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.

High-temperature superconductors (HTS) are promising candidates for use in the high-field magnets needed in particle accelerators and fusion reactors. HTS conductor on round core (CORC^®) cables and wires wound from ReBa₂Cu₃O_7-x (REBCO) coated tapes are being developed for use in high-field magnet applications including fusion magnets operating at currents beyond 80 kA, requiring them to be bundled into cable-in-conduit conductor (CICC) configurations. The use of HTS cables enable demountable superconducting fusion magnets that would allow easier access to the fusion machine for maintenance and parts replacement. Such demountable magnets require practical, low-resistance joints, capable of injecting current uniformly into the many REBCO tapes that make up different cable designs. Optimization steps on CORC^® cables have resulted in high-current terminations and joints with a joint resistance measured between a pair of 30-tape CORC^® cables of 51 nΩ at 76 K and 1.9 nΩ at 4 K. Demountable joints between CICCs consisting of six CORC^® cables arranged in flat and round configurations were also tested and compared to joints between low-temperature superconducting (LTS) CICCs consisting of NbTi Rutherford cables. Samples were paired into two configurations (LTS-to-LTS and HTS-to-HTS) with a demountable joint between them that were each tested in series with currents up to 10 000 A in an applied background magnetic field of up to 8 T. The total loop resistance of the HTS-to-HTS sample, including their terminations and joint, was about 4 nΩ at 4 K in self-field with the resistance of the copper pressed joint being less than 1 nΩ. At 8 T, the total loop resistance increased to 6.9 nΩ with the pressed joint contributing 1.4 nΩ. These initial tests prove the feasibility of producing remountable (dry) joints with low resistance between superconducting magnet windings in future compact fusion machines.

In recent years, high temperature superconductors (HTSs) have been developed intensively for different applications such as high field magnets, fault current limiters, and current leads. Two-dimensional HTS-based conductors, the coated conductors, used for these applications are multilayered structures on substrate tapes, which can be laminated in addition by a metal, e.g., by copper or stainless steel. Depending on the constituents and the conductor architecture, the thermal conductivity of the whole conductor can vary over several orders of magnitude. Thermal conductivity data are required for calculation of the temperature distribution in a superconducting tape or cable, and for prediction of quench propagation in the conductor itself, and finally in a superconducting device (e.g., coil arrangement). Thermal conductivity data should be measured first of all for the HTS tape, which is used for production of the superconducting appliance. In this paper, we present results of thermal conductivity measurements of the yttrium-barium-copper-oxide copper stabilized superconducting tape in the direction along the tape. Moreover, measurements of the thermal conductivity of the tape in the normal direction are presented.

High Temperature Superconductors are a promising option for many superconducting devices like high current conductors for large high-field magnets, current leads, or superconducting coils. Quench propagation in these devices is currently under intensive investigation, and it can be predicted by modelling the entire structure. However, along with thermal material properties, the thermal properties of contact interfaces between structural materials as well as electrical stabilizers are highly needed. So far, a lack of such data in the literature made the analysis of quench propagation very difficult. Thermal resistance of copper-copper and copper-stainless steel interfaces were characterized for different pressure and temperature ranges. Therefore, thermal conductivity was measured with the axial heat flow method within the Physical Property Measurement System of Quantum Design using Thermal Transport Option in the steady-state measurement mode in the temperature range from 4 K to 300 K. For the required investigation, the Thermal Transport Option was extended: the sample puck was equipped with an additional copper frame allowing measurement of the thermal conductance of a stack of metallic plates pressurized by a screw. The method was further extended to allow the measurement of thermal conductance at different applied force values. For this, the copper frame was equipped with strain gauges, and calibrated for measuring the force applied to the stack at different temperatures. Further steps included the systematic measurement of thermal conductance versus temperature of the stacks for a range of applied force values. Two copper stacks composed of copper plates of 100 μm and 200 μm of thickness, and one stack composed of copper and stainless steel plates of 100 μm thickness were investigated. From the measured conductance values, the thermal contact resistance between contacting materials was evaluated.

High-temperature superconductors (HTSs) are used in a growing number of applications. Technical HTSs usually consist of superconducting and metallic parts. HTS conductors are characterized by their critical current density and critical temperature defined by the superconducting layer, but other quantities such as the current transfer length (CTL) and the interfacial resistance between the superconducting part and surrounding metals are also important characteristics. Since the CTL and the interfacial resistance cannot be measured directly, a method is known to evaluate these parameters indirectly from the characterization of the spatial variation of the surface voltage of the metallic surrounding that is in direct contact with the superconducting material. In this article, we present the details of the experimental method developed to determine the CTL and the interfacial resistance of commercial second-generation HTS conductors of any type of architecture. We apply this method to quantify the interfacial resistance of ten HTS tapes from six major tape manufacturers. Also, since the architecture of a commercial HTS tape is usually more complicated than an “idealized” superposition of two flat layers (i.e., a metallic layer and a superconducting layer), recourse to three-dimensional numerical modeling has been required in order to understand in details the current transfer mechanisms taking place within the tape. We also show that the method can be applied directly to more complicated structures encountered in HTS cables, namely to determine the barrier resistance between a metallic structure in contact with a superconducting tape, providing important information required in numerical modeling when designing superconducting devices.

HTS CrossConductor (abbreviated as "HTS CroCo") is a twisted stacked tape conductor optimized for a high current den-sity and simple long-length fabrication. We report on recent achievements in the fabrication of HTS CrossConductors of only 3.6 mm diameter made from 3 mm and 2 mm wide REBCO tapes. These conductors showed a critical current of 1.5 kA at T = 77 K, self-field in agreement with expectations from single tapes. In order to investigate the performance also at low temper-atures and high magnetic fields, a dedicated sample holder was designed to allow temperature- and field-dependent measure-ments in the FBI test facility. We observed critical currents of nearly 5 kA at T = 4.2 K and B = 12 T leading to an engineering critical current density of 482 A/mm$^{2}$. At elevated temperatures, the sample's performance followed well the expectation from the individual tapes. With the availability of tapes with thinner sub-strates, it can be extrapolated that the current density could reach more than 1000 A/mm$^{2}$ at T = 4.2 K and B = 12 T.