Christoph M. Bayer

Overview of Progress on the EU DEMO Reactor Magnet System Design

The DEMO reactor is expected to be the first application of fusion for electricity generation in the near future. To this aim, conceptual design activities are progressing in Europe (EU) under the lead of the EUROfusion Consortium in order to drive on the development of the major tokamak systems. In 2014, the activities carried out by the magnet system project team were focused on the toroidal field (TF) magnet system design and demonstrated major achievements in terms of concept proposals and of consolidated evaluations against design criteria. Several magnet system R&D activities were conducted in parallel, together with broad investigations on high temperature superconductor (HTS) technologies. In this paper, we present the outcomes of the work conducted in two areas in the 2014 magnet work program: 1) the EU inductive reactor (called DEMO1) 2014 configuration (power plant operating under inductive regime) was the basis of conceptual design activities, including further optimizations; and 2) the HTS R&D activities building upon the consolidated knowledge acquired over the past years.

HTS CroCo: A Stacked HTS Conductor Optimized for High Currents and Long-Length Production

In order to build high-current cables from second-generation high-temperature superconductor (HTS) rare-earth barium-copper-oxide (REBCO) tapes, numerous approaches were studied, such as conductor on round core, Roebel cable, and several versions of twisted stacked-tape cable types. Based on the work of Takayasu et al. and Uglietti et al., we developed and tested a modified type of stacked HTS tape arrangement optimized for high engineering critical current density. Key aspects were the implementation of a simple and reliable continuous fabrication routine for production of application-relevant lengths of twisted conductors in the range of 100 m and above and the subsequent enveloping by a seamless copper tube. Several samples of this HTS Cross-Conductor (HTS CroCo) were prepared successfully, both partially and fully equipped with REBCO tapes in untwisted and twisted configurations. The critical current of the samples was measured at T = 77 K and self-field conditions. The measurements showed the expected critical currents calculated from the individual tape values if taking into account the enhanced self-field in the tape arrangement. Furthermore, one untwisted partially REBCO-equipped sample was tested in the FBI facility at KIT at T = 4.2 K and in magnetic fields up to B = 12 T, showing good performance with no degradation even at high Lorentz forces. In addition, the mechanical performance of this sample was studied under tensile loads. No degradation could be observed, and the strain dependence was equal to that of single REBCO tapes. Due to the combination of excellent mechanical and electrical performance, the HTS CroCo is a promising candidate as a strand for long-length high-current cables, for example, with Ic(4.2 K, self-field) ≈ 30 kA for power transmission or with Ic(4.5 K, 14 T) ≈ 8 kA as a strand for high-current cables targeting large high-field magnets.

Electrical Characterization of ENEA High Temperature Superconducting Cable

ENEA is currently involved in the design and manufacture of a fully high temperature superconductor (HTS) cable in the cable-in-conduit conductor (CICC) configuration exploiting commercial second generation ReBaCuO (Re: Rare Earth and Y) coated-conductors. The final cable will be composed of five slots obtained in a helically twisted aluminum central core and filled with 2G tape stacks. This conductor is designed to operate above 10 kA in 12 T background field at 4.2 K or at about 10 kA in self-field at 77 K. A first sample of about 1-m length with one fully superconductive slot has been manufactured using 15 tapes provided by Superpower, Inc. and 12 tapes from the SuNAM Company grouped in two sub-stacks divided by a Kapton foil. Each tape of the stack has been characterized individually by measuring critical current values Ic at 77 K (liquid N2 bath) in self-field and n-index. Results revealed that the tapes showed no degradation of critical current values when compared with suppliers specifications confirming that the proposed manufacturing process is fully compatible with commercial coated-conductors. Inter-tape resistance(Rinter) has also been measured and the observed dependence of Rinter on the tape position in the stack has been put in correlation with transverse stress distribution calculated by finite element models. A second sample with a full superconducting slot has been manufactured using 18 SuNAM tapes. Preliminary results on the stack transport measurements performed at 77 K in self-field will be presented and discussed. All the samples were manufactured by using already existing industrial equipments at Tratos Cavi SpA.

Conceptual Design of a Toroidal Field Coil for a Fusion Power Plant Using High Temperature Superconductors

Taking a step further from the International Thermonuclear Experimental Reactor (ITER), the next step will be a demonstration fusion power plant, DEMO, i.e., a fusion power plant (FPP) prototype. As a part of European Union (EU) DEMO studies, the result of the so-called PROCESS system code has been taken as the basis to design a toroidal field coil (TFC) winding pack with high temperature superconductor (HTS) REBCO, which is a promising HTS candidate. From this, the cable space area, the winding pack current density, and the total current in one TFC has been obtained. In this paper, a conceptual design of a HTS TFC is presented, and related parameters, such as peak magnetic field at the conductor, conductor current, and coil inductance, are calculated. The results have been used to evaluate the temperature margin and the hot spot temperature in case of a quench. With the calculated results, it is shown that at 4.5 K, the actual available HTS conductor can be used to design a TFC for DEMO within the available space given by PROCESS code.

FBI Measurement Facility for High Temperature Superconducting Cable Designs

High temperature superconductor (HTS) cables are sophisticated and promising cable designs for a variety of feasibilities. However, as any technical development, HTS cables need to be designed, tested and evaluated. For HTS cables, the most important parameter is the critical current, which depends on the operating parameters magnetic field, temperature and strain. To analyse the full application spectrum of an HTS cable, it needs to be tested under the influence of its self-magnetic field, different external magnetic fields, varying temperatures and mechanical stresses. The FBI (F force, B magnetic field, I current) measurement facility at the Karlsruhe Institute of Technology (KIT) provides the possibility of measuring the critical current of a cable up to 10 kA. With an LTS split coil magnet of 12 T and a sample gap of 80×40 mm2, different HTS cable types can be measured. Mechanical forces can be applied up to 100 kN in axial direction of a cable. This paper explains the structural assembly of the FBI measurement facility, demonstrates its applicable measuring parameters and shows hitherto measurement results.

Mechanical reinforcement for RACC cables in high magnetic background fields

Operable in liquid helium, liquid hydrogen or liquid nitrogen, high temperature superconductor (HTS) cables are investigated as future alternatives to low temperature superconductor (LTS) cables in magnet applications. Different high current HTS cable concepts have been developed and optimized in the last years—each coming with its own benefits and challenges. As the Roebel assembled coated conductor (RACC) is the only fully transposed HTS cable investigated so far, it is attractive for large scale magnet and accelerator magnet applications when field quality and alternating current (AC) losses are of highest importance. However, due to its filamentary character, the RACC is very sensitive to Lorentz forces. In order to increase the mechanical strength of the RACC, each of the HTS strands was covered by an additional copper tape. After investigating the maximum applicable transverse pressure on the strand composition, the cable was clamped into a stainless steel structure to reinforce it against Lorentz forces. A comprehensive test has been carried out in the FBI facility at 4.2 K in a magnetic field of up to 12 T. This publication discusses the maximum applicable pressure as well as the behaviour of the RACC cable as a function of an external magnetic field.

Toward a High-Current Conductor Made of HTS CrossConductor Strands

Over the last decade, several approaches for the fabrication of high-current cables from second-generation high-temperature superconductor (HTS) REBCO tapes were investigated, among them Roebel cable, conductor on round core, and several twisted stacked tape cable types. We introduced a novel type of stacked HTS tape arrangement, which is called HTS CrossConductor or HTS CroCo, aiming for high engineering critical current densities je that can be scaled to application-relevant lengths of conductors. Measurements of the critical current of a triple-HTS CroCo superstrand at T = 77 K and self-field conditions and at T = 4.2 K, as well as in fields up to B = 12 T, show the expected current calculated from the individual tape critical currents. Twisting experiments of the triple-HTS CroCo superstrand indicate a critical bending strain of around 0.6%. Based on these results, a Rutherford cable design for a cable with a critical current of 80 kA is suggested.

Characterization of High Temperature Superconductor Cables for Magnet Toroidal Field Coils of the DEMO Fusion Power Plant

Nuclear fusion is a key technology to satisfy the basic demand for electric energy sustainably. The official EUROfusion schedule foresees a first industrial DEMOnstration Fusion Power Plant for 2050. In this work several high temperature superconductor sub-size cables are investigated for their applicability in large scale DEMO toroidal field coils. Main focus lies on the electromechanical stability under the influence of high Lorentz forces at peak magnetic fields of up to 12 T.