The DEMO magnet system - Status and future challenges

We present the pre-concept design of the European DEMO Magnet System, which has successfully passed the DEMO plant-level gate review in 2020. The main design input parameters originate from the so-called DEMO 2018 baseline, which was produced using the PROCESS systems code. It defines a major and minor radius of 9.1 m and 2.9 m, respectively, an on-axis magnetic field of 5.3 T resulting in a peak field on the toroidal field (TF) conductor of 12.0 T. Four variants, all based on low-temperature superconductors (LTS), have been designed for the 16 TF coils. Two of these concepts were selected to be further pursued during the Concept Design Phase (CDP): the first having many similarities to the ITER TF coil concept and the second being the most innovative one, based on react-and-wind (RW) Nb3Sn technology and winding the coils in layers. Two variants for the five Central Solenoid (CS) modules have been investigated: an LTS-only concept resembling to the ITER CS and a hybrid configuration, in which the innermost layers are made of high-temperature superconductors (HTS), which allows either to increase the magnetic flux or to reduce the outer radius of the CS coil. Issues related to fatigue lifetime which emerged in mechanical analyses will be addressed further in the CDP. Both variants proposed for the six poloidal field coils present a lower level of risk for future development. All magnet and conductor design studies included thermal-hydraulic and mechanical analyses, and were accompanied by experimental tests on both LTS and HTS prototype samples (i.e. DC and AC measurements, stability tests, quench evolution etc.). In addition, magnet structures and auxiliary systems, e.g. cryogenics and feeders, were designed at pre-concept level. Important lessons learnt during this first phase of the project were fed into the planning of the CDP. Key aspects to be addressed concern the demonstration and validation of critical technologies (e.g. industrial manufacturing of RW Nb3Sn and HTS long conductors, insulation of penetrations and joints), as well as the detailed design of the overall Magnet System and mechanical structures.

Advance in the conceptual design of the European DEMO magnet system

Pierluigi Bruzzone, Ortensia Dicuonzo, Elena Gaio, Roberto Guarino, Mithlesh Kumar, Kamil Sedlák, Boris Stepanov, Davide Uglietti, Rainer Wesche

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 DEMO baseline has major and minor radius of 9.1 m and 2.9 m, plasma current 17.9 MA, toroidal field on the plasma axis 5.2 T, and the peak field in the toroidal-field (TF) conductor 12.0 T. The 900 ton heavy TF coil is prepared in four low-temperature-superconductor (LTS) variants, some of them differing slightly, other significantly, from the ITER TF coil design. Two variants of the CS coils are investigated-a purely LTS one resembling the ITER CS, and a hybrid coil, in which the innermost layers made of HTS allow the designers either to increase the magnetic flux, and thus the duration of the fusion pulse, or to reduce the outer radius of the CS coil. An issue presently investigated by mechanical analyzes is the fatigue load. Two variants of the poloidal field coils are being investigated. The magnet and conductor design studies are accompanied by the experimental tests on both LTS and HTS prototype samples, covering a broad range of DC and AC tests. Testing of quench behavior of the 15 kA HTS cables, with size and layout relevant for the fusion magnets and cooled by forced flow helium, is in preparation.

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Winding Pack Proposal for the TF and CS Coils of European DEMO

Pierluigi Bruzzone, Kamil Sedlák, Rainer Wesche

The design of the European DEMO, i.e., the future fusion tokamak planned after ITER, is being developed under the coordination of the EUROfusion Consortium. This paper reports the design optimization of the toroidal field (TF) winding pack and its corresponding react-and-wind conductor, and a new design study of the central solenoid (CS). The optimization of the TF coil is driven by the results of the mechanical analysis that revealed an unacceptable stress accumulation in some locations of the previous proposal of the TF winding pack. The design study of the CS coil is done with the aim of minimizing the outer radius, while maintaining the magnetic flux defined in the PROCESS system code. The results of the design study, namely the optimized CS coil radius, current density, hoop stress, and the field map, define the initial information that will be needed in future for designing the DEMO CS winding pack and conductor. Contrary to the former similar studies, no upper limit is set for the peak field of the CS, implicitly allowing the use of high-temperature superconductors wherever the current density of Nb3Sn at the operating field is too low.

2016

Central solenoid winding pack design for DEMO

Pierluigi Bruzzone, Kamil Sedlák, Boris Stepanov, Davide Uglietti, Rainer Wesche

The present study aims to reduce the outer radius of the central solenoid (CS) with respect to its nominal size specified by EUROfusion for a maintained CS magnetic flux. A reduced outer CS radius would allow the reduction of the overall size and cost of the DEMO magnet system. The proposed outline design of the winding pack for the CS1 module is based on layer winding. To achieve the same magnetic flux in a CS coil of significantly reduced outer radius the peak magnetic field at the CS conductors needs to be substantially increased. The use of high-temperature superconductors is therefore envisaged in the highest field sections of the CS coil. It is planned to use react & wind Nb3Sn conductors for intermediate field sections and NbTi at the lowest fields. In order to make a most economic use of the superconductors the proposed winding pack design considers a superconductor grading.

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