E. Clark

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.

Multiphysics Analysis of the Wendelstein 7-X Actively Cooled Scraper Element

Abstract The Wendelstein 7-X stellarator experiment is scheduled to start operation in mid-2015, and to move to steady-state operation in 2019. During this steady-state operation, certain plasma scenarios have been shown to produce heat fluxes that exceed the technological limits on the edges of the divertor target elements. The addition of a so-called scraper element (SE) in the ten divertor locations is being investigated in order to reduce the heat load on these divertor target edges. The ANSYS commercial multiphysics package is used to model the performance of the SE under predicted operational conditions. Computational fluid dynamics (CFD) modeling is performed to analyze the hydraulic and thermal characteristics of the water-cooled SE under thermal loading using the ANSYS CFX software. This multiphysics modeling is performed for the entire SE to ensure that the total pressure drop in the cooling water circuits, the increase in water temperature, and the peak temperature in the CFC all satisfy the design requirements. Because the contour of the SE surface must be machined to a sub-millimeter precision, it is important to determine the amount of thermal expansion experienced by the entire SE. The thermal-hydraulic results are imported into ANSYS Mechanical to perform the thermal-structural analysis. The thermal deformation of the SE is examined to confirm that the component’s position will remain within its operational limits.

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.

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.

Computational Investigation of the Thermal-Hydraulic Performance for Twisted Tape Enabled High Heat Flux Components

Abstract High heat flux thermal management is an important challenge for upcoming nuclear fusion and plasma physics experiments. The plasma facing components (PFCs) in devices such as ITER or Wendelstein 7-X (W7-X) will be subjected to extreme heat loads on the order of 10–20 MW/m2 in the divertor region. The heat dissipation issue will become critical in this next generation of experiments, and active cooling will be necessary. The current state-of-the-art water cooled technologies can accommodate extreme heat fluxes and often utilize passive heat transfer enhancement techniques, such as swirl flow, to decrease the thermal loading on PFCs. Swirling flow is commonly induced with a twisted tape that is inserted into a circular tube. Twisted tape devices are planned for use in both W7-X and ITER. Computational modeling was performed to investigate the thermal-hydraulic performance for single-phase, turbulent flow of water through a twisted tape device. This study exploited the advantage of computational simulations by analyzing local flow information. It was shown that points of low wall shear stress corresponded to locations of low heat transfer coefficient and high surface temperatures. Thus, decreased wall shear stress could be an indicator for early burnout in twisted tape geometries. This analysis was the first step towards informing the design of twisted tape devices utilized in PFCs.

Overview of activities for the wendelstein 7-X scraper element collaboration

The Wendelstein 7-X (W7-X) stellarator is in final stages of commissioning, and will begin operation in the last half of 2015. In this 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 may 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 US 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.