Thermo-mechanical analysis of the Wendelstein 7-X divertor

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Abstract

The stellarator Wendelstein 7-X (W7-X) has a divertor consisting of 10 units installed inside the plasma vessel (PV). It was decided not to install the long pulse high-heat flux (HHF) divertor targets at the first two years stage of W7-X operation and to start with an adiabatically cooled test divertor unit (TDU) and shorter plasma pulses operation. This allows to accumulate operation experience with much simpler components, and as a result to adjust accurately the actively cooled HHF divertor which replaces the TDU for the stationary operation. Finite element (FE) analyses have been performed for better understanding of thermo-mechanical problems of divertor targets, and to guide the design of the TDU and HHF divertors. This paper presents the detailed results of the temperature response, the deformation and thermal stress of the divertor components.

Introduction

W7-X (major radius R = 5.5 m, minor radius a = 0.55 m, five periods) is designed for stationary operation (30 min) at Bo  3 T with continuous wave heating by ECRH with up to 10 MW power, and additional heating by NBI and ICRH with power up to 14 MW for 10 s. W7-X is now under construction at Max-Planck Institute for Plasma Physics in Greifswald. The goal of the experiment is to demonstrate the suitability of the advanced stellarator concept as a desirable alternative to the tokamak for a future fusion reactor [1], [2].

W7-X has a divertor consisting of 10 units installed inside the PV along the helical edge of the plasma contour to control the energy and particle flow at the plasma boundary under steady state operation. For long-pulse plasma operation of W7-X, the actively water cooled HHF target plates are designed to withstand thermal loads up to 10 MW/m2 [3], [4]. However, it has been decided for the first stage of W7-X operation to start with TDU for shorter plasma pulse operation. Afterward, the TDU will be replaced with the actively cooled HHF divertor for stationary operation. In order to accumulate operation experience during TDU phase as relevant as possible, the geometry features of the TDU and the HHF are largely similar [5]. The TDU, like the HHF divertor, consists of four distinct areas, see Fig. 1. These are the horizontal target, the vertical target, the high Iota tail and the low heat-flux region between the high Iota tail and the horizontal target [1].

One of the most serious challenges for the design of plasma facing components is the thermo-mechanical problem [6], [7]. In order to develop and confirm the design of the TDU and the HHF divertors, FE calculations with ANSYS® code have been performed to resolve the following thermo-mechanical issues: (a) Pulse length of plasma operation and temperatures of the in-vessel components for the TDU phase, (b) Temperature distribution and maximum temperature of the target elements, (c) Bowing and overall deflection of the target elements, cooling pipes, and so on, (d) Thermal stresses in each component.

Section snippets

Thermal analysis of TDU

During the first pulse operation stage of W7-X experiments, passive cooling is to be used not only for TDU but also for some other in-vessel components. For high power operation up to 8–10 MW, the un-cooled divertor target is rapidly heated up and becomes a heat source for other in-vessel components due to thermal radiation. Therefore it is very important to analyse the thermal behaviour of the passive target and other in-vessel components since the allowable temperature limits strictly define

HHF

The HHF divertor targets are made of 3D reinforced CFC NB31 as plasma facing material which is bonded via an active metal casting (AMC) copper interlayer to the water cooled CuCrZr structure [3]. The NB31 presents better mechanical properties to guarantee the reliability of the joint to the heat sink. The CuCrZr elements are grouped into modules (between 8 and 12 elements per module) in which the elements are supported on a SS module frame. Special pipes connect the elements to water manifolds

Summary

FE thermo-mechanical analyses have been carried out for the W7-X divertors. The design of the TDU without active cooling has been verified by FE analyses with the main conclusion that the passively cooled TDU meets the main requirements for the first stage pulse operation of W7-X. However, some limitations summarised in Section 2.1 are to be taken into account. For the HHF divertor the FE calculations indicate that the temperatures of all components of the target element remain within an

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