J. Tretter

Modeling and Analysis of the W7-X High Heat-Flux Divertor Scraper Element

The Wendelstein 7-X stellarator experiment is scheduled for the completion of device commissioning and the start of first plasma in 2015. At the completion of the first two operational phases, the inertially cooled test divertor unit will be replaced with an actively cooled high heat-flux divertor, which will enable the device to increase its pulse length to steady-state plasma performance. Plasma simulations show that the evolution of bootstrap current in certain plasma scenarios produce excessive heat fluxes on the edge of the divertor targets. It is proposed to place an additional scraper element in the 10 divertor locations to intercept some of the plasma flux and reduce the heat load on these divertor edge elements. Each scraper element may experience a 500-kW steady-state power load, with localized heat fluxes as high as 20 MW/m2. Computational analysis has been performed to examine the thermal integrity of the scraper element. The peak temperature in the carbon-carbon fiber composite, the total pressure drop in the cooling water, and the increase in water temperature must all be examined to stay within specific design limits. Computational fluid dynamics modeling is performed to examine the flow paths through the multiple monoblock fingers as well as the thermal transfer through the monoblock swirl tube channels.

Wendelstein 7-X high heat flux components

The actively water-cooled In-Vessel Components (IVCs) of the stellarator Wendelstein 7-X consist of the divertor, the first wall protection components, the port liners, each designed for different loading conditions, and the associated pipework, the control coils, the cryo-pump system, the Glow discharge electrodes, and a set of diagnostics. The divertor, designed for high heat fluxes, is a set of 10 target and baffle units arranged along the plasma surface. The design and production of these high heat flux (HHF) components is a challenging task. The divertor target elements, which are based on flat CFC (carbon-carbon fibre composite) tiles bonded via active metal casting onto CuCrZr cooling structures required intensive development and testing to reach a reliable performance; removing, under stationary conditions, 10 MW/m2. Industrially manufactured high quality target elements have been delivered and assessed, and the process of incorporating them into assembly units, so-called modules, has begun. The time scale for the completion of the HHF divertor has been held for the last four years and the final delivery of the HHF divertor is still planned in 2017. In parallel to the realization of the divertor the remaining IVCs have been defined, developed, designed and fabricated and the installation of many of these components has begun. Some of these components can also be expected, for a short period of time, to receive high heat loads approaching those of the divertor. These components will be described, in detail, from conception to realization.

Status of High Heat Flux Components at W7-X

The actively water-cooled in-vessel components (IVCs) of the stellarator Wendelstein 7-X consist of the divertor, the first wall protection components, the port liners, each designed for different loading conditions, and the associated pipework, the control coils, the cryo-pump system, the Glow discharge electrodes, and a set of diagnostics. The divertor, designed for high heat fluxes (HHFs), is a set of 10 target and baffle units arranged along the plasma surface. The design and production of these HHF components is a challenging task. The divertor target elements, which are based on flat carbon-carbon fiber composite tiles bonded via active metal casting onto CuCrZr cooling structures required intensive development and testing to reach a reliable performance; removing, under stationary conditions, 10 MW/m2. Industrially manufactured high quality target elements have been delivered and assessed, and the process of incorporating them into assembly units, so-called modules, has begun. The time scale for the completion of the HHF divertor has been held for the last four years and the final delivery of the HHF divertor is still planned in 2017. In parallel to the realization of the divertor, most of the remaining IVCs have been defined, developed, designed, and fabricated and the installation of many of these components has begun. Some of these components can also be expected, for a short period of time, to receive high heat loads approaching those of the divertor. These components will be described, in detail, from conception to realization.

Actively Water-cooled Plasma Facing Components of the Wendelstein 7-X Stellarator

Abstract The actively water-cooled plasma facing components (PFCs) of the Wendelstein 7-X stellarator consisting of the first wall protection and the divertor systems have a total surface area of about 265m2. The complex 3D geometry of the plasma and plasma vessel with 244 vessel ports dedicated to diagnostics, heating systems and water-cooling pipe-work together with the need to minimize the space taken and the significant heat loads expected on the components presents significant design and manufacturing challenges. The actively water-cooled divertor, made of 100 target modules, has an area of 19 m2. Each target module is formed from target elements made of CFC flat tiles bonded with the bi-layer technology to CuCrZr heat sinks. In total 16,000 tiles are bonded to the 890 target elements. A full-scale target module prototype has been manufactured to validate the design, the selected technological solutions and the inspection methods to be used in the serial module fabrication. About 30% of the target elements have been delivered and the production of the remaining elements should be completed by 2014. The fabrication of the components of the first wall protection, 320 stainless steel panels and 170 heat shields, is almost completed.

Configuration space control of in-vessel components for Wendelstein 7-X

Concurrent engineering with the design of increasingly complex components requires addition tools to avoid space conflicts. Configuration Space Control is a key technology necessary to achieve the required design efficiency and product development of a complex experiment. Easily accessible solutions available within CAD Frameworks, Product Data Management, and Configuration Management Systems currently only solve part of this task. Therefore, it has been vital to develop and control a set of procedures which handle the concurrent engineering issues and manage the compatibility of the various components being designed, manufactured and assembled. In addition a defined set of procedures are required to control the changes, additions and non-conformities to the design of components which occur in a developing experiment. To cope with these tasks, sophisticated tools and procedures have been adapted, developed and implemented. This paper covers the Configuration Space Control process for In-Vessel Components of Wendelstein 7-X, and demonstrates its application in the control of the as-assembled components.

Overview of Design and Analysis Activities for the W7-X Scraper Element

The Wendelstein 7-X stellarator is in final stages of commissioning, and will begin operation in late 2015. In the first phase, the machine will operate with a limiter, and will be restricted to low power and short pulse. But in 2019, plans are for an actively cooled divertor to be installed, and the machine will operate in steady state at full power. Recently, plasma simulations have indicated that, in this final operational phase, a bootstrap current will evolve in certain scenarios. This will cause the sensitive ends of the divertor target to be overloaded beyond their qualified limit. A high heat flux scraper element (HHF-SE) has been proposed in order to take up some of the convective flux and reduce the load on the divertor. In order to examine whether the HHF-SE will be able to effectively reduce the plasma flux in the divertor region of concern, and to determine how the pumping effectiveness will be affected by such a component, it is planned to include a test divertor unit scraper element (TDU-SE) in 2017 during an earlier operational phase. Several U.S. fusion energy science laboratories have been involved in the design, analysis (structural and thermal finite element, as well as computational fluid dynamics), plasma simulation, planning, prototyping, and diagnostic development around the scraper element program (both TDU-SE and HHF-SE). This paper presents an overview of all of these activities and their current status.

Thermal-fluid modeling of single- and two-phase flows in the W7-X high heat flux divertor scraper element

The Wendelstein 7-X (W7-X) stellarator experiment is scheduled to begin operation in 2015 with steady-state operation planned for 2019. During the steady-state operation, certain simulated plasma scenarios have been shown to produce heat fluxes that surpass the technological limits on the edges of the divertor target elements. To reduce the heat load on the target elements, the addition of a “scraper element” (SE) in ten divertor locations is under investigation. Two aspects of thermal-fluid modeling for the W7-X SE components are analyzed in this paper using the ANSYS CFX 15.0 software: (1) sensitivity to turbulence models in single-phase and (2) two-phase modeling with the standard k-epsilon model. The results from the single-phase survey of seven turbulence models reveal a significant sensitivity to the selected model in the thermal-hydraulic analysis. Two-phase modeling is completed with the k-epsilon turbulence model, but no vapor generation is recorded. A significant temperature gradient between the liquid-solid interface indicates the need for a turbulence model that can achieve higher mesh resolution in the near-wall region.

A proposed scraper element to protect the end of the W7-X divertor target elements

The three main areas of the W7-X divertor target are: the horizontal target, the vertical target and the high iota tail, with a pumping gap between the horizontal and vertical targets. For each of the standard operational scenarios the target has been designed so that, based on vacuum field calculations, the plasma strike points lie away from the ends of the target. Each target consists of CFC armoured water cooled poloidally running target elements capable of operating reliably in these central regions at up to 10MW/m2 in steady state operation, as necessary for the standard configurations. Due to the U-turn shape of the cooling channel at the pumping gap end of the elements the cooling is not optimal and a reduced performance is expected. However, recent studies of experiment scenarios, also now taking into account the influence of bootstrap currents, showed that the strike point drifts over the ends of the target elements adjacent to the pumping gap on the L/R time scale; potentially leading to damage of the elements in these scenarios. Various operational methods have been studied to satisfactorily avoid this situation without success. The paper describes design changes being made to improve the ability of the end of the elements to accept reasonable power levels, however it is not expected that the elements in these regions will meet the full power requirements. Hence, this paper also describes the design of a protection element; the purpose of this so-called “scraper element” is to intercept the power to the targets before the full power reaches the sensitive end of the elements. This is done without reducing significantly the pumping at the pumping gap and addresses issues such as how to control local impurity flows. The programme of work concerning design and technology qualification still needed to realise this scraper element is described.

Wendelstein 7-X high heat-flux divertor scraper element

The Wendelstein 7-X stellarator experiment is scheduled to complete construction in 2014 and begin operation in 2015. After the first operational phase, the inertially cooled test divertor unit will be replaced with an actively cooled high heat-flux divertor which will enable the device to increase its pulse length and its steady-state plasma performance. Plasma simulations show that the evolution of bootstrap current in certain plasma scenarios produce excessive heat fluxes on the divertor edge elements. It is proposed to place an additional “scraper element” in the ten divertor locations that will capture some of the plasma flux and reduce the heat load on these divertor edge elements. Each scraper element may experience a 500 kW steady-state power load, with localized heat fluxes as high as 20 MW/m2. Computational modeling has also been performed in order to model the thermal and structural integrity of the scraper element. The peak temperature in the CFC, the total pressure drop in the cooling water, and the increase in water temperature must all be examined to stay within specific design limits. Computational fluid dynamics (CFD) modeling is performed to examine the flow paths through the multiple monoblock fingers as well as the thermal transfer through the monoblock swirl tube channels. Finite element analysis is integrated into the CFD results in order to ensure the structural integrity of the component.

Design and Test of Wendelstein 7-X Water-Cooled Divertor Scraper

Heat load calculations have indicated the possible overloading of the ends of the water-cooled divertor facing the pumping gap beyond their technological limit. The intention of the scraper is the interception of some of the plasma fluxes both upstream and downstream before they reach the divertor surface. The scraper is divided into six modules of four plasma facing components (PFCs); each module has four PFCs hydraulically connected in series by two water boxes (inlet and outlet). A full-scale prototype of one module has been manufactured. Development activities have been carried out to connect the water boxes to the cooling pipes of the PFCs by tungsten inert gas internal orbital welding. This prototype was successfully tested in the GLADIS facility with 17 MW/m2 for 500 cycles. The results of these activities have confirmed the possible technological basis for a fabrication of the water-cooled scraper.