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Reliable analyses of thermal-hydraulic (TH) transients are becoming fundamental to supporting the design of magnets for fusion applications based on high temperature superconducting (HTS) cable-in-conduit-conductors (CICCs). However, currently available TH codes were developed and validated only for the analysis of magnets wound with low temperature superconductor (LTS) CICCs. In order to confirm the applicability of such tools to HTS CICCs, a two-step strategy is presented in this paper. First, a qualitative assessment of the characteristic time and spatial scales in HTS CICCs, based on the twisted-stacked-tape cable (TSTC) concept, has been performed. It shows that the different geometry and materials of TSTC strands lead to heat transfer time scales on the conductor cross-section, which are comparable to those of fast transients, ranging from milliseconds to few seconds, e.g. quench initiation and propagation as well as AC losses. Therefore, the assumptions of temperature uniformity on the cross-section, typical of the well-established 1D codes developed for LTS magnets, become questionable. A second, quantitative assessment, based on detailed electro-thermal models of the conductor and of the single TSTC strand cross-section, provides guidelines for the development of more reliable 1D models for the analysis of TH transients in HTS fusion magnets.

The European DEMO, i.e. the demonstration fusion power plant designed in the framework of the Roadmap to Fusion Electricity by the EUROfusion Consortium, is approaching the end of the pre-conceptual design phase, to be accomplished with a Gate Review in 2020, in which all DEMO subsystems will be reviewed by panels of independent experts. The latest 2018

Bulky metallic structures are needed in the toroidal field (TF) superconducting magnets for fusion applications to withstand the large Lorentz forces acting on the winding. The pulsed coil operation during a plasma scenario and the fast current discharge, or a plasma disruption event in off-normal operating conditions, cause transient magnetic fields, inducing eddy currents in the TF structures. The eddy currents generate heat in the structures heating in turn the winding pack, eroding the temperature margin: such power deposition is a key input for thermal-hydraulic (TH) analyses. However, the computation of eddy currents in fusion magnets is a challenging topic since a transient, fully 3D electromagnetic (EM) model is required. The EM problem is solved here by means of the finite element (FE) open source code FreeFEM++. First, the correct implementation of the EM problem is verified by means of suitable benchmarks against both simple analytical cases and the results obtained with state-of-the-art FE commercial codes on the DTT TF coil, used as a reference geometry. Then the EM code is applied to the evaluation of the magnetic fields and eddy currents induced in the same reference coil during the normal (static) and off-normal (transient) operation; the output of the EM analysis is used as input to the TH analysis carried out with the 4C code, aimed at computing the temperature margin evolution during the transient.

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