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The detachment regime has a high potential to play an important role in fusion devices on the road to a fusion power plant. Complete power detachment has been observed several times during the experimental campaigns of the Wendelstein 7-X (W7-X) stellarator. Automatic observation and signaling of such events could help scientists to better understand these phenomena. With the growing discharge times in fusion devices, machine learning models and algorithms are a powerful tool to process the increasing amount of data. We investigate several classical supervised machine learning models to detect complete power detachment in the images captured by the Event Detection Intelligent Camera System (EDICAM) at the W7-X at each given image frame. In the dedicated detached state the plasma is stable despite its reduced contact with the machine walls and the radiation belt stays close to the separatrix, without exhibiting significant heat load onto the divertor. To decrease computational time and resources needed we propose certain pixel intensity profiles (or intensity values along lines) as the input to these models. After finding the profile that describes the images best in terms of detachment, we choose the best performing machine learning algorithm. It achieves an F1 score of 0.9836 on the training dataset and 0.9335 on the test set. Furthermore, we investigate its predictions in other scenarios, such as plasmas with substantially decreased minor radius and several magnetic configurations.

To support the scenario design for the upcoming long-pulse high-performance campaign of Wendelstein 7-X, this work presents a study of high-beta full-field 3D equilibria obtained with the HINT code. For three magnetic configurations of different edge-ι, the effects of both overall pressure and pressure profile changes on the magnetic topology are analyzed. Anisotropic diffusion modeling is used to obtain estimates of the conductive heat load distribution both on the divertor and other plasma-facing components in finite-beta magnetic configurations. For the magnetic standard configuration, limitations of the model are outlined by comparing measured and predicted heatloads by performing a linear regression of the main strike-line position against various plasma parameters in both the experimental and the simulated device.

One set of horizontal target elements of the Test Divertor Units (TDU), retrieved from the Wendelstein 7-X (W7-X) vessel after the end of second divertor Operation Phase (OP1.2B) in Hydrogen (H), were investigated by picosecond Laser-Induced Breakdown Spectroscopy (ps-LIBS). The Boron (B) distribution, H pattern and the material erosion/deposition pattern on these target elements were analyzed with high depth resolution and mapped in the poloidal direction of W7-X. From the spectroscopic analysis, B, H, Carbon (C) and Molybdenum (Mo) were clearly identified. A non-uniformly distributed B pattern on these divertor target elements was determined by the combination of B layer deposition during the three boronizations and W7-X plasma operation with multiple erosion and deposition steps of B. Like the TDU, the analyzed target elements are made of fine grain graphite, but have two marker layers which allow us to determine the material migration via the ps-LIBS technique. Two net erosion zones including one main erosion zone with a peak erosion depth of 6.5 μm and one weak erosion with a peak erosion of 1.3 μm were determined. Between two net erosion zones, a net deposition zone with width of 135 mm and a thickness up to 3.5 μm at the peak deposition location was determined by the ps-LIBS technique. The B distributions are correlated with the erosion/deposition pattern and the operational time in standard magnetic configuration of W7-X in the phases after the boronizations. The thickness of the containing B layer on these target elements also correlates with the erosion/deposition depth, in which the thickness of the containing B layer varies spatially in poloidal direction between 0.1 μm and 6 μm. Complementary, Focused Ion Beam combined with Scanning Electron Microscopy (FIB-SEM) was employed also to verify and investigate the deposition layer thicknesses at typical net erosion and net deposition zones as well as to identify the three boronizations in depth.

In electron (cyclotron) heated plasmas, in both ASDEX Upgrade (L-mode) and Wendelstein 7-X, clamping of the ion temperature occurs at T i ∼ 1.5 keV independent of magnetic configuration. The ions in such plasmas are heated through the energy exchange power as n e 2 ( T e − T i ) / T e 3 / 2 , which offers a broad ion heating profile, similar to that offered by alpha heating in future thermonuclear fusion reactors. However, the predominant electron heating may put an additional constraint on the ion heat transport, as the ratio T e/T i > 1 can exacerbates ITG/TEM core turbulence. Therefore, in practical terms the strongly ‘stiff’ core transport translates into T i-clamping in electron heated plasmas. Due to this clamping, electron heated L-mode scenarios, with standard gas fueling, in either tokamaks or stellarators may struggle to reach high normalized ion temperature gradients required in a compact fusion reactor. The comparison shows that core heat transport in neoclassically optimized stellarators is driven by the same mechanisms as in tokamaks. The absence of a strong H-mode temperature edge pedestal in stellarators, sofar (which, like in tokamaks, could lift the clamped temperature-gradients in the core), puts a strong requirement on reliable and sustainable core turbulence suppression techniques in stellarators.

Sawtooth-like crashes were observed during electron cyclotron current drive experiments for strikeline controls at the optimised superconducting stellarator Wendelstein 7-X (W7-X). The majority of the crashes did not have a relevant impact on plasma performance. However, a limited number of events, characterised by a large plasma volume affected by the instability, have been related to a strong deterioration performance and even to the premature termination of the plasma. The hot plasma core expelled during these sawtooth crashes can reach the plasma edge, where plasma surface interaction can occur and impurities can be released. The x-ray tomography shows a strong radiation increase starting from the edge and moving towards the inner plasma regions. This results in the cooling down and shrinking of the plasma, which eventually leads to a poor coupling of the ECRH to the electrons, that can in turn result in a plasma loss. A relation between the size and amplitude of the sawtooth crashes and the impurity increase is reported.

Electron cyclotron resonance heating (ECRH) is a powerful and flexible plasma heating technique that serves as the main heater at Wendelstein 7-X (W7-X) and will be used at ITER for start-up, heating, current drive and mitigation of plasma instabilities. In the case of poor or degraded microwave absorption, which is expected in the O2-mode heating scenario, a significant part of the beam directly hits the wall, leading to local overheating and potential damage. The ECRH shine-through power is mostly reflected onto the targets; only a small fraction is really absorbed through ohmic losses (typically 3% for graphite at 140 GHz). The ohmic losses do not only depend on the material properties and the frequency, but also on the polarization of the wave and the angle of incidence. This paper presents a thermographic analysis of ECRH experiments at W7-X, including heat load and temperature simulations of the first wall that include ECRH shine through. Two O-mode ECRH experiments with both a high temperature rise of the first wall and different angles of beam incidence on the wall’s surface are depicted. One experiment has 775 kW of power modulation (5 Hz) with mixed polarization (45% O-mode, 55% X-mode) and an EC beam angle almost normal to the first wall. The second has 550 kW of steady EC power with O-mode polarization, a shallow beam angle and increased power absorption by the material. It is shown that infrared thermography is a useful tool for measuring shine-through power and protecting wall components.

Pulsed injections of boron carbide granules into Wendelstein 7-X stellarator (W7-X) plasmas transiently increase the plasma stored energy and core ion temperatures above the reference W7-X experimental programs by up to 30%. In a series of 4 MW electron cyclotron resonance heating experiments, the PPPL Probe Mounted Powder Injector provided 50 ms bursts of 100 μm granules every 350 ms at estimated quantities ranging from approximately 1 mg/pulse to over 30 mg/pulse. For each injection, the stored energy was observed to initially drop and the radiated power transiently increased, while the radial electron density profile rose at the edge as material was assimilated. Once the injected boron carbide was fully absorbed, the density rise transitioned to the core while the stored energy increased above the previous baseline level by an amount linearly correlated with the injection quantity. During the injection, the ion temperature gradient steepened with peak core ion temperatures observed to increase from a nominal 1.7 keV to over 2.6 keV for the largest injection amounts. Enhanced performance is accompanied by a reversal of the radial electric field at ρ < 0.3, indicating that the core transport has switched to the ion root. These observations are suggestive of a change in transport and provide further evidence that externally induced profile modifications provide a possible path to enhanced W7-X performance metrics.I. INTRODUCTIO

Investigations of particle parallel flow velocities have been carried out for the scrape-off layer (SOL) of the Wendelstein 7-X (W7-X) stellarator, in order to gain insights on the SOL transport properties during attached and detached plasma scenarios. The experimental evidence is based on the coherence imaging spectroscopy (CIS) diagnostic, able to measure 2D impurity emission intensity and flow velocity. The impurity monitored by CIS is C2+, characterized by a line-emission intensity observed to be linearly proportional to the total plasma radiated power in both attached and detached plasmas. The related C2+ velocity shows a strong dependence on the line-averaged electron density while remaining insensitive to the input power. During attached plasmas, the velocity increases with increasing line-averaged density. The tendency reverses in the transition to and during detachment, in which the velocity decreases by at least a factor of 2. The sharp drop in velocity, together with a rise in line-emission intensity, is reliably correlated to the detachment transition and can therefore be used as one of its signatures. The impurity flow velocity appears to be well coupled with the main ions’ one, thus implying the dominant role of impurity-main ion friction in the parallelimpurity transport dynamics. In view of this SOL impurity transport regime, the CIS measurement results are here interpreted with the help of EMC3-Eirene simulations, and their major trends are already explainable with a simple 1D fluid model.

A method is presented for the swift reconstruction of electron density profiles measured by the alkali beam emission spectroscopy. It is based on the linearization of the governing rate equations and leads to a direct calculation for obtaining the profiles. The uncertainties of the measurement are incorporated into the problem through the utilization of Tikhonov regularization and the generalized least squares method. An approximation for the uncertainty of the reconstructed density data is calculated as well. The applicability of the method is tested against both simulated and real experimental results of the W7-X stellarator.

The W7-X stellarator has so far performed experiments under both limiter and divertor conditions. The plasma is mostly generated by ECR-heating with powers up to 6.5 MW, and the plasma density is usually limited by the radiation losses from low-Z impurities (such as carbon and oxygen) released mainly from the graphite targets. The present work first summarizes the radiation loss fractions f rad achieved in quasi-stationary hydrogen plasmas in both operational phases, and then shows how impurity radiation behaves differently with the two different boundary conditions as the plasma density increases. The divertor operation is emphasized and some beneficial effects (with respect to impurity radiation) are highlighted: (1) intensive radiation is located at the edge (r/a > 0.8) even at high radiation loss fractions, (2) the plasma remains stable up to f rad approaching unity, (3) the reduction in the stored energy is about 10% for high f rad scenarios. Moreover, effects of wall boronisation on impurity radiation profiles are also presented.

A good understanding of the confinement of energetic ions in non-axisymmetric magnetic fields is key for the design of reactors based on the stellarator concept. In this work, we develop a model that, based on the radially-local bounce-averaged drift-kinetic equation, classifies orbits and succeeds in predicting configuration-dependent aspects of the prompt losses of energetic ions in stellarators. Such a model could in turn be employed in the optimization stage of the design of new devices.

The optimized stellarator Wendelstein 7-X (W7-X) is designed to have an approximately quasi-isodynamic magnetic configuration with reduced neoclassical transport in comparison to a classical stellarator, and turbulent transport is expected to be a significant source of anomalous heat transport across the plasma minor radius. The ion temperature gradient driven mode and the trapped electron mode (TEM) are thought to be responsible for the ion-scale turbulence in W7-X plasmas with volume averaged pressure below 1%. In this work, the electron temperature gradient driven turbulence is shown to be a good candidate for the explanation of the observed electron heat flux, in the inner plasma region where the density gradient is weak (in the outer region, a relatively stronger density gradient would drive additional TEM turbulence). The experimental electron heat transport measured during electron cyclotron resonant heating power and plasma density scans is compared to neoclassical predictions, and the stiffness in the electron heat transport measured during transient transport experiments is presented in three common magnetic configurations of W7-X. In low-⟨β⟩ plasma discharges, the stiffness in the electron heat flux, quantified by the ratio of the heat pulse to power balance diffusivity, χ e H P / χ e P B , is measured to be less than 2, and trend downwards with increasing collisionality.

The effect of magnetic islands on plasma flow and turbulence has been experimentally investigated in the stellarator W7-X. Magnetic configurations with the 5/5 magnetic island positioned at the plasma edge, inside the last closed flux surface, are studied. The main diagnostic used in the present work is a V-band Doppler reflectometer that allows the measurement of the perpendicular plasma flow and density fluctuations with good spatial resolution. A characteristic signature of the 5/5 magnetic island is clearly detected in the perpendicular flow profile. The comparison of the experimental flow and the neoclassically driven E × B flow indicates that the island contribution to the flow is maximum at the island boundaries and close to zero at the island O-point. Besides, a reduction in the density fluctuation level is found nearby the island O-point. The similarities between these observations and those found in other devices and in gyrokinetic simulations are discussed.

In this work, a new version of KNOSOS is presented. KNOSOS is a low-collisionality radially-local, bounce-averaged neoclassical code that is extremely fast, and at the same time, includes physical effects often neglected by more standard codes: the component of the magnetic drift that is tangent to the flux-surface and the variation of the electrostatic potential on the flux-surface. An earlier version of the code could only describe configurations that were sufficiently optimized with respect to neoclassical transport. KNOSOS can now be applied to any large aspect ratio stellarator, and its performance is demonstrated by means of detailed simulations in the configuration space of Wendelstein 7-X., Comment: IAEA paper, submitted to Nuclear Fusion

Tracer-encapsulated solid pellet (TESPEL) was injected from the outboard midplane into Wendelstein 7-X (W7-X) plasmas. Viewing from behind the flight path, the distribution of the light emissions from the TESPEL cloud particles (H, C and C2+) was recorded using an ultrafast visible camera with a temporal resolution up to 2 µs, which allows resolving both the pellet cloud evolution and the detachment of the drifting cloud. It was observed that both the B┴ and the B║ aligned dimensions of the pellet cloud fluctuate quasi-periodically, this being associated with a vertical cloud movement and an eruption/ejection of part of the cloud. It is found that, first, the pellet cloud expands parallel to the magnetic field lines for about 10 µs. Then, when the cloud reaches a certain B║ size the ionized part of the cloud (plasmoid) moves vertically (typically upwards) and detaches itself from the pellet within 10 µs. One possible explanation for the upward movement of the pellet cloud is the presence of a vertical component of grad(B) pointing downward resulting in an upward cloud drift.

In the Wendelstein 7-X stellarator, the main locations of particle sources are expected to be the carbon divertors, baffles and graphite heat shield first wall. In this paper, the heat shield is implemented in EMC3-EIRENE to understand the expected areas and magnitudes of the recycling flux to this component. It is found that in the simulation the heat shield is not a significant source of recycling neutrals. The areas of simulated recycling flux are shown to correlate well with footprints of plasma-wetting seen in post-experimental campaign in-vessel inspection photos. EMC3-EIRENE reconstruction of line-integrated H-alpha measurements at the heat shield indicate that the majority of emission does not come from local recycling neutrals. Rather, the H-alpha signals at the heat shield are dominated by ionization of neutrals which have leaked from the divertor/baffle region into the midplane. The magnitude of the H-alpha line emission from the synthetic reconstruction is consistent with the experiment, indicating that a large overestimation of heat shield recycling would occur if these measurements were assumed to be from local recycling sources. In the future, it may be possible to obtain some information of local recycling from the heat shield since it was found that the majority of the recycling flux occurs on two well-localized areas.

Turbulent transport is widely considered to be the main driver for cross-field transport in the scrape-off layer (SOL) of toroidal magnetized plasmas. Here, reciprocating Langmuir probes are employed to measure both the plasma profiles and the turbulent particle transport in the SOL of the Wendelstein 7-X stellarator. The relation between turbulent radial particle flux Γr and the local pressure gradient is often approximately linear across the entire SOL width, indicating that radial turbulence spreading is absent. This observation holds across a wide range of magnetic configurations and different plasma heating and density scenarios. The magnitude of the turbulent transport for a given gradient reveals a dependence on the magnetic configuration and the position in the SOL, which we relate to the cross-spectral characteristics of multi-tip floating potential measurements. Magnetic islands can add further complexity due to non-monotonic SOL profiles and the breaking of the transport-gradient relation. Finally, anomalous diffusion coefficients are determined from the probe measurements.

The perpendicular velocity (v ⊥) of turbulence is measured by means of a hopping poloidal correlation reflectometer diagnostic for the study of turbulence rotation in the plasma edge region in Wendelstein 7-X. An elliptic approximation (EA) model is applied for calculating the space-time correlation in the edge region. The v ⊥ profile is compared in the high-ι (edge ι ≈ 1.2) and limiter (edge ι < 1) configurations. It is found that the v ⊥ gradient is inversely proportional to the heating power in the high-ι configuration, which could be attributed to the turbulence activities in the plasma edge. In the limiter configuration, the v ⊥ spike is observed near the island boundary, which is corresponded to the plasma current crash. In the standard configuration (edge ι ≈ 1), the v ⊥ profile in the island region distributes as a function of the perpendicular separation. With increasing of the plasma current, the island enlarges the size as is referred from the v ⊥ scope, accompanying with steepening of the v ⊥ gradient at the boundary of the island.

The optimised superconducting stellarator W7-X generates its rotational transform by means of external coils, therefore no toroidal current is necessary for plasma confinement. Electron cyclotron current drive experiments were conducted for strikeline control and safe divertor operation. During current drive experiments periodic and repetitive crashes of the central electron temperature, similar to sawtooth crashes in tokamaks, were detected. Measurements from soft x-ray tomography and electron cyclotron emission show that the crashes are preceded by weak oscillating precursors and a displacement of the plasma core, consistent with a (m, n) = (1, 1) mode. The displacement occurs within 100 μ s , followed by expulsion and redistribution of the core into the external part of the plasma. Two types of crashes, with different frequencies and amplitudes are detected in the experimental program. For these non-stationary parameters a strong dependence on the toroidal current is found. A 1-D heuristic model for current diffusion is proposed as a first step to explain the characteristic crash time. Initial results show that the modelled current diffusion timescale is consistent with the initial crash frequency and that the toroidal current rise shifts the position where the instability is triggered, resulting in larger crash amplitudes.

Coated glass targets are a key component of the Wendelstein 7-X laser blow-off system that is used for impurity transport studies. The preparation and analysis of these glass targets as well as their performance is examined in this paper. The glass targets have a high laser damage threshold and are coated via physical vapor deposition with µm thick films. In addition, nm-thin layers of Ti are used as an interface layer for improved ablation efficiency and reduced coating stress. Hence, the metallic or ceramic coating has a lateral homogeneity within 2% and contaminants less than 5%, being optimal for laser ablation processing. With this method, a short (few ms) and well defined pulse of impurities with about 1017 particles can be injected close to the last closed flux surface of Wendelstein 7-X. In particular, a significant amount of atoms with a velocity of about 1 km/s enters the plasma within 1 ms. The atoms are followed by a negligible concentration of slower clusters and macro-particles. This qualifies the use of the targets and applied laser settings for impurity transport studies with the laser blow-off system in Wendelstein 7-X.

The Charge Exchange Recombination Spectroscopy (CXRS) diagnostic has become a routine diagnostic on almost all major high temperature fusion experimental devices. For the optimized stellarator Wendelstein 7-X (W7-X), a highly flexible and extensive CXRS diagnostic has been built to provide high-resolution local measurements of several important plasma parameters using the recently commissioned neutral beam heating. This paper outlines the design specifics of the W7-X CXRS system and gives examples of the initial results obtained, including typical ion temperature profiles for several common heating scenarios, toroidal flow and radial electric field derived from velocity measurements, beam attenuation via beam emission spectra, and normalized impurity density profiles under some typical plasma conditions.

Equilibrium analysis in fusion devices usually relies on plasma pressure profiles and magnetic measurements outside the plasma. The kinetic profiles can give indirect information about the equilibrium magnetic field, while the stationary magnetic diagnostics cannot resolve current distributions on a smaller scale. This work presents a reciprocating magnetic probe, designed to provide direct plasma response measurements of the magnetic field in the scrape-off layer of Wendelstein 7-X. Hardware design and frequency characteristics are discussed, and a post-processing technique for extending the lower frequency cutoff of the integration scheme is presented.

Numerical calculations based on nonlinear two-fluid equations have been carried out to understand the sawtooth-like crashes of electron temperature observed in Wendelstein 7-X stellarator experiments with electron cyclotron current drive (ECCD). The application of ECCD leads to non-monotonic radial profiles of the safety factor q with two q = 1 surfaces in the plasma core region. Using input parameters similar to the experiment, numerical calculations show two types of crashes of the central electron temperature depending on the growth of the internal kink mode coupled to double tearing modes. When the distance between two equilibrium q = 1 surfaces is sufficiently large, the internal kink mode leads to a fast radial displacement of the hot core from the center to the region of the q = 1 surfaces. In agreement with experimental observation, this causes a full crash of central electron temperature in about 20–30 microseconds. When the distance between the two q = 1 surfaces is not large enough, partial crashes of the central electron temperature are found. During these crashes the hot core only temporally moves outwards. Then it moves back, because the double tearing mode perturbation leads to an increase of the minimum q value about one.

In this paper, the plasma volume averaged impurity confinement of selected charge states and impurity species has been characterized for the Stellarator Wendelstein 7-X (W7-X), covering a wide range of atomic charges ( Z = 12–44) and atomic masses ( M = 28–184). A comparison of the experimental findings to theoretical neoclassical and turbulent transport expectations suggests, aside from/in addition to the neoclassical transport, an additional significant anomalous transport mechanism, which is not inconsistent with the predictions of a turbulence dominated impurity transport and is in agreement with the experimental results from recent transport studies based on the direct measurements of impurity diffusion profiles, performed at W7-X.

openaire: EC/H2020/633053/EU//EUROfusion The 2018 operation phase (OP 1.2b) of the stellarator Wendelstein 7-X (W7-X) included, for the first time, neutral beam injection (NBI) to heat the plasma. Since the injection geometry at W7-X is not parallel, this generates both passing and trapped fast particles. During longer phases of NBI injection, with the primary purpose to study the heating efficiency of this system, Alfven eigenmodes (AEs) were observed by a number of diagnostics, including the phase contrast imaging (PCI) system, the magnetic pick-up coils (Mirnov coils), and the soft x-ray multi-camera tomography system (XMCTS). Alfven eigenmodes are of great interest for future fusion reactors as it has been shown that the resonant interaction of fast ions with self-excited AEs can lead to enhanced transport of fast ions and potentially to energy losses. This is especially true for so-called gap-modes, Alfven eigenmodes with frequencies in gaps of the continuous spectrum, since they lack continuum damping. These modes are commonly known to be excited by fast ions, but other destabilizing mechanisms, e.g. the electron-pressure gradient are also possible. In this article we present a first analysis of the experimentally observed frequencies from the theoretical side. The calculation of shear Alfven wave continua for selected cases and the assignment of observed frequencies to the gaps of the continuous spectra are presented. Using the ideal-MHD code CKA (Konies A. 200710th IAEA TM on Energetic Particles in Magnetic Confinement System), we find gap modes that match the experimental measurements in terms of the observed frequencies. We emphasize the crucial roles played by the coupling of sound and Alfven waves as well as of the Doppler shift arising as a consequence of the radial electric field in W7-X. We employ the perturbative gyrokinetic code CKA-EUTERPE (Feh ' er 2013 Simulation of the interaction between Alfv ' en waves and fast particles), using a slowing-down distribution function for the fast ions as calculated by the Monte-Carlo particle following code ASCOT (Hirvijokiet al2014Comput. Phys. Commun.185 1310-21) to assess the fast-ion drive. We find that the fast-ion drive is insufficient to overcome the background-plasma damping. The fact that unstable modes were observed experimentally may point to problems with the modelling or indicate the existence of other destabilizing mechanisms, e.g. associated with the electron-pressure gradient (Windischet al2017Plasma Phys. Control. Fusion59 105002) that sensitively depend on the profiles of the background plasma.

Comprehensive linear stability studies of resistive modes are presented for Wendelstein 7-X-type stellarator equilibria with electron cyclotron current drive (ECCD). The external co-current drive leads to an increase of the rotational transform and the formation of one or two ι = 1 rational flux surfaces. Using the 3D linear stability CASTOR3D code, low n *-type resistive modes (n * = 1,2...,8 with n * being the dominant toroidal Fourier harmonic contributing to the mode) are investigated. We studied the dependence of their growth rates on plasma resistivity, parallel viscosity, and shape of the rotational transform profile (especially various distances between two ι = 1 flux surfaces). Similarly to tokamak configurations, single and double tearing modes, and internal resistive kink modes are found. In addition, modes oscillating between two n *-types of the same mode family are observed. The frequencies of those modes are in the range of ≈ 10–260 Hz. Equilibria with either a large distance between the ι = 1 flux surfaces, or a single ι = 1 surface are most unstable with respect to n * = 1 resistive kink modes. The latter finding fits to the experimental observation of sawtooth-like oscillations followed by a thermal quench in W7-X discharges with ECCD.

In the last decade, Coherence Imaging Spectroscopy (CIS) has shown distinctive results in measuring ion flow velocities in the edge of magnetically confined plasma devices. Its 2D spatially resolved measurement capabilities and its high optical throughput are ideal for investigating the impurity behavior in the complex 3D magnetic island topology edge of Wendelstein 7-X (W7-X). However, a highly precise and stable calibration method is required for a reliable diagnostic operation. A new level of precision and stability has been achieved for the two CIS systems installed at W7-X with the use of a new calibration source, a continuous tunable laser commercially available only since 2015. A specific prototype model was successfully adapted to the challenging requirements of W7-X, granting high accuracy (±0.01 pm) and flexibility (spectral range: 450-650 nm) in the wavelength calibration required for measuring low-Z impurity ion flow velocities. These features opened up new investigation possibilities on temperature stability and wavelength response of the CIS components, allowing to fully characterize and validate the W7-X systems. The CIS diagnostic was operational throughout the last W7-X experimental campaign. Measured velocities on the order of ∼20-30 km/s were observed, corroborated by comparisons with measurements with Mach probes.

In stellarators, increasing the density is beneficial for the energy confinement. While there is no single reason for this observation, it is still very robust across different devices and this is reflected in the empirical energy confinement time scaling for stellarators, ISS04. In order to study whether this is also true for Wendelstein 7-X, the density scaling of the energy confinement time is analyzed and compared to ISS04 for the first divertor experiments. When the density is increased beyond a critical density, however, radiative collapses are frequently observed. Existing analytical models for the critical density are revisited to assess whether they can predict the accessible density range. Furthermore, since close to the collapse the radiation losses increase substantially, the impact on the global energy confinement is investigated. It is found that in plasmas with high radiation the density scaling of the energy confinement time becomes weaker, the reason for this observation is not yet clear. In the second half of the first divertor campaign, boronization was applied to W7-X for the first time. This broadened the operational window, allowing for operation at higher density and, hence, higher stored energy.

The design and evaluation of the Langmuir probe system used in the first divertor operation phase of Wendelstein 7-X is described. The probes are integrated into the target plates and have individually facetted surfaces to keep the angle of incidence of the magnetic field within an appropriate range for different magnetic configurations. Multiple models for the derivation of plasma parameters from current–voltage characteristics are introduced. These are analyzed with regard to their assumptions and limitations, generalized, and adapted to our use case. A detailed comparison is made to determine the most suitable model. It is found that the choice of model has a large impact, for example, resulting in a change in the inferred temperatures of up to a factor two. This evaluation is implemented in a Bayesian modeling framework and automated to allow for joint analysis with other diagnostics and a replacement of ad hoc assumptions. We rigorously treat parameter uncertainties, revealing strong correlations between them. General and flexible model formulations permit an expansion to additional effects.

A set of dedicated marker samples consisting of fine-grain graphite as substrate, an interlayer of 0.2–0.4 μm molybdenum (Mo) employed as marker, and a 5–10 μm thick carbon (C) marker layer on top were installed in Wendelstein 7-X (W7-X) to investigate locally the C erosion and deposition. In this study, a set of five individual marker tiles, installed in a vertical divertor element of the test divertor unit in half-module 50, and exposed to about 40 min of plasma predominant in the standard magnetic divertor configuration in the first year of divertor operation in W7-X (OP1.2A), were retrieved from the vessel for post-mortem analysis. Picosecond laser induced breakdown spectroscopy (ps-LIBS) was applied on these marker tiles in order to determine the local erosion/deposition pattern caused by plasma impact. The general erosion/deposition pattern on the vertical target element was studied with the aid of depth-profiling by Mo line emission due to ps-LIBS with the number of applied laser pulses (355 nm, 2.3 J cm−2, 35 ps) at one probing location. Several potential asymmetry factors which avoid a perfect layer-by-layer ablation process in the laser ablations are proposed and discussed when a rough layered structure sample with a rough surface is analysed by the ps-LIBS technique. Thereby, a simulation model was developed to correct the measurement error of the ps-LIBS method caused by the non-perfect rectangle profile of the applied laser beam. The depth resolution of the applied ps-LIBS system was determined by quantification of the laser ablation rates of the different layers and the C substrate which were measured utilising profilometry and cross comparison with the thicknesses of the C and Mo marker layers determined by a combined focused ion beam and scanning electron microscopy technique. For the first time, the erosion/deposition pattern on the vertical target was mapped and quantified by ps-LIBS technique. A relatively wide net erosion zone with a poloidal extend of about 200 mm was identified which can be correlated to the main particle interaction zone at the magnetic strike-line of the dominantly applied standard magnetic divertor configuration. At the position of peak erosion, not only 7.6 × 1019 C atoms/cm2 but also 2 × 1018 Mo atoms/cm2 which results can be extrapolated to total 15 × 1019 C atoms/cm2, were eroded due to plasma fuel particle (H, He) and impurity (O, C) ion impact.

Due to its twisted magnetic field configurations, the orientation of the plasma-facing components (PFCs) of the Wendelstein 7-X stellarator is changed from bean shape to triangular shape along the plasma axis within a module. The PFCs consist of graphite (test divertor units, baffles, head shield tiles, toroidal divertor closures) and stainless steel components (wall panels, pumping gap panels, poloidal closures). In order to document the status of these components as well as to get the first footprints of the plasma-wall interactions (PWI) that occurred during the plasma campaign, a detailed inspection of the whole plasma vessel after the plasma campaign is of vital importance. Such an inspection was carried out after the operation phase OP1.2b completed in October 2018. A number of observations were made, such as, molten stainless steel drops, melting of a long term probe, deposition on steel panels, arc traces, loosely bound particles and deposition stripes on the plasma vessel hidden behind the graphite tiles. In order to understand the possible causes, their implication and steps to avoid some of these problems for the next phase of machine operation, an assessment was made. The melting events should be avoided in the future, the others are due to usual PWI mechanisms and will be monitored after each campaign.

A series of ice pellets was injected into the advanced stellarator Wendelstein 7-X (W7-X). Although the pellets were small and slow, deep and efficient particle fueling could be observed experimentally. The most striking feature appearing after the injection of the pellets, however, was a transient increase in the energy confinement time. This transient phase resembled in several aspects modes of enhanced confinement after gas-puff or pellet injection, as observed in other fusion experiments. All experimental attempts, to prolong this phase, failed. In this paper, discharges are described that show the enhanced energy confinement, and some conditions are summarized which seem to be essential in order to generate it. The focus here is on deep particle fueling by pellets, and shaping of the density profiles during and after the series of pellets. During this time, neutral gas particle re-fueling at the plasma edge is reduced, while density profile peaking and low impurity radiation losses are present.

A new type of in-vessel Penning gauge, the Wisconsin In Situ Penning (WISP) gauge, has been developed and successfully operated in the Wendelstein 7-X (W7-X) island divertor baffle and vacuum vessel. The capacity of the quantitative measurements of the neutral reservoir for light impurities, in particular, helium, is important for tokamaks as well as stellarator divertors in order to avoid fuel dilution and radiative energy loss. Penning gauges assisted by spectroscopy are a powerful tool to obtain the total neutral pressure as well as fractional neutral pressures of specific impurities. The WISP gauge is a miniaturized Penning gauge arrangement, which exploits the ambient magnetic field of magnetic confinement fusion experiments to establish the Penning discharge. Then, in situ spectroscopy is conducted to separate the fractional neutral pressures of hydrogen, helium, and possibly also other impurities. The WISP probe head was qualified using the magnetic field of the Magnetized Dusty Plasma Experiment at Auburn University between 0.25 T and 3.5 T [E. Thomas et al., J. Plasma Phys. 81, 345810206 (2015)]. The in-depth quantitative evaluation for hydrogen and helium will be shown as well as an exploration of nitrogen, argon, and neon. A power law scaling between current I and pressure p, I = f(Gas,V) · pn(Gas, B), was shown. The factor f is gas and anode potential dependent, while n is gas and magnetic field strength dependent. Pressure measurements from 0.1 mbar and down to 1 × 10−5 mbar were achieved, demonstrating a reliable operating range for relevant pressure levels in the divertor and main vessel regions in current and future fusion devices, with a time resolution of up to 1 kHz. The lowest achievable pressure measurement increases with an increase in B and can be shifted with the anode potential V. At W7-X, the WISP probe head was mounted on an immersion tube setup that passes through the cryostat and places the probe head close to the plasma. Two probe heads were positioned in different divertor pump gaps, top and bottom, and one close to the plasma on the midplane in one module. The gauges were in situ calibrated together with the ASDEX pressure gauges [G. Haas and H.-S. Bosch, Vacuum 51, 39 (1998)]. Data were taken during the entire operation phase 1.2b, and measurements were coherent with other neutral gas pressure gauges. For the spectroscopic partial pressure measurements, channels of a spectroscopic detection system based on photo-multipliers, a so-called filterscope [R. J. Colchin et al., Rev. Sci. Instrum. 74, 2068 (2003)], provided by the Oak Ridge National Lab were used.

Complex experiments such as the stellaratorWendelstein 7-X demand efficient experimental planning, oper-ation, and data evaluation. To assist the process organization atW7-X, a metadata framework has been implemented. It annotatesa physics program with tags describing its intention, mainparameters, boundary conditions, and evaluation results. Theapproach enables relevant metadata to be collected exactly whereinformation is available and to forward it from experimentalplanning and execution to the related entry in the central W7-XLogbook. In consequence, these metadata offer possibilities forclassifying, searching, filtering, and so on, both in the database ofthe prepared programs and in the logbook of the executed pro-grams. Wherever possible, metadata are generated automatically.Starting from the parameters of the program, the frameworkallows typical physics or technical quantities or any programmat-ically extractable information describing the programs intentionto be extracted already while editing. During discharge prepa-ration, metadata are retrieved from the related session planningdocuments and from the actual session environment. Supplemen-tary tags can be added manually from an extendible tag catalogto categorize experimental programs or to provide metadata fornot fully integrated diagnostics. All metadata are logged togetherwith status and progress information during execution and areavailable within the program’s log—where the metadata can befinally completed, for example, to manually qualify the success ofthe program or by adding programmatically produced metadatausing the logbook’s programming interface.

The statistical characteristics of the scrape-off layer (SOL) turbulence have been investigated in the first divertor plasma operation of W7-X using a reciprocating probe. The turbulence spectra, auto-correlation time and the statistical parameters are analyzed in three magnetic configurations. In standard and high mirror configurations, the SOL turbulence can be classified into four patterns from the outer SOL to the inner SOL, and each pattern is characterized by the poloidal cross correlation spectrum and the turbulence propagation property. A strong dependence of the SOL turbulence on the magnetic topology is demonstrated experimentally. In high iota configuration, the SOL island is relatively narrow and consequently the fluctuation level of turbulence is weak in the far SOL but enhanced significantly in the near SOL. In the reversed field case, the SOL turbulence reveals similar propagation properties and statistical parameters with those in the normal field case, demonstrating the same turbulence structure in both toroidal field directions.

Wendelstein 7-X (W7-X) is an optimized stellarator with a 3-dimensional five-fold modular geometry. The plasma-wall-interaction (PWI) investigations in the complex 3D geometry of W7-X were carried out by in situ spectroscopic observations, exhaust gas analysis and post-mortem measurements on a large number of plasma-facing components extracted after campaigns. The investigations showed that the divertor strike line areas on the divertor targets appeared to be the major source of carbon impurities. After multistep erosion and deposition events, carbon was found to be deposited largely at the first wall components, with thick deposits of >1 μm on some baffle tiles, moderate deposits on toroidal closure tiles and thin deposits at the heat shield tiles and the outer wall panels. Some amount of the eroded carbon was pumped out via the vacuum pumps as volatile hydrocarbons and carbon oxides (CO, CO2) formed due to the chemical processes. Boron was introduced by three boronizations and one boron powder injection experiment. Thin boron-dominated layers were found on the inner heat shield and the outer wall panels, some boron was also found at the test divertor unit and in redeposited layers together with carbon. Local erosion/deposition and global migration processes were studied using field-line transport simulations, analytical estimations, 3D-WallDYN and ERO2.0 modeling in standard magnetic field configuration.

Carbon impurity transport and deposition were investigated in the Wendelstein 7-X stellarator by injecting isotopically labelled methane ((CH4)-C-13) into the edge plasma during the last plasma operations of its Operational Phase (OP) 1.2B experimental campaign. C-13 deposition was measured by secondary ion mass spectrometry (SIMS) on three upper divertor tiles located on the opposite side of the vessel to the(13)CH(4) inlet. The highest C-13 inventories were found as stripe-like patterns on both sides of the different strike lines. These high deposition areas were also analysed for their impurity contents and the depth profiles of the main elements in the layers. Layered deposition of different impurity elements such as Cr, Ni, Mo and B was found to reflect various events such as high metallic impurities during the OP1.2A and three boronizations carried out during OP1.2B.

First results from the optimized helias Wendelstein 7-X stellarator (W7-X) have shown that core transport is no longer mostly neoclassical, as is the case in previous kinds of stellarators. Instead, turbulent transport poses a serious limitation to the global performance of the machine. Several studies have found this particularly relevant for ion transport, with core ion temperatures becoming clamped at relatively low values of $T_{i} \simeq 1.7$ keV, except in the few scenarios in which turbulence can be suppressed. In order to understand turbulent mechanisms at play, it is important to have a clear understanding of the parametric dependencies of turbulent fluctuations, and the relation between them and turbulent transport. In this work we use Doppler reflectometry measurements carried out during a number of relevant operational scenarios to provide a systematic characterization of ion-scale ($k_\perp\rho_i\simeq 1$) density fluctuations in the core of W7-X. Then, we study the relation between fluctuation amplitude and plasma profiles and show how distinct regimes can be defined for the former, depending on normalized gradients $a/L_{ne}$ and $a/L_{Ti}$. Furthermore, we discuss the importance of other potentially relevant parameters such as $T_e/T_i$, $E_r$ or collisionality. Comparing the different regimes, we find that turbulence amplitude depends generally on the gradient ratio $\eta_i=L_{ne}/L_{Ti}$, as would be expected for ITG modes, with the exception of a range of discharges, for which turbulence suppression may be better explained by an ITG to TEM transition triggered by a drop in collisionality. Finally, we show a number of scenarios under which $T_{i,core} > 1.7$ keV is achieved and how core fluctuations are suppressed in all of them, thus providing experimental evidence of microturbulence being the main responsible for the limited ion confinement in W7-X., Comment: Manuscript submitted to Nuclear Fusion in May 2021

The energetic particle slowing down model in the BEAMS3D stellarator neutral beam code is compared to analytic models and experimental data from the Wendelstein 7-X experiment (W7-X). Recently, the first neutral beam experiments were performed in W7-X, providing validation of neutral beam deposition codes (Lazerson S.A. et al 2020 Nucl. Fusion 60 076020). This work builds upon that work, and follows the gyro-center orbits of the neutral-beam-generated fast ions to the plasma boundary. Slowing down times based on measurements of diamagnetic energy changes are compared to simulation data. A discharge solely heated by neutral beam injection is used to compare neoclassical heat flux estimates to neutral beam fueling, heating, and current drive. Experimental estimates of electron heat diffusivity suggest that electron turbulence is destabilized by density peaking in the discharge. Neutral beam current drive dominates over bootstrap current, resulting in a reversal of the toroidal current, as seen experimentally. Particle losses and heat flux through the equilibrium boundary are described. The effects of the magnetic configuration and plasma density on such parameters are also assessed. Benchmarking based on analytic estimates and other energetic particle codes is presented.

The Wendelstein 7-X project is aimed at demonstrating that an optimised stellarator is an attractive candidate for a fusion reactor. This requires the achievement of a number of technical and physics goals. Several of these goals have already been achieved in the first three experimental campaigns. We shall exemplify this by a number of results. One important goal is the demonstration of quasi-steady-state operation at high plasma density and temperature for half an hour, which encompasses the physical and technical requirements of stable operation with density and impurity control, cw heating, water-cooled targets, particle exhaust, and an appropriate control and data acquisition system for long-pulse operation. In the previous experimental campaigns, with uncooled targets, stationary discharges for up to 25 s with 5 MW heating power and up to 100 s with 2 MW heating power were achieved. For the next operational phase, to start in 2022, water-cooled targets are being installed, water cooling will be provided for all first-wall components, and a number of further upgrades to plasma heating, vacuum and fuelling systems as well as diagnostics will become available. With this equipment, the further goals of the project will be tackled, including the stepwise extension of discharge intervals to the multi-minute range. The experimental results so far have confirmed the neoclassical theory underlying several of the optimisation goals. At the same time, they showed that an optimisation is also required with respect to anomalous transport. In the future operational phases we aim at building a solid theoretical, experimental and technical foundation for the design of a next-step stellarator device.

The European 1 MW, 170 GHz industrial CW prototype gyrotron has been designed within EGYC (European GYrotron Consortium) in collaboration with the industrial partner Thales Electron Devices (TED) and under the coordination of Fusion for Energy (F4E). This is a conventional (hollow) cavity gyrotron that is based on the 1 MW, 170 GHz short-pulse (SP) modular gyrotron, which has been designed and manufactured by KIT in collaboration with TED. The SP prototype has been tested in multiple experimental campaigns since 2015 and the nominal cavity mode TE32,9 is exited at 170.1 GHz, producing RF power above 1 MW with 35 % interaction efficiency. The first phase of the experiments with the CW industrial gyrotron was successfully completed at KIT in 2016, verifying most of the ITER specifications. Short pulses (

Coaxial-cavity gyrotrons are microwave sources that can extend the possible power levels of hollow cavity gyrotrons significantly and make them attractive for future fusion experiments and power plants. KIT already demonstrated operation of a modular short-pulse 170 GHz coaxial-cavity gyrotron with an output power of 2.2 MW, operating in the TE34,19mode. Today’s focus of KIT is the verification of this technology at longer pulses, which will prove the long-pulse capabilities also. At the moment, the current KIT prototype has been extended with cooling capabilities for all critical, highly loaded components of the tube and two new, different electron guns are available for operation. This paper will give an overview about the recent status and the future prospects connected to the coaxial-cavity gyrotron development at KIT, including both experimental and theoretical activities.

Studies towards a 1.5 MW, 140 GHz CW gyrotron, with the capability of MW-class operation also at 175 GHz, are ongoing at Karlsruhe Institute of Technology in view of a possible future upgrade of the ECRH system of the stellarator W7-X. The upgrade of the existing 1.0 MW, 140 GHz European gyrotron for W7-X has been chosen as a development path. Detailed designs of the cavity, the non-linear uptaper, and the quasi-optical launcher for the upgraded gyrotron have been obtained and have been validated numerically. In parallel, a mode generator, intended for low-power tests of the quasi-optical mode converter system of the upgraded gyrotron, has been designed, manufactured, and successfully tested.

This paper reports about a novel approach to the absolute intensity calibration of an electron cyclotron emission (ECE) spectroscopy system. Typically, an ECE radiometer consists of tens of separated frequency channels corresponding to different plasma locations. An absolute calibration of the overall diagnostic including near plasma optics and transmission line is achieved with blackbody sources at LN2 temperature and room temperature via a hot/cold calibration mirror unit. As the thermal emission of the calibration source is typically a few thousand times lower than the receiver noise temperature, coherent averaging over several hours is required to get a sufficient signal to noise ratio. A forward model suitable for any radiometer calibration using the hot/cold method and a periodic switch between them has been developed and used to extract the voltage difference between the hot and cold temperature source via Bayesian analysis. In contrast to the classical analysis which evaluates only the reference temperatures, the forward model takes into account intermediate effective temperatures caused by the finite beam width and thus uses all available data optimally. This allows the evaluation of weak channels where a classical analysis would not be feasible, is statistically rigorous, and provides a measurement of the beam width. By using a variance scaling factor, a model sensitive adaptation of the absolute uncertainties can be implemented, which will be used for the combined diagnostic Bayesian modeling analysis.

A forward modeling technique is developed for determining the characteristic features of observed MHD modes from the line-of-sight data of the soft x-ray (SXR) tomography diagnostics in the Wendelstein 7-X (W7-X) stellarator. In particular, forward modeling is used to evaluate the poloidal mode numbers m, radial location, poloidal rotation direction and ballooning character of the MHD modes. The poloidal mode structures have been modeled by the radially localized Gaussian-shaped emission regions rotating along the magnetic surfaces. In the present study the cases of rigid-shape emission regions and flexible emission regions are modeled. Various mode phase velocity dependences on the magnetic surface position are simulated. The modeled phase dynamics of line-integrated oscillations and the distribution of oscillation amplitudes are compared with the experimental signals of the SXR cameras which observe the plasma at various viewing angles in the poloidal cross-section. Application of this technique enables describing of the 1–50 kHz modes. In particular, in the discharge W7X-PID 20180918.045 three identified branches with the poloidal mode numbers m= 8, m= 10 and m= 11 localized at ρ ≈ 0.3 are rotating in the clockwise poloidal direction. The present paper reports the first application of the forward modeling technique to the data from the SXR diagnostics in W7-X. The high m-modes are identified by forward modeling in W7-X.

Radial profiles of density fluctuations and the radial electric field, E r, have been measured using Doppler reflectometry during the post-pellet enhanced confinement phase achieved, under different heating power levels and magnetic configurations, during the 2018 W7-X experimental campaign. A pronounced E r-well is measured with local values as high as −40 kV m−1 in the radial range ρ ∼ 0.7–0.8 during the post-pellet enhanced confinement phase. The maximum E r intensity scales with both the plasma density and electron cyclotron heating power level, following a similar trend to the plasma energy content. A good agreement is found when the experimental E r profiles are compared to simulations carried out using the neoclassical codes, the drift kinetic equation solver (DKES) and kinetic orbit-averaging solver for stellarators (KNOSOS). The density fluctuation level decreases from the plasma edge toward the plasma core and the drop is more pronounced in the post-pellet enhanced confinement phase than in reference gas-fuelled plasmas. Besides, in the post-pellet phase, the density fluctuation level is lower in the high iota magnetic configuration than in the standard one. To determine whether this difference is related to the differences in the plasma profiles or to the stability properties of the two configurations, gyrokinetic simulations have been carried out using the codes stella and EUTERPE. The simulation results point to the plasma profile evolution after the pellet injection and the stabilization effect of the radial electric field profile as the dominant players in the stabilization of the plasma turbulence.