Thermo-mechanical analysis of Wendelstein 7-X plasma facing components

The stellarator experiment Wendelstein 7-X (W7-X) is designed for stationary plasma operation (30 min). Plasma facing components (PFCs) such as the divertor targets, baffles, heat shields and wall panels are being installed in the plasma vessel (PV) in order to protect it and other in-vessel components. The different PFCs will be exposed to different magnitude of heat loads in the range of 100 kW/m²–10 MW/m² during plasma operation. An important issue concerning the design of these PFCs is the thermo-mechanical analysis to verify their suitability for the specified operation phases. A series of finite element (FE) simulations has been performed to achieve this goal. Previous studies focused on the test divertor unit (TDU) and high heat flux (HHF) target elements. The paper presents detailed FE thermo-mechanical analyses of a prototype HHF target module, baffles, heat shields and wall panels, as well as benchmarking against tests.     

Manufacturing of the Wendelstein 7-X divertor and wall protection

The in-vessel components of Wendelstein 7-X (W7-X) with a total surface of 265 m² comprise the divertor and the wall protection. The high heat flux (HHF) and lower heat flux (LHF) target, the baffle, the end plates closing the divertor chamber, a cryo vacuum pump (CVP) and a control coil form one divertor unit. Steel panels and the graphite heat shield protect the wall, including the ports. The HHF target elements, the steel panels and the control coils are manufactured by industry. The remaining components will be manufactured by the Max-Planck-Institute für Plasmaphysik (IPP) at its Garching workshops. For all components the final acceptance tests will be performed by IPP. This paper summarizes the main aspects for manufacturing, the preceding development and qualification tests as well as the final acceptance tests for the in-vessel components.     

Design improvement of the target elements of Wendelstein 7-X divertor

The actively cooled high-heat flux divertor of the Wendelstein 7-X stellarator consists of individual target elements made of a water-cooled CuCrZr copper alloy heat sink armored with CFC tiles. The so-called “bi-layer” technology developed in collaboration with the company Plansee for the bonding of the tiles onto the heat sink has reliably demonstrated the removal of the specified heat load of 10 MW/m² in the central area of the divertor. However, due to geometrical constraints, the loading performance at the ends of the elements is reduced compared to the central part. Design modifications compatible with industrial processes have been made to improve the cooling capabilities at this location. These changes have been validated during test campaigns of full-scale prototypes carried out in the neutral beam test facility GLADIS. The tested solution can remove reliably the stationary heat load of 5 MW/m² and 2 MW/m² on the top and on the side of the element, respectively. The results of the testing allowed the release of the design and fabrication processes for the next manufacturing phase of the target elements.     

Mechanical analysis of the joint between Wendelstein 7-X target elements and the divertor frame structure

The target elements of the actively cooled high heat flux (HHF) divertor of Wendelstein 7-X are made of CFC (carbon fiber-reinforced carbon composite) tiles bonded to a CuCrZr heat sink and are mounted onto a support frame. During operation, the power loading will result in the thermal expansion of the target elements. Their attachment to the support frame needs to provide, on the one hand, enough flexibility to allow some movement to release the induced thermal stresses and, on the other hand, to provide enough stiffness to avoid a misalignment of one target element relative to the others. This flexibility is realized by a spring element made of a stack of disc springs together with a sliding support at one of the two or three mounting points. Detailed finite element calculations have shown that the deformation of the heat sink leads to some non-axial deformation of the spring elements. A mechanical test was performed to validate the attachment design under cyclic loading and to measure the deformations typical of the expected deformation of the elements. The outcome of this study is the validation of the design selected for the attachment of the target elements, which survived experimentally the applied mechanical cycling which simulates the thermal cycling under operation.     

Hydraulic analysis of the Wendelstein 7-X cooling loops

Actively water cooled in vessel components (IVC) are required for the long pulse operation of the stellarator Wendelstein 7-X (W7-X). In total, the cooling pipes have a length of about 4.5 km, supplying the coolant via 304 cooling circuits for the IVC. Within each cooling loop, the IVC are organized mostly in parallel. A homogeneous flow through all branches or at least the minimum specified flow in all of the branches of a circuit is crucial for the IVC to withstand the loading conditions. A detailed hydraulic simulation model of the W7-X cooling loops was built with the commercial code Flowmaster, which is a 1-D computational fluid dynamics software. In order to handle the huge amount of pipe-work data that had to be modelled, a pre- and post-processing macro was developed to transfer the 3D Catia V5 CAD model to the 1-D piping model. Within this model, the hydraulic characteristics of different types of first wall components were simulated, and compared with their pressure drop measurements. As a result of this work, the need for optimization of some cooling loops has been identified and feasible modified solutions were selected.     

Spectroscopic studies of fuel recycling and impurity behaviors in the divertor region of Wendelstein 7-X

The first divertor operation phase(OP1.2 a) was carried out on Wendelstein 7-X in the second half of 2017.Fuel recycling and impurity behaviors in the divertor region were investigated by employing a newly built ultraviolet–visible–near infrared overview spectroscopy system.The characteristic spectral lines of the working gases(hydrogen and helium),intrinsic impurities(carbon,oxygen and iron),and seeded impurities(neon and nitrogen) were identified and analyzed.The divertor electron temperature and density were measured using He I(667.8,706.5,and 728.1 nm) line intensity ratios.The Hα(656.3 nm),He I(587.6 nm),C II(514.5 nm),and O I(777.2 nm) emissions were investigated over a wide range of operating conditions.The results showed that fuel and impurity emissions in the divertor region exhibit a strong dependence on magnetic topology and plasma conditions.The levels of Hα,He I,C II,and O I emissions are all reduced moving from the standard configuration to the high mirror configuration,and even further reduced for the high iota configuration,which is associated with decreasing connection length in these island divertor configurations.The H/He influx ratio shows that the plasma is a mixture of helium and hydrogen.The neutral and impurity influxes from the divertor target tend to increase with increasing divertor electron temperature.     

Thermal analysis of the Mirnov coils of Wendelstein 7-X

Mirnov coils are used to measure fluctuations of the magnetic field which are in particular generated by magnetohydrodynamic (MHD) modes. The underlying plasma currents have a multipolar structure in a poloidal cross-section. Therefore the amplitude of the magnetic fluctuations decays quickly with increasing distance from the plasma edge. It is hence important to place the Mirnov coils as close to the plasma edge as possible where they are exposed to high thermal loads. Two types of Mirnov coils are proposed to be used in Wendelstein 7-X (W7-X). Type 1 (44 Mirnov coils) should be mounted on the plasma side of wall protection panels with a graphite cap to shield them from direct plasma exposure. Type 2 (137 Mirnov coils) will be located behind the tiles of the heat shields. An important issue concerning the design of these Mirnov coils is to verify their suitability for steady state operation from the thermal point of view. Both steady state and transient finite element thermal analyses were performed for the Mirnov coils under different conditions and with different designs. The paper presents detailed thermal analyses of the Mirnov coils.     

Status of Wendelstein 7-X construction

Wendelstein 7-X (W7-X) represents the continuation of fusion experiments of the stellarator type at the Max-Planck Institute for Plasma Physics (IPP). The aim of W7-X is to demonstrate the suitability for a fusion reactor of this alternative type of magnetically confined plasma experiment. W7-X is being built at Greifswald in the northeast of Germany. The size of device (725 tons, height of 5 m, diameter 16 m) and the superconductive magnet system distinguish W7-X from earlier stellarators at IPP. The paper provides a summary of the status of the main components, the mastering of the technical challenges during component acceptance testing and during machine assembly. Latest results of the assembly work are especially highlighted. The scope of the construction of W7-X was modified and additional acceleration measures were implemented to mitigate risks and delays. Some aspects of these changes are explained in this paper.     

Results and consequences of high heat flux testing as quality assessment of the Wendelstein 7-X divertor

The series manufacturing of the first 282 Wendelstein 7-X divertor elements was concluded in 2011. The divertor is designed to remove a steady-state heat load of 10 MW/m². 940 target elements of five different types made of CuCrZr heat sinks and covered with 16,000 CFC NB31 flat-tiles have to be produced. Additional to quality assessment during the manufacturing process, a final assessment of the delivered elements with operational heat load is indispensable to ensure a constant high thermal performance of the installed divertor.Based on the results of the pre-series testing a statistical quality assessment method has been developed for the series production. The application of this method to the series elements ensures their thermal performance with reasonable high heat flux test effort.     

Thermo-mechanical behavior of retro-reflector and resulting parallelism error of laser beams for Wendelstein 7-X interferometer

A 10 channels interferometer will be used in the Wendelstein 7-X (W7-X) for plasma density control and density profile tracking with laser beams passing through the plasma. Due to complex shape of non-planar modular coils and divertor structure, there are no large poloidally opposite ports on the plasma vessel (PV). Therefore 10 in-vessel Corner Cube Retro-reflectors (CCRs) will be used. The CCRs are integrated in the water cooled heat shield and exposed directly to thermal loads from plasma radiation. Thermo-mechanical issues are very important for the design of the CCR because deformation and flatness as well as mutual angles of the three reflecting surfaces would affect the parallelism of the laser beams and the functionality of the interferometer. Intensive work has been done to explore a suitable design for the CCR concerning thermo-mechanical behavior. Previous studies Ye et al. (2008, 2009) and Köppen et al. (2011) focused on structural optimization to decrease thermal stress in the reflecting plates under the thermal loads, and on computation and check of curvature radii of the deformed reflecting surfaces with the design criterion that the curvature radius must be bigger than 200 m. The paper presents detailed thermo-mechanical analysis of the current improved CCR under thermal loads and bolt preloads. The results of the thermo-mechanical analysis were used for the study of the resulting parallelism error of the laser beams with newly developed and more reasonable design criterion.     

Configuration Management for Wendelstein 7-X

A complex system like the large superconducting Wendelstein 7-X stellarator necessitates a dedicated organizational structure which assures permanent consistency between the requirements of its system specification and the performance attributes of all its components throughout its life time. This includes well-defined processes and centrally coordinated information structures. For this purposes the department Configuration Management (CM) has recently been established at W7-X. The detailed tasks of CM for W7-X are oriented along common CM standards and comprise configuration identification, change management, configuration status accounting and configuration verification. While the assembly of W7-X is proceeding some components are still under procurement or even under design. Thus design changes and non-conformances may have a direct impact on the assembly process. Highest priority has therefore been assigned to efficient control of change and non-conformance processes which might delay the assembly schedule.     

Thermal and mechanical analysis of Wendelstein 7-X thermal shield

The thermal insulation of Wendelstein 7-X cryostat consists of multi-layer insulation (MLI) and a thermal shield. The shield is cooled by helium gas flowing in pipes which are attached to the shields via copper strips or braids. The paper presents the basic thermal and mechanical layout of the thermal shield. The design is strongly influenced by the tight design space.Main mechanical loads on the shield are electromagnetic forces resulting from rapid shut down of the magnet system and the self weight. Design and calculations were performed iteratively. Copper and brass were checked in combination with different electrical isolation variants. The induced eddy currents will be reduced if the upper and the lower half shells of the cryostat are electrically isolated against each other. The cryostat shield and the port shields are made of brass.The expected heat loads on the shield were estimated. The resulting temperature distribution was then calculated for brass and copper shield panels. The average shield temperature is below 85 K and fulfills the thermal requirements.     

Performance and statistical quality assessment of CFC tile bonding on the pre-series elements of the Wendelstein 7-X divertor

Extensive high heat flux (HHF) testing of pre-series IV targets was performed to establish the industrial process for the ongoing production of the actively water-cooled target elements which will be needed for the installation of the Wendelstein 7-X (W7-X) divertor. Finally, 890 components covered with about 18,000 CFC tiles will be installed.The examinations of the elements with 10 MW/m² cycling up to 10,000 pulses, 16 MW/m² cycling and screening tests up to 32 MW/m², confirm the robustness of the design and in particular of the applied CFC bonding technology. The results of the IR examination of the initial tests have been assessed statistically. The paper presents a detailed statistical analysis based on the Six-Sigma method of the surface temperature increase of the CFC tiles tested for 100 cycles at 10 MW/m². Assuming that the series elements will behave in a similar fashion to the pre-series elements this statistical assessment can also be performed for the series elements.     

Configuration space control for Wendelstein 7-X

The Wendelstein 7-X stellarator (W7-X) is a superconducting fusion experiment, presently under construction at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik. W7-X is a device with high geometrical complexity due to the close packing of the components in the cryostat and their complex 3D shape e.g. of the superconducting coils. The tasks of configuration space control are to ensure that all these components do not collide with each other under a set of defined configurations, i.e. at the time of assembly, at 4 K or for various coil currents. To fulfill these tasks sophisticated tools and procedures were developed and implemented within the realm of a newly founded division that focuses on design, configuration control and configuration management.     

Quality assurance during assembly of Wendelstein 7-X

To control all the work and test steps during assembly of Wendelstein 7-X for each major assembly task Quality Assurance and Assembly Plans are used as the central managing instrument. These documents ensure that the order of all steps is carried out as planned and that the envisaged quality will be met. The confirmation of a successful working step often is done by tests and measurement. For each test special instructions were prepared to ensure reproducible and correct results. The tests are either carried out by the certified QA inspectors of the project or by specially qualified internal inspectors. The most important tests and measurements are outlined briefly. All quality deviations are assessed in relation of consequences for later operation.     

Design and analysis of the W7-X divertor scraper element

Thehigh heat-flux divertor of the Wendelstein 7-X large stellarator experiment consists of 10 divertor units which are designed to carry a steady-state heat flux of 10 MW/m². However, the edge elements of this divertor are limited to only 5 MW/m², and may be overloaded in certain plasma scenarios. It is proposed to reduce this heat by placing an additional “scraper element” in each of the ten divertor locations. It will be constructed using carbon fiber composite (CFC) monoblock technology. The design of the monoblocks and the path of the cooling tubes must be optimized in order to survive the significant steady-state heat loads, provide adequate coverage for the existing divertor, be located within sub-millimeter accuracy, and take into account the boundaries to other in vessel components, all at a minimum cost. Computational fluid dynamics modeling has been performed to examine the thermal transfer through the monoblock swirl tube channels for the design of the monoblock orientation. An iterative physics modeling and computer aided design process is being performed to optimize the placement of the scraper element within the severe spatial restrictions.     

Challenges in the realization of the In-Vessel-Components of Wendelstein 7-X

The In-Vessel Components (IVC) for the Wendelstein 7-X stellarator at the Institute for Plasma-Physics (IPP), to be installed for the initial phase of operation, are nearing completion and a significant fraction of the components was delivered in 2011 and 2012. Due to the considerable amount of different components including many variants, the timely realization required a comprehensive management approach, not only covering the demanding technology and system requirements, but also coordination, planning and control issues. A variety of tools were set up to address the technical, financial and timescale challenges. The implementation of this comprehensive management approach is illustrated by the production of the water-cooling system of the IVC. Careful design and manufacture of these components is needed to fulfil the cooling function under high vacuum conditions within very restricted available space. The evolution of the complexity of these components together with changes of boundary conditions had to be managed, integrated into the overall project planning and adequately resourced.     

On the accuracy of port assembly at Wendelstein 7-X

Wendelstein 7-X uses 254 ports for diagnostic and supply purposes. Actually 176 ports are final adjusted and welded. The major number of ports meets the general position tolerances of typically 4, …, 8 mm after assembly without countermeasures. 3D metrology turned out to be an essential factor to achieve required adjustment accuracy as well as to control welding process. The measurement accuracy of typically 0.3, …, 0.6 mm proved to be appropriated for all adjustment and control processes inside the experimental hall. A consequent application of 3D metrology can substitutes trail assembly steps and saves process time. Even reduced tolerances of special ports (AEV and AEK-V2) are achieved using appropriated assembly, welding and metrology procedures.     

Influence of construction errors on Wendelstein 7-X magnetic configurations

Wendelstein 7-X, currently under construction at the Max-Planck-Institut für Plasmaphysik in Greifswald, Germany, is a modular advanced stellarator, combining the modular coil concept with optimised properties of the plasma. The magnet system of the machine consists of 50 non-planar and 20 planar superconducting coils which are arranged in five identical modules, forming a toroidal five-fold symmetric system. The majority of operational magnetic configurations will have rotational transform ι/2π = 1 at the boundary. Such configurations are very sensitive to symmetry breaking perturbations, which are the consequence of imprecisely manufactured coils or assembly errors. To date, all 70 coils have been fabricated, and the first two half-modules of the machine have been assembled. The comparative analysis of manufactured winding packs and estimates of the corresponding level of magnetic field perturbation are presented. The dependency of the error fields on the coil assembly sequence is considered, as well as the impact of the first assembly errors. The influence of different construction uncertainties is discussed, and measures to minimise the magnetic field perturbation are suggested.     

Reverse engineering process of cryostat components of Wendelstein 7-X

The Wendelstein 7-X stellarator is a superconducting fusion experiment, presently under construction at the Greifswald branch of the Max-Planck-Institut für Plasmaphysik. This paper gives an overview of the reverse engineering processes applied on cryostat components of the W7-X superconducting magnet system.