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The optimized superconducting stellarator device Wendelstein 7-X (with major radius , minor radius , and plasma volume) restarted operation after the assembly of a graphite heat shield and 10 inertially cooled island divertor modules. This paper reports on the results from the first high-performance plasma operation. Glow discharge conditioning and ECRH conditioning discharges in helium turned out to be important for density and edge radiation control. Plasma densities of with central electron temperatures were routinely achieved with hydrogen gas fueling, frequently terminated by a radiative collapse. In a first stage, plasma densities up to were reached with hydrogen pellet injection and helium gas fueling. Here, the ions are indirectly heated, and at a central density of a temperature of with was transiently accomplished, which corresponds to with a peak diamagnetic energy of and volume-averaged normalized plasma pressure . The routine access to high plasma densities was opened with boronization of the first wall. After boronization, the oxygen impurity content was reduced by a factor of 10, the carbon impurity content by a factor of 5. The reduced (edge) plasma radiation level gives routinely access to higher densities without radiation collapse, e.g. well above line integrated density and central temperatures at moderate ECRH power. Both X2 and O2 mode ECRH schemes were successfully applied. Core turbulence was measured with a phase contrast imaging diagnostic and suppression of turbulence during pellet injection was observed.

The Multi-Purpose-Manipulator (MPM) situated at the outboard mid-plane is a versatile carrier system for diagnostic probes at Wendelstein 7-X (W7-X). It is able to probe into the last scrape-off (SOL) layer close to the last closed flux surface. The manipulator is also has the capability to puff gas for well-localized fueling/impurity seeding into the SOL. In the previous operational campaign of OP 1.2a and 1.2b both hydrogen and methane were injected from the manipulator mounted probes. The effect of hydrogen puffing on the main plasma was observed to be much stronger than that of the methane puffing. This difference is related to the difference in ionization lengths between the two gas species. To support this assumption a puffing of a similar amount of methane and hydrogen was modelled with ERO.

Electron- (ECWC) and ion- (ICWC) cyclotron wall conditioning are essential means for controlled fusion to modify the surface state of plasma-facing components in order to reduce impurity generation and fuel accumulation in the wall. Development of ECWC and ICWC requires characterization of neutral particle fluxes generated in discharges, because neutrals enhance the homogeneity of the conditioning, which may contribute to remote or shadowed areas, especially in the presence of a permanent magnetic field (e.g. W7-X, ITER). A time-of-flight neutral particle analyzer (ToF-NPA) with 4.07 m flight distance is employed to measure time- and energy-resolved low energetic (

In this work we present an analysis of fast-camera imaging of Alcator C-Mod's divertor region and of the plasma cross section in the W7-X stellarator. In C-Mod, the dynamics of intermittent edge plasma filaments are imaged at 397,100 frames per second in an oblique view. An automatic pipeline was developed to reconstruct the filament motion in the R, Z plane of the divertor using EFIT field line projections. Next, we added our fast-framing camera to W7-X's diagnostic suite for the 2017 run campaign, the first with a divertor. It has a tangential view of the machine cross section, including part of a divertor module. The view is coupled to the camera via a coherent fiber bundle. The spatial resolution is 2–4 cm and the typical plasma brightness limits the fastest usable frame rate to 30k frames/s. We observe filaments in the plasma boundary that rotate around the magnetic axis during radiative collapse; filaments were observed in steady state in the limiter configuration of W7-X by Kocsis et al. [1]. We also observe for the first time a 1–2 kHz quasi-coherent fluctuation in divertor target emissions in the W7-X “standard” magnetic configuration. Finally, a rough analysis of edge gas fueling efficiency using fast camera data and S/XB coefficients finds that fueling is less efficient at densities higher than 4 × 1019 m−3. Keywords: C-Mod, W7-X, Filaments, Blobs, Turbulence, Fluctuation

The control of rotational transform in Wendelstein 7-X (W7-X) is key to both the island divertor operation and safety of plasma facing components. The island divenor concept in W7-X relies on an edge flux surface with rotational transform of t = 1 resonating with an intrinsic n/m = 5/5 resonance to form a five lobed island chain. This island chain intersects with divertor plates to give rise to the island divertor. Changes in the relative position of the rational surface and the divertor plates can result in changes in divenor performance, thus the control of the rotational transform is essential to operation of the W7-X device. During the first divertor campaign electromagnetic loads resulted in elastic deformations of the shaped modular stellarator coils. Such deformations made these coils more planar, reducing the vacuum rotational transform, subsequently shifting the t = 1 resonance outward. Unintended plasma wall interactions provided the first clear evidence of this effect during plasma operation. Flux surface measurements were utilized to estimate the correct level of current in the planar coils for correction of t, and found to be around similar to-100 A. Scans the planar coil current for iota correction were performed during plasma operation. These measurements suggest planar coil currents between -250 and similar to 0 A would place the strike lines at the designed distance from the pumping gaps. Divertor Langmuir and upstream probe measurements confirm these estimates along with measurements of divertor neutral gas pressure.

For the study of electrostatic and magnetic properties of filaments characterizing the edge region of the stellarator experiment W7-X, a specifically designed insertable probe head was constructed within the framework of EUROfusion WP.S1 work package in collaboration between Consorzio RFX, IPP Greifswald and Forschungszentrum Mich. The probe head, named High Resolution Probe (HRP), was conceived to be installed on the mid-plane multi-purpose fast reciprocating manipulator on W7-X. Electromagnetic filamentary turbulent structures are found to characterize the edge region of different magnetic configurations including Reversed Field Pinch, stellarator and tokamak, where strong currents are associated also to ELM filamentary structures. The study of those phenomena in W7-X stellarator is of particular interest as the electromagnetic features of filaments are expected to become more relevant with the increase of the local plasma beta. In particular the aim is to provide information about the presence and the features of parallel current density associated to filamentary turbulent or ELM-like structures. Furthermore, the possibility to measure the time evolution of the flow radial profiles using the Mach probe array was considered as a further interesting part of the study, given the strong interplay expected between the turbulent fluctuations and the average flows. Further important information provided is the radial propagation of turbulent flux. The contribution will present the design development, the R&D studies and the applied solutions for the sensors embedded in the probe head. In particular, the presence of 140 GHz ECRH plasma environment represents one of the main challenges for reliable magnetic fluctuation measurements. First measurements were performed during the W7-X experimental campaign OP1.2b, where electromagnetic features of turbulence were measured with the probe, at different magnetic configurations.

The paper reports on the optimization process of the soft X-ray pulse height analyzer installed on the Wendelstein 7-X (W7-X) stellarator. It is a 3-channel system that records X-ray spectra in the range from 0.6 to 19.6 keV. X-ray spectra, with a temporal and spatial resolution of 100 ms and 2.5 cm (depending on selected slit sizes), respectively, are line integrated along a line-of-sight that crosses near to the plasma center. In the second W7-X operation phase with a carbon test divertor unit, light impurities, e.g., carbon and oxygen, were observed as well as mid- to high-Z elements, e.g., sulfur, chlorine, chromium, manganese, iron, and nickel. In addition, X-ray lines from several tracer elements have been observed after the laser blow-off injection of different impurities, e.g., silicon, titanium, and iron, and during discharges with prefill or a gas puff of neon or argon. These measurements were achieved by optimizing light absorber-foil selection, which defines the detected energy range, and remotely controlled pinhole size, which defines photon flux. The identification of X-ray lines was confirmed by other spectroscopic diagnostics, e.g., by the High-Efficiency XUV Overview Spectrometer, HEXOS, and high-resolution X-ray imaging spectrometer, HR-XIS.

In the first experimental campaign of Wendelstein 7-X (W7-X), a combined probe head mounted on the multi-purpose manipulator has been used to measure the scrape-off layer (SOL) turbulence characteristics. The preliminary experimental results are summarized to illustrate the SOL turbulence properties in the limiter configuration on W7-X. In a standard limiter configuration, significant electrostatic fluctuations can be found in the near SOL, and the dominant frequency of fluctuation power is below 100 kHz. The auto-correlation spectrum power law decay factor is α ≈ −1 below 40 kHz and α ≈ –2 between 50 and 200 kHz. A broadband spectrum appears between 240 and 380 kHz with a low spectral power density, but a high cross-correlation coefficient. The statistical characteristics of turbulence are calculated by the two-point cross-correlation technique. A clear poloidal dispersion relation is found in the spectrum S(kθ, f), propagating along the ion diamagnetic drift direction with a group velocity (below 100 kHz) about 0.56 km/s in the near SOL in the laboratory frame. The poloidal correlation length is around 5–10 mm in SOL. The turbulence phase velocity is about 0.5–1 km/s when close to the last closed flux surface, which is comparable with the poloidal E × B drift speed.

A group of edge diagnostics and modelling has been developed for investigation of synergy between 3D edge physics and plasma-wall interactions on Wendelstein 7-X (W7-X). Two endoscopes have been designed for visible and ultraviolet spectroscopy and tomography of the plasma edge, along with infrared thermography of the divertor tiles. 2D profiles of impurities (e.g. helium, carbon) will be measured by two endoscopes viewing the island divertor region in the plasma edge with a spatial resolution of

The investigation of edge plasmas at the stellarator W7-X requires a flexible tool for integration of a variety of different diagnostics as e.g. electrical probes, probing magnetic coils, material collection, or material exposition probes, and gas injection. A multi-purpose manipulator (MPM) system has been developed and attached to the W7-X vessel before the operational phase 1.1. The system was designed as user facility for many diagnostics, which can be mounted on a unique interface without breaking the W7-X vacuum. The manipulator system, located in the equatorial plane, transports the inserted diagnostic probe to the edge of the inner vacuum vessel. From there, the probe can be moved over a maximum distance of 350 mm to different positions inside the plasma with a maximum acceleration and deceleration of 30 m/s2. Acceleration, speed, and stroke depth are individually adjustable and programmable by a PLC system within predetermined limits. The MPM system can be equipped with multifunctional probes and is prepared for cooling/heating of the probe head, gas injection, and a flexible setting of the electrical diagnostic. In the paper, the MPM components, their functionalities, and the interface to the probe head are described. The scope of physical parameters is presented for the development of diagnostic probes applicable in the MPM user facility. This also includes the operational parameters for the movements and the electrical environment for measurements. The operation of the MPM during the OP1.1 is demonstrated on the application of a multipurpose probe head, including electric, magnetic, and thermal probes.

With the objective of understanding the energetic-particle loss mechanism in three-dimensional plasmas, a Faraday-cup-based fast-ion loss detector (FILD) was developed and installed in OP1.2b in Wendelstein 7-X (W7-X) as a collaboration between the National Institute for Fusion Science and Max-Planck-Institute for Plasma Physics. The FILD, which consists of double apertures and thin aluminum foils, was based on a magnetic spectrometer using the magnetic field of the fusion device. The double aperture limits the thermal ion, but allows the energetic ion to enter the FILD box. The thin aluminum foils serve as the ion collector. Orbit-following calculations were performed in order to find a suitable position for the FILD. The results indicated that barely co- and counter-going transit ions reached the FILD position mounted on the multi-purpose manipulator installed on W7-X. Moreover, because the injection angle of neutral beams injector installed on W7-X was relatively perpendicular, the target range of pitch angle was set to from 91 degrees to 150 degrees. An energy-and-pitch-angle map was created using a grid calculation code in order to decide on the position/size of aperture and aluminum foil. The grid calculation code indicates the position where the energetic ion would strike on the aluminum foil. Here, the map was used as the basis for designing an aluminum foil pattern with two energies and four pitch angle ranges. Also, note that the lower energy range was designed so that it accommodates a neutral beam injection energy of 55 kV. Measurements of beam ion losses were performed in neutral beam (NB) blip experiments, where concurrent increases and decreases of barely co-going transit beam ion losses due to NB injections were observed. Furthermore, this study validated that the FILD installed on a midplane manipulator probe is capable of probing a range of radii spanning 1.0–1.5 cm, over which the beam ion loss current would vary significantly.

Within the development of new fusion devices like ITER and DEMO, material testing becomes more and more relevant. The material ion beam test facility MARION which was primarily designed for beam source development and commissioning is also used for high heat flux testing up to 120 MW/m2 power density. In order to improve this second field of activity the concept of MARION is adapted to material testing. To increase the flexibility and to decrease the costs for material testing operation the vacuum system has been changed by excluding the cryogenic pumps combined with an enhancement of the conventional vacuum pumping part. The reduction in pumping capacity has made a new optimization of the operation parameters necessary. The experimental results of this optimization can be used as calibration base for future heat load tests. They are shown in this paper, combined with the changes in operation range for this new, additional mode.

For the development of the tritium monitoring system in ITER the hydrogen isotope release by Laser-Induced Desorption (LID) from Be layers is studied to determine the laser parameters for a high desorption efficiency while minimising dust production and surface modifications is also pursued. Be layers of 1 µm thickness with 25–30 at% D and 3 × 1022 D/m2 comparable to JET-ILW areal concentrations [1] have been produced by High Power Impulse Magnetron Sputtering (HiPIMS) on ITER grade W. Laser pulses of 1, 5 and 10 ms duration heat the layer in vacuum in the Fuel REtention DIagnostic Setup (FREDIS) and release the retained D thermally. By mass spectrometry in FREDIS and subsequent Nuclear Reaction Analysis (NRA) inside the laser spot the desorbed and remaining D is quantified. While a pulse duration of 1 ms cannot fully desorb the deuterium, it is found that a single 5 or 10 ms laser pulse with an absorbed energy density of ca. 1.5 MJ/m2 corresponding to a heat flux factor around 20 MW√s/m2 leads to nearly complete desorption of the retained D. This encourages the development of a useful tritium monitoring system, although the present layers produce some dust due to local delamination of the layer on at least 11% of the heated surface (at 1.4 MJ/m2 absorbed energy within 5 ms) and lead to unavoidable crack formation. Keywords: Fuel retention, Beryllium, Tritium monitoring, Laser, Desorption, FREDIS

The Multi-Purpose-Manipulator (MPM) has been operated, as a versatile carrier system for probes, since the first campaign OP. 1.1 in 2015 at Wendelstein 7X. The MPM is mounted at the outboard midplane and is able to perform fast plunges through the entire scrape off layer up to the last closed flux surface. In addition, a system for gas injection was commissioned that allowed the use of a dedicated probe head and a port on the existing probes for fueling and impurity seeding experiments. A combined probe, a combination of Langmuir, Mach and magnetic probes, was used. For the second campaign OP. 1.2a in 2017, with an island divertor, an upgraded combined probe, a fluctuation probe, a retarding field analyzer, a Mach probe array, a laser blow off target and a material probe holder were added to the range of plasma edge diagnostics. The new diagnostic probe heads were designed to address specific tasks, such as measuring the edge fluctuations, the ion temperature and the radial Mach number profiles. However, all fast plunging probes were designed such that they provide the most important quantities for SOL characterization (electron temperature, density, radial electric field).

A bi-directional multi-channel retarding field analyzer (RFA) probe has been successfully developed for the first time on the Wendelstein 7-X (W7-X) stellarator boundary plasma. Modifications to the RFA prototype hardware and its upgrade for the two W7-X island divertor campaigns are presented, including the electronics. In this paper the experiences and challenges operation and customizing an RFA at W7-X are discussed, as well as the data analysis using a maximum coefficient of determination method to obtain the ion temperature based on the measured modulated ion current. Edge ion temperature profiles have been measured in the standard and high iota configurations. Previous article in issue

This paper reports on the design and the performance of the recently upgraded X-ray imaging spectrometer systems, X-ray imaging crystal spectrometer and high resolution X-ray imaging spectrometer, installed at the optimized stellarator Wendelstein 7-X. High resolution spectra of highly ionized, He-like Si, Ar, Ti, and Fe as well as H-like Ar have been observed. A cross comparison of ion and electron temperature profiles derived from a spectral fit and tomographic inversion of Ar and Fe spectra shows a reasonable match with both the spectrometers. The also measured impurity density profiles of Ar and Fe have peaked densities at radial positions that are in qualitative agreement with the expectations from the He-like impurity fractional abundances, given the measured temperature profiles. Repeated measurements of impurity decay times have been demonstrated with an accuracy of 1 ms via injection of non-recycling Ti, Fe, and Mo impurities using a laser blow-off system.

Knowledge of the edge ion temperature (T i) is important to understand the edge physics in fusion plasmas. The T i edge profile has been successfully measured at W7-X in an island divertor configuration using a retarding field analyzer probe on a reciprocating manipulator. The experimental observations show that in the standard configuration, an ion temperature shoulder at the plasma boundary has been observed near the location where a sudden change of the magnetic field connection length appears in the scrape-off layer (SOL) region. The value of the T i shoulder was observed to be dependent on the edge island structure modified by the control coils, SOL kinetic power, and electron density. A significant reduction and flattening in the edge ion temperature profile has been also observed during the nitrogen seeding from the upper horizontal divertor port. In addition, the edge ion-to-electron temperature ratio has been calculated, which gradually increases with the major radius and decreases with the normalized ion collisionality. An extended two-point model including upstream ion temperature is introduced to understand the experimental measurement.