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.

Several new conductor designs for fusion applications, based on high temperature superconducting (HTS) materials, have been recently proposed worldwide. Most of them are based on the twisted-stacked-tape cable (TSTC) idea: a stack of HTS tapes is twisted, soldered and embedded in a copper tube, constituting what we identify here as a ‘strand’. Few strands are then twisted around a central core to make an HTS cable. The cable is finally inserted in a stainless-steel jacket, which provides mechanical support and confinement for the forced flow of the supercritical helium coolant through the interstitials among the strands. The thermal gradients that build up in a TSTC make the 1D thermal-hydraulic models developed for low temperature superconducting magnets unsuitable to study fast transients such as quench propagation. A new model is presented here, which allows describing each strand, and separately accounting for the respective superconducting (SC) stack and copper tube. Compressible Euler-like equations describe the He flow and thermodynamic state in each interstitial fluid region. The distribution of the transport current is computed with a distributed parameter approach, self-consistently with the temperature distribution in the different so-called thermal components of the TSTC. The model is applied to the study of quench propagation in a conductor made of CroCo strands—a recent design proposed by the Karlsruhe Institute of Technology and based on the TSTC idea. A comparison with the modelling approach adopted for quench simulations in LTS conductors is carried out, showing that, when the heating is localized, the new model predicts, for the average temperature on the cross section at the hottest location along the conductor and for the voltage along the conductor, an evolution much slower than that predicted by a standard LTS-conductor model.

The Karlsruhe Institute of Technology (KIT) agreed to construct and test the high-temperature superconductor (HTS) current leads for the tokamak JT-60SA presently under construction in the frame of the Broader Approach agreement between Europe and Japan. In total 6 HTS-CLs for 25.7 kA for the TF coils and 20 HTS-CLs for 20 kA for the PF and CS coils are needed. In addition, the acceptance tests of the HTS current leads were performed in the test facility CuLTKa at KIT. The manufacturing started in 2014 and the last current leads were tested in July 2017. The main parts of the acceptance test are the determination of the heat load at 4 K and the 50 K He mass flow rate through the heat exchanger as well as the simulation of a loss of flow accident. One test of the PF current leads also included a pulse test at 20 kA to demonstrate their PF operation capability. In this paper, an overview of the manufacture and acceptance tests of the HTS current lead is given. The results for the different current leads are summarized and compared to the specifications.

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.

In the frame of the EU DEMO Program within the EUROfusion consortium, the potential use of the High Temperature Superconductor (HTS) material REBCO for a high current conductor of a future Toroidal Field Coil is being investigated. The structural analysis of the winding pack imposed limits on the aspect ratio of the cable space within the jacket, leading to a round ca-ble of 31.2 mm diameter. The HTS CrossConductor (HTS CroCo) developed at the Karlsruhe Institute of Technology KIT is used as a base strand for the cable. During the cable design, the number and the composition of HTS CroCo strands, the amount of additional copper, e.g. a central copper former, is varied with respect to its performance during quench. The cable is embedded in a stainless steel jacket and cooled by 4.5 K supercritical helium. The thermal hydraulic analysis has been done using the code THEA and determines the quench performance of the HTS conductor. The one-dimensional model consists of four homogenized components electrically and thermally coupled via thermal resistances R $_th$ and the heat transfer coefficient h. In the model a quench is initiated by heat pulses E $_init$ over a length l $_INZ$ and for a time t $_Q$ . Important outcomes of the simulations are the hot spot temperatures T $_hot$ of HTS CroCo, copper core and jacket during a fast discharge of the magnet with a time constant of $\tau$ = 27 s after reaching the quench detection voltage V $_QD$ of 100 mV. The dependence of T $_hot$ on different geometrical, material and thermal parameters is studied to determine the critical ones and to derive general design principles for HTS TF coils for fusion magnets. Finally, the performance of a full conductor length is investigated.