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

Abstract

The target elements of the actively cooled high heat flux (HHF) divertor of Wendelstein 7-X are made of CFC (carbon fibre-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.

Progress in the production of the W7-X divertor target modules

“…The TEs were manufactured with two CuCr1Zr studs and screwed onto the rails (Fig. 3) via disc spring stacks with a 7 Nm torque [11]. The rails were joined by a stiffening support plate made of stainless steel forming a frame.…”

Mechanical examination and analysis of W7-X divertor module sub-structures

“…2. The module is comprised of target elements, mechanically attached to a module frame; the behavior of this attachment under heat load and cyclic mechanical load was already discussed in prior work [2]. The frames shown in Fig.2 connect on their lower Figure 2: Degrees of freedom of the divertor frame side to the consoles that attach them to the plasma vessel wall, on the upper side they provide a hook and clamp mechanism for easy installation of the module holder to the adjustment frame.…”

“…Fig.5 shows two thermal results for the module 1h. The module is mechanically fixed on the module holder only on the water connectors side, described in [2], the other two mount stubs are sliding points. Two thermal load cases have been calculated, the centre loading above the stud and loading between the two sliding studs.…”

“…The divertor is made of 10 similar and discrete units aligned along the field lines. Each unit consists of four main areas, three HHF areas; the vertical target, the horizontal target and the high iota tail together with an intermediate section designed for a lower heat flux [2]. Each HHF target module consists of eight to twelve cooled target elements.…”