Klaus-Peter Weiss

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.

High-Current HTS Cables: Status and Actual Development

Second-generation high-temperature superconductors (HTS) REBCO are promising for high-field application due to the excellent Jc(B) performance at low temperatures and for power transmission at liquid nitrogen temperature. For power transmission with HTS, the formation of high-current HTS cables from single HTS tapes is desirable for cable currents of several 10 kA up to more than 100 kA. On the other hand, to avoid high inductances, which would cause high-voltage problems in case of quench or fast shutdown, high-current HTS cables are needed for larger high-field magnets, too, typically operated at 4.5 K or lower. In the last years, several HTS cable designs have been proposed, such as twisted stacked cable, CORC cable, and Roebel cable. This talk will give an overview of such proposals and highlight actual developments in HTS cable design and REBCO tape optimization, e.g., for highest field. In addition, an optimization of round TSTC strand will be introduced, which is designed for simple fabrication in long lengths for versatile use either for the production of cables to be used at high fields or for power transmission.

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.

Mechanical-Electrical Modeling of Stretching Experiment on 45

Cable-In-Conduit Conductors made with Nb3Sn strands will be used in ITER magnets. The current carrying capability of these Nb3Sn strands is known to be highly dependant on the strain state resulting from mechanical loading. The intricate cabling pattern of CICC, added to the thermal differential shrinkage between conductor jacket and Nb3Sn filaments induce complex strand trajectories and a highly inhomogeneous strain state. This “cable strain map” also evolves with operating loads (Lorentz force/hoop stress). The SAMAN experiment, conducted in the FBI facility at Karlsruhe Institute of Technology, aimed to stretch subsize, ITER-like conductors, in order to observe the evolution of the critical current associated with these loadings. The application of the Multifil finite element code, developed at Ecole Centrale de Paris, has helped quantifying the local strains along every individual strand, and their evolutions during cooldown (from heat treatment), energizing and stretching phenomena. Using Multifil output mechanical data as input in the CEA electrical code CARMEN has allowed computing the critical current in every strand, thus leading to an understanding of the critical current degradation of such subsize conductors. This paper shows, for two SAMAN samples, what is the impact of bending strain concentration on a CICC current transport capability.

{\rm Nb}_{3}{\rm Sn}

In the cable-in-conduit-conductor (CICC) design of the toroidal field system for the international thermonuclear reactor (ITER) Nb3Sn is used as superconductor material. Considering the single strand performance, the crucial characteristic is the strain dependence of the critical current. Within this context, the performance of the CICC under strain is determined by the behaviour of the single strands and additional effects related to the manufacturing process. In the framework of the European fusion technology program a task has been started to investigate single strands as well as sub-size CICC performance using different cable layouts (9, 45 and 180 strands). For this systematic approach, parameters such as the void fraction, the number of pure copper strands, the void fraction or the cabling pattern have been varied. To examine the critical properties in detail, the available test facility, consisting of two experimental setups, is capable to measure the strain dependence in magnetic fields up to 14 T at 4.2 K, by applying an axial load to the samples. Measurements on such sub-size CICC samples are presented and compared to the expected performance.

Strands CICCs

The superconducting magnet system of the W7-X stellarator presently under construction at the Max-Planck-Institute for Plasmaphysics in Greifswald, Germany, requires 7 pairs of current leads for a maximum current of 18.2 kA. The Forschungszentrum Karlsruhe is responsible for construction, manufacturing and performance testing of the current leads prior to installation in the W7-X machine. To reduce the overall cooling power for the refrigerator as well as the free convection problem due to the upside-down orientation of the leads, it was decided to use HTS material in the temperature range between approx. 5 K and 60 K. The optimization of the HTS part requires to know exactly the electrical, mechanical and thermal parameters of the HTS material. Actually two suppliers, EHTS and Sumitomo offer such Bi-2223/AgAu material. Single tapes as well as stacks were used to measure the critical current as a function of applied magnetic field for various temperatures. The thermal conductivity, the stress-strain dependence and the thermal contraction were also measured. In the paper the electrical, mechanical and thermal performance of different tapes and stacks will be described and discussed.

Systematic Approach to Examine the Strain Effect on the Critical Current of Nb

Within the growing field of cryogenic applications, there is an increasing demand on cryogenic material characterization. The Cryogenic Material Test Facility Karlsruhe CryoMaK is able to perform a variety of tests. The facility has a long standing tradition in cryogenic testing going back to 20 years in the Institute of Technical Physics at the Karlsruhe Institute of Technology (KIT). Mainly focused on mechanical testing also thermal conductivity and expansion measurements are performed. Additional test rigs are used to examine superconducting performance under applied tensile load in magnetic field.

_{3}

High temperature superconductor (HTS) tapes, e.g. Bi2223, are increasingly used for technical applications. A high variety of tape designs and compositions exists according to the special demands, e.g. for magnet coils or current leads. To use this material it is necessary to determine the thermal, mechanical and electrical properties in the temperature range planned for the application. Several types of available tapes are examined with respect to their mechanical properties at room temperature and at 77 K. Thermal properties, like thermal expansion and heat conductivity, in the temperature range from 5 K up to 300 K are given. In addition the effect of tensile loads in tape direction on the critical current is investigated.

Sn Cable-in-Conduit-Conductors

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.

Electrical, Mechanical and Thermal Characterization of Bi-2223/AgAu Material for Use in HTS Current Leads for W7-X

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.