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Mechanics of Magnetic Fusion Reactors  Chapter | 12    373


             intensity/deformation areas in windings are located at the interface of ‘aver-
             aged’ material and the coil case external insulation. Therefore, sub-modelling
             is used in the modelling of stellarator MS instead of the standard ‘backward
             passage’ approach, common in the tokamak design. Displacements determined
             when analysing the global model are used as boundary conditions for a refined
             sub-model in the area of interest.
                For the purposes of the MS mathematical modelling, it is convenient to iso-
             late a regular part of the structure. In stellarators the regular part is either a
             72-degree (in machines based on a five field-period Helias configuration, such
             as W7-AS and W7-X) or a 90-degree (in four-period machines, such as TJ-II
             and HSX) azimuthal sector, containing 8 to 12 coils.
                The design of a superconducting MS should account for the degradation of
             the physical and mechanical properties of materials at cryogenic temperatures.
             An important concern is the ‘intermittent plasticisation’ phenomenon that pro-
             gresses depending on the loading rate [12]. Due to potential loss of performance
             through this effect, plastic deformations are not allowed by design standards for
             many types of cryogenic equipment.
                The  mechanical  strength analysis  of the W7-X stellarator MS construc-
             tional elements, loaded in the elastic–plastic range, was based on the follow-
             ing postulate: these elements meet the prescribed static strength criterion even
             when local plastic deformations in some areas are greater than 1%, if, hypo-
             thetically, they are replaced with structural materials with characteristics only
             set at room temperature, whose load-bearing capacity is adequate according to
             elastic–plastic calculations. The only exception are cryopipelines, for which
             acceptable strain is selected with consideration for the material’s cryogenic
             enforcement.
                The stellarator’s vacuum chamber is affected by a smaller range of pondero-
             motive forces compared with the tokamak’s VV. Coil current ramp-up and dis-
             charge in the stellarator are slower than in the tokamak. Current disrupture and
             coil conductors superconducting-to-resistive state transition are unlikely. Halo
             currents and plasma vertical displacements are negligible. The primary sources
             of ponderomotive forces to be accounted for in the structural and durability
             analyses of the VV and the in-vessel components are the eddy currents induced
             during coil current discharge, plasma bootstrap and diamagnetic current varia-
             tions, as well as current flowing from the VV to the electrically conductive in-
             vessel components.
                Strength and stiffness calculation standards for stellarators are yet to be de-
             fined. That is why the W7-X design process was governed by ITER design
             guidelines. The latter are intended for much more heavy-duty operation, for
             example, tens of times greater number of working cycles, and have been some-
             what modified (relaxed) for W7-X. These easing of design standards cannot
             extend to stellarators based on the DT fuel cycle. Exception is also valid for
             W7-X welding processes and welded joints testing. The W7-X’s relaxed design
             standards permit the following:
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