Search

The Wendelstein 7-X experimental device has completed the third plasma operation campaign in November 2018. The running, two-year shut down phase is being used to install new diagnostics, new in-vessel-components (steady-state divertor and cryopumps), and their auxiliary supply systems. In particular, the installation of the ten cryopumps requires a substantial upgradation of the cryosystems. The work package comprises the installation of a 55-m long new transfer line from the cryoplant to the new valve box and the installation of ten transfer lines with lengths up to 15 m from the valve box to the ten cryovacuum pumps (CVPs) in the plasma vessel. The tasks have been split into two main contracts in industry. Both contracts are presently running, the production of the transfer lines and the valve box per September 2019 is in an advanced status. The aim is to complete the production in early 2020 and the installation work by the end of 2020. A second task regarding the cryosystems is the upgrade with respect to the foreseen long pulse operation of W7-X. Therefore, new storage tanks for helium gas and for liquid nitrogen are necessary. The helium gas storage has been installed at the end of 2018 whereas the liquid nitrogen storage tank is presently under design. This article gives an overview about individual upgrade tasks of the W7-X cryosystems, highlights the special design requirements, and reports the status of the contracts.

A deep understanding of the plasma-wall interaction processes in fusion devices like Wendelstein 7-X is necessary for an efficient plasma operation and a long lifetime of the plasma-facing components.In this work we present an approach employing residual gas analysis after picosecond laser-induced ablation (ps LIA-QMS) of graphite limiter tiles, exposed in the first plasma operational phase of Wendelstein 7-X, for depth-resolved and quantitative hydrogen content analysis. A series of poloidal and toroidal locations are analyzed at three of the five limiters, showing up to 2.3 × 1022 hydrogen atoms/m2 in net-deposition areas after a total plasma exposure of about 311 s in mixed hydrogen and helium operation. Shallow implantation of hydrogen is observed in erosion zones, where a low fuel content is present due to the high surface temperature during plasma operation. The hydrogen content spans between (1.1 and 3.7) × 1021 hydrogen atoms/m2 in the net-erosion areas. Moreover, oxygen has been analyzed and its appearance in both the implantation and deposition zone was verified. Results are compared to thermal desorption spectrometry and to simultaneously performed laser-induced breakdown spectroscopy (LIBS) measurements. Keywords: Laser-induced ablation, Quadrupole mass spectrometry, Laser-induced breakdown spectroscopy, Graphite plasma-facing components, Wendelstein 7-X, Limiter

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 operational regimes in the first divertor campaign of W7-X were limited by unexpectedly high heat loads on certain baffle tiles. In the high-mirror configuration, a permanent hotspot on the baffle plate was detected by the thermographic system, which was further confirmed during the post-campaign inspections of the plasma-facing components. The maximum heat load on three baffle tiles reached about 4.5 MW m−2, which is almost an order of magnitude above its designed value of 0.5 MW m−2. The paper presents a detailed analysis to understand how the baffle plate, which is originally designed to screen the recycling neutrals and is therefore hidden from the hot plasma, can receive such a high heat load – a level that is expected for target plates. Three main causes have been identified: 1) the three baffle tiles are radially only about 5 mm away from a main heat channel towards a target, which is much shorter than the radial power decay length of several centimeters derived from the thermographic measurements in this region; 2) the three baffle tiles are intersected with field-lines of about 30 m length, which are long enough to collect considerable amount of heat via cross-field transport; 3) the baffle tiles have locally large grazing angles. In addition, the analysis method shown in this paper is of general use to estimate the heat flows in shadow areas of targets and thus to evaluate heat loads on the potentially critical components occurring in these areas, especially required for the design of complex 3D divertors like the one in W7-X.

3-Dimensional (3D) analysis is crucial for many materials and can be used to study their structure and properties. Laser-Induced Breakdown Spectroscopy (LIBS) is a versatile tool to get depth information quickly, but the poor depth resolution and in general a difficult quantification are the two main drawbacks. To solve these problems, a LIBS method based on picosecond-laser pulses is introduced. The ablation depth is measured and associated with the laser pulse number. A series of layer-structured graphite samples was tested by this method and the 2D and 3D layer structures of these samples were identified with a resolution of up to 24 and 102 nm for Mo and C elements under a residual pressure of 1 × 10−5 Pa, respectively. This shows the great potential of picosecond Laser-Induced Breakdown Spectroscopy (ps-LIBS) in the field of depth analysis.

A key question of long-pulse operation in the stellarator Wendelstein 7-X (W7-X) is the net erosion of plasma-facing components (PFCs), which determines W7-X PFC lifetimes as well as the impact on the impurity influx to the plasma. In order to judge the erosion and deposition balance at the passively cooled graphite divertor, a set of exchangeable graphite tiles with marker layer stripes consisting of 5–10 μm C deposited on a 200 nm thick Mo interlayer were installed as an integral part of the divertor surface and removed at the end of the first operational phase for post-operational analysis. Laser-induced breakdown spectroscopy (LIBS) combined with laser-induced ablation-quadrupole mass spectrometry (LIA-QMS) were used for further analysis. The LIBS results show that the erosion-dominated regions are close to the location of the strike lines in standard magnetic configuration of W7-X optimised for the island divertor concept. Almost the complete C/Mo marker layer is eroded in these regions, providing an average erosion rate of 4.3 nm s−1 in the 2481 s plasma seconds of the campaign. In parallel, only a little deposition at the edge of the divertor tiles, where Mo is observed on the bulk graphite material was found. Furthermore, LIA-QMS showed a high hydrogen content on uncoated bulk graphite areas, while a lower hydrogen concentration was detected in the erosion-dominated zone, caused by the high surface temperature at the strike line during plasma operation.

In order to get first insight into net tungsten erosion in W7-X, tungsten (W) marker layers were exposed during the operational phase OP 1.2b at one position of the Test Divertor Unit (TDU), at 21 different positions of the inner heat shield, and at two scraper elements. The maximum tungsten erosion rate at the TDU strike line was 0.13 nm s−1 averaged over the whole campaign. The erosion was inhomogeneous on a microscopic scale, with higher erosion on ridges of the rough surface inclined towards the plasma and deposition of hydrocarbon layers in the recessed areas of the rough surface. The W erosion at the inner heat shield was below the detection limit of 3–6 × 1012 W-atoms/cm2s, and all inner heat shield tiles were covered with a thin B/C/O layer with thickness in the range 2 × 1017–1018 B + C atoms/cm2 (about 20–100 nm B + C). W-erosion of the marker layers on the scraper elements was also below the detection limit.

Analysis of elemental distributions in plasma-facing components (PFCs) is vital for the study of material erosion, deposition, and fuel retention in Wendelstein 7-X (W7-X) stellarator operating in full 3D geometry. In this work, we report the results on the application of picosecond laser-induced breakdown spectroscopy (ps-LIBS) combined with laser-induced ablation quadrupole mass spectrometry (LIA-QMS) to quantitatively investigate the elemental depth distribution and fuel retention on the screws PFCs from the divertor baffle of W7-X. Depth profiles of H, C, Ti and Mo intensities on the screws with specific multilayer structures have been achieved by LIBS combined with LIA-QMS. A linear correlation approach has been applied for identifying the interface between the C layer and the Ti layer and shows that a laser ablation rate of (82.9 ± 0.9) nm/shot can be achieved on C layer thickness measurement by a laser energy density of 6.5 J/cm2. Moreover, quantitative H retention content on the surface of the screws with stellarator discharge plasma exposure is approximately 1.1 × 1021 atoms/m2 which is higher than approximately 3.5 × 1020 atoms/m2 on the surface of the new screw without plasma exposure. The studies well demonstrate the ability to determine elementary resolved layer thicknesses and H retention measurements by depth analysis of LIBS on PFCs exposed in W7-X.

In the stellarator Wendelstein 7-X with its twisted 3D magnetic field geometry, studies of material migration with respect to first wall components become very important in view of the envisioned long-pulse operation. A variety of erosion/deposition samples were installed on the plasma-facing components exposed at three different nominal heat load levels between 0.1 and 10 MW/m2. After the first successful operation phase in divertor configuration, all the probes at higher and lower load levels were removed, whereas at the intermediate load levels, 352 out of 30 000 screws have been exchanged at selected locations along the toroidal and poloidal directions. The exchanged probes have been analyzed by various measurement techniques. At the higher load levels where the probes were installed within the divertor, heavy erosion has been observed presumably at the strike line positions. Both, erosion and deposition phenomena have been found on the screw heads. The optical reflection measurement profile of the whole plasma vessel show the deposition patterns at similar locations in all the five modules. At the low load level, the Si-wafer probes are under investigation.

Wendelstein 7-X (W7‐X) is the world’s largest superconducting nuclear fusion experiment of the optimized stellarator type. In the first Operation Phase (OP1.1) helium and hydrogen plasmas were studied in limiter configuration. The heating energy was limited to 4 MJ and the main purpose of that campaign was the integral commissioning of the machine and diagnostics, which was achieved very successfully. Already from the beginning a comprehensive set of diagnostics was available to study the plasma. On the path towards high-power, high-performance plasmas, W7‐X will be stepwise upgraded from an inertially cooled (OP1.2, limited to 80 MJ) to an actively cooled island divertor (OP2, 10 MW steady-state plasma operation). The machine is prepared for OP1.2 with 10 inertially cooled divertor units, and the experimental campaign has started recently. The paper describes a subset of diagnostics which will be available for OP1.2 to study the plasma edge, divertor and scrape-off layer physics including those already available for OP1.1, plus modifications, upgrades and new systems. The focus of this summary will be on technical and engineering aspects, like feasibility and assembly but also on reliability, thermal loads and shielding against magnetic fields.

In the Wendelstein 7-X stellarator with its twisted magnetic geometry the investigation of plasma wall interaction processes in 3D plasma configurations is an important research subject. For the upcoming operation phase i.e. OP1.2, three different types of material probes have been installed within the plasma vessel for the erosion/deposition investigations in selected areas with largely different expected heat load levels, namely, ≤10 MW m−2 at the test divertor units (TDU), ≤500 kW m−2 at the baffles, heat shields and toroidal closures and ≤100 kW m−2 at the stainless steel wall panels. These include 18 exchangeable target elements at TDU, about 30 000 screw heads at graphite tiles and 44 wafer probes on wall panels, coated with marker layers. The layer thicknesses, surface morphologies and the impurity contents were pre-characterized by different techniques and subjected to various qualification tests. The positions of these probes were fixed based on the strike line locations on the divertor predicted by field line diffusion and EMC3/EIRENE modeling calculations for the OP1.2 plasma configurations and availability of locations on panels in direct view of the plasma. After the first half of the operation phase i.e. OP1.2a the probes will be removed to determine the erosion/deposition pattern by post-mortem analysis and replaced by a new set for the second half of the operation phase, OP1.2b.

Laser-induced breakdown spectroscopy (LIBS) is a well-established elemental composition analysis method as well as one of the most promising candidates for in situ first wall diagnosis of fusion devices. In this work, limiter graphite tiles, which were exposed in the initial operational phase (OP1.1) of the Wendelstein 7-X stellarator to He and H plasma, are analyzed ex situ by LIBS employing a picosecond pulsed laser for the first time and compared with post mortem analysis techniques. Depth profiles of each element and 2D profile of the ratio of H and C atoms on the surface are investigated. Both H content and retention depth on the deposition dominated zone are higher than on the erosion dominated zone due to the formation of C-H co-deposition layer. The results from LIBS are in agreement with those from the cross-sectional scanning electron microscopic image and electron dispersive x-ray spectroscopy.

The first operational campaign of Wendelstein 7-X (W7-X) provided an excellent environment for the study of plasma-surface interaction (PSI) in a stellarator. In situ spectroscopic analysis via a combined visible/infrared camera system and a filterscope system revealed that the primary erosion zone was correlated with the high heat flux regions on the limiter. This analysis matched to where the erosion zone was found in the post-mortem analysis, which was done with scanning electron microscopy/focused ion beam/electron dispersive x-ray spectroscopy imaging. Additionally, a region of prompt deposition was found to the inside of these high heat flux zones. A region of far scrape-off layer (SOL) deposition was found at the edges of the limiter tiles. All deposition regions were identified by their homogeneous, increased oxygen content compared to the pure carbon makeup of the limiters. Poloidal variation of the impinging heat flux follow the imprint of the 3D SOL flux tubes. In how far this reflects in the PSI will require further analysis and modeling.

Commissioning [1] and the first operational phase of the stellarator Wendelstein 7-X (W7-X) have been accomplished successfully at the Greifswald branch of the Max-Planck-Institut fur Plasma Physik. First helium plasma was achieved on 10th of December 2015 followed by the first hydrogen plasma in February 2016. The plasma is confined by a magnetic field of 2.5 T on the plasma axis created by a superconducting magnet system of 70 coils. The coils are located in a cryostat and protected against thermal radiation by vacuum and a thermal shield. Cooling of the coils and of the shield is provided with a helium refrigerator keeping the magnet system at 4 K and the shield at 70 K. The paper presents the cooling concept of the thermal shield and the coils with its structures. It describes the results of the first cool down of the cryostat to 4 K and the resulting heat loads at the different temperature levels. The experimental data show that the heat loads are well below the plant capacity allowing safe magnet operation.

In February 2015 began the cool-down of about 450 tons cold mass of Wendelstein 7-X i.e. 70 superconducting coils, 14 currents leads, massive support structure and the thermal shield, enclosed within a vacuum vessel of about 16 m outer diameter. After a smooth cool-down, the temperatures around 5 K were achieved within about 4 weeks, in the short standby mode with the thermal shield return temperature

Ten divertor cryo-vacuum pumps (CVP) will be installed in the plasma vessel of Wendelstein 7-X stellarator. The inner part of CVP is used for the condensation of hydrogen, deuterium, etc. and chevrons are used to protect the inner part from the radiation and gas loads. The inner part is cooled with cold helium (He) and operated at 3.4 K and 3.9 K during peak power and standard modes of operation. The chevrons are operated at 80 K and supplied with liquid nitrogen (LN2). Three different helium cooling schemes were worked out using forced flow supercritical helium, namely, parallel flow, parallel flow with intermediate sub-cooling and series flow with intermediate sub-cooling. For the chevrons, three cooling schemes, i.e. bath cooling, forced flow two phase and single phase with pump, were worked out. The analysis involved the process calculations and thermo-hydraulic calculations considering the operational requirements of CVP. The optimisation was also coupled with the LN2 heat exchangers of the helium refrigerator to find the optimum flow scheme. The regeneration of inner part up to LN2 temperatures and the complete CVP up to ambient conditions is foreseen without affecting the other cooling applications connected to refrigerator.

Publisher Summary This chapter gives an overview on the cooling requirements as well as on the refrigeration system chosen for the superconducting magnet system and cryo vacuum pumps of WENDELSTEIN 7-X, a new fusion experiment of the stellarator type. A flexible refrigerator was developed which is able to run in quite different working conditions. Special features are single phase forced flow cooling with adjustable supply temperatures down to 3.4 K using cold circulators and cold compressors. The various operating modes, combined with the requirement to limit the power input, day-night load leveling, and the spatial boundary conditions for the system layout were a challenge for process and construction designers. With the W7-X refrigerator, Linde Kryotechnik AG and the Max Planck Institute for Plasma Physics broke new grounds. With the well directed use of the cold buffer as well as the extremely optimized sub-cooling system the refrigerator will save costs and energy. Because of the application of cold compressors and time distributed loads the warm compressor system will be comparably small. Apart from saving costs, space within the buildings is saved as well.

Following successful installation and commissioning, final acceptance tests were carried out on the helium refrigerator for Wendelstein 7-X.The tests were carried out for the normal operating modes i.e. peak power mode (3.4K), standard mode(3.9K), short standby mode (< 10K) and long standby mode (< 100K).Besides the normal modes, the transient modes including, cool-down and warm-up, auto interchanging between various modes and the purging modes were tested.In addition, the handling of quench, emergency signals such as cryostat vacuum break,utility failure, alarm and trip signals were checked.

Fourteen pieces of high temperature superconducting current leads (CL) arranged in seven pairs, will be installed on the outer vessel of Wendelstein 7-X (W7-X) stellarator. In order to support the CL, it is provided with two glass fiber reinforce plastic (GFRP) flanges, namely, the lower cryostat flange (CF) remaining at room temperature and upper radial flange (RF) at about 5 K. Both the flanges i.e. CF & RF experience high mechanical loads with respect to the CL, due to the evacuation of W7-X cryostat, cool-down of cold mass including the CL, electro-magnetic forces due to current & plasma operations and self weight of CL. In order to check the integrity of these flanges for such mechanical loads, thermo-mechanical tests were carried out on these flanges at room temperatures and at liquid nitrogen (LN2) temperatures. The details of test set-up, results and modeling are described in the paper.