (7%). Combining these source terms with their individual fueling efficiencies resolves the core fueling distribution. Due to the higher fueling efficiency in the main chamber, up to 51% of the total 5.1 × 10 21 s −1 core fueling particles are entering the confined plasma from the main chamber. τ p values in the range of 260 ms were extracted for these discharges. Together with τ p , the global recycling coefficient R ¯ was resolved for every τ p * measurement and a typical value close to unity was obtained. An increase of the island size, resulted in no change of τ p , but doubled τ p * , indicating the feasibility of the control coils as an actuator to control exhaust features without affecting core confinement properties.

We present novel experimental evidence of parametric decay instability of microwave beams in the plasma edge of the Wendelstein 7-X stellarator. We propose that the instability is sustained by trapping of only one daughter wave in the non-monotonic density profile measured with high spatial resolution within a stationary magnetic island. The power levels and spectral shapes of the detected microwave signal are reproduced by numerical modelling and a theoretical power threshold is predicted around 300 kW, comparable with observations. We predict a fraction of power drained by daughter waves around 4% in the experiments, potentially increasing above 50% for more hollow edge density profiles. Such absorption levels could significantly reduce the efficiency of the microwave heating and current-drive system in tokamaks and stellarators.

Detached divertor plasmas has been achieved by density ramp in W7-X. During the transition into detachment, the main divertor radiation region moves from the region close to the divertor targets to the region around LCFS. The typical spatial distributions of carbon and hydrogen Balmer lines in detached divertor plasma are presented. They show that the line emissions of CH produced by chemical sputtering is still strong close to the targets. The C I fills up the whole detached divertor plasma and its intensity increases when approaching the targets due to the dissociation from hydrocarbon and self-sputtering. The C II and C III are mainly located at around LCFS region. Meanwhile, the distribution of hydrogen Balmer lines is considerably flatter in detached divertor plasma. The Balmer line ratios indicate that the detached divertor plasma seems to be still in excitation or partial recombination state. This is consistent with the divertor Langmuir probe measurements which shows that the electron temperature near strike line is around 6 eV. Finally, the electron pressure at upstream and downstream position is measured and its ratio shows that the electron pressure drops significantly along the magnetic field lines. However, the pressure can not drop further due to the lack of the volume recombination in detached divertor plasma.