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Design and feasible verification of the round high-temperature superconducting (HTS) strand with REBCO tapes is the key technology for future fusion magnet application such as China Fusion Engineering Test Reactor (CFETR). Flat tape configuration of REBCO tapes and incompatibility with cable design based on round low-temperature superconducting (LTS) strand lead to the development of new cable technology. In this article, a HTS strand variant with soldered multilayer copper tapes as filler and tube called SaMiT is developed following the technology path of twisted stacked-tape cable (TSTC). Due to the use of a milling process to form the round strand, the probability of REBCO tapes damage is reduced greatly in the manufacturing process. Meanwhile, the tapes delamination problem and solution is also discussed. The bending and twisting test of the samples are performed to verify the electromechanical characteristics. The measurement result shows the SaMiT has great potential in HTS cable application. Future tests at transverse load and high field are needed for fusion magnet application.

Various strand concept for high-temperature superconductor (HTS) flat tapes have been developed in the past few years for fusion magnet applications. We developed a round HTS strand with soldered multilayer copper tapes as filler and support called SaMiT following the technic path of twisted stacked-tape cable. DC tests performed under bending and twisting condition demonstrated that the proposed design of the HTS strand is promising in the high current cable and magnet application, but the study on electromechanical properties against the transverse and cyclic load is still needed. A press device was designed to measure the critical current of SaMiT sample at 77 K and self-field as a function of transverse and cyclic load to simulate the electromagnetic force load during the magnet operation. Transverse load amplitudes have been applied up to 40 MPa and cyclic load up to 1000 cycles at certain load amplitude. The experimental results are presented and structural optimization suggestions are discussed with visual inspection and analysis.

The 25 T all superconducting magnet designed as a combination of 14 T low temperature superconductor (LTS) background field magnet and a 11 T YBCO insert coil has been developed at Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). The prototype insert coil with no-insulation winding technique has the inner and outer diameters of 16 mm and 54 mm, respectively, and consists of ten single pancakes wound with about 220 m of YBCO conductors produced by Shanghai Superconducting Technologies Co., Ltd (SSTC). The preliminary tests were carried out at 77 K and 4.2 K. The test results show that the performance of coil did not degrade during the manufacturing and impregnation process, and it can generate a steady-state central magnetic field of 9.7 T in the 14.3 T background field, and 10.6 T in self-field. The detailed design, construction, and preliminary test results of the YBCO insert coil will be fully introduced in this paper.

A great progress in the electrical performance of high-temperature superconductor (HTS) REBCO tapes allows for a much increased transport current capability in cables especially at high magnet fields. This is of interest to future fusion magnet applications. A variant type of twisted stacked-tape cable called quasi-round twisted stacked-tapes conductor (QisTac) is proposed and developed for the superconducting conductor of the China fusion engineering test reactor. The stack of the twisted HTS tapes is wrapped by a tight spiral made with copper wires. The stack with the copper wire is, then, soldered to form the solid round conductor, which is reliable and easy to manufacture in long lengths. A QisTac sample with four REBCO tapes was prepared and measured at 77 K and in the self-field condition. The measurement result shows the expected critical current calculated from the individual tapes considering the self-field effect. The bending characteristic of the sample was also tested under different bending radii. The critical current is reduced by roughly 5% at 0.54% bending strain. Some residual bending strain is maintained when the maximal bending force is released. This result is promising, but reproducibility testing and tests at high field and current are needed for future high field application.

With the continuous reduction of the production costs and improvement of the transport performance, YBCO coated conductor is the most promising candidate for the high field magnet application due to its high irreversibility field and strong mechanical properties. Presently a stable production capacity of the YBCO conductors has been achieved by Shanghai Superconducting Technology Co., Ltd (SSTC) in China. Therefore, the demand in high field application with YBCO conductors is growing in China. This paper describes the design, fabrication and preliminary experiment of a solenoid coil with YBCO conductors supplied by SSTC to validate the possibility of high field application. Four same double pancakes were manufactured and assembled for the YBCO coil where the outer diameter and height was 54.3 and 48 mm respectively to match the dimensional limitation of the 14 T background magnets. The critical current (Ic) of YBCO conductors was obtained by measuring as a function of the applied field perpendicular to the YBCO conductor surface which provides the necessary input parameters for preliminary performance evaluation of the coil. Finally the preliminary test and discussion at 77 and 4.2 K were carried out. The consistency of four double pancakes Ic was achieved. The measured results indicate that the fabrication technology of HTS coil is reliable which gives the conference for the in-field test in high field application. This YBCO coil is the first demonstration of the SSTC YBCO coated conductors.

This paper details the design and construction of a non-contact apparatus that can measure the longitudinal critical current of REBCO tapes based on magnetic circuit method. A 3D finite element model was proposed to study the relation between Ic (critical current of the REBCO tape) and BH (magnetic field at the Hall sensor). The proportionality coefficient of Ic and BH was obtained by using the said model. Several experiments on the REBCO tapes were conducted to determine the repeatability, reliability, and accuracy of the apparatus. Four parameters (average value, minimum value, the range, and the coefficient of critical current variation (COV)) were used to determine the repeatability and accuracy of the apparatus. It was observed that transmission speed affects test results because of relaxation of the magnetic flux. Another tape was tested at different transmission speeds to study the relation between BH and decay time. From the results, a principle formula was established to explain the relation between BH and decay time, and it was shown that the test results also take the same form as the principle formula.

The construction of a new hybrid magnet, consisting of a 11 T superconducting outsert and a 34 T resistive insert magnet, has been finished at the Chinese High Magnetic Field Laboratory (CHMFL) in Hefei. With a room temperature bore of 800 mm in diameter, the hybrid magnet superconducting outsert is composed of four separate Nb 3 Sn-based Cable-in-Conduit Conductor (CICC) coils electrically connected in series and powered by a single power supply system. The power supply system for the superconducting outsert consists of a 16 kA DC power supply, a quench protection system, a pair of 16 kA High Temperature Superconducting (HTS) current leads, and two Low Temperature Superconducting bus-lines. The design and manufacturing of the power supply system have been completed at the CHMFL. This paper describes the design features of the power supply system as well as the current fabrication condition of its main components.

An 11 T superconducting magnet has been manufactured for a 45 T hybrid magnet, the superconducting magnet has a large room temperature bore of 800 mm, so it can provide an 11 T background field for performance test of cable in conduit conductors (CICC). A test facility for superconducting CICCs has been designed as an insert combined with the superconducting magnet, wherein an 80 kA superconducting transformer has been designed to provide high operating current for the CICC samples. The CICC sample was designed to be a bifilar non-inductive solenoid coil. This paper will describe the principle design of the test facility and the overview of the 80 kA superconducting transformer.

Bending tolerance is one of the most fundamental mechanical properties of REBCO coated conductors for application to a superconducting coil. In this study, we investigated critical current degradation of REBCO coated conductors under bending and uniaxial tensile strain at 77 K and self-field with three different kinds of production from SuperPower Inc, Suzhou Advanced Materials Research Institute (SAMRI) and Shanghai Superconductor Technology (STT), respectively. All the tapes showed better bending tolerance under compressive strain than tensile strain, particularly for STT tapes, which showed no Ic degradation in compressive bending strain even with bending radius of 4.5 mm, but a 5% Ic degradation in tensile strain with bending radius of 6–6.5 mm. A uniaxial tensile strain test was also carried out to verify the dependence of Ic under tensile bending strain, the test results showed a good agreement between them. The tensile strain tolerance was 0.28–0.29% for STT tapes, 0.4–0.43% for SAMRI tapes, and 0.5–0.55% for SuperPower tapes.

A hybrid magnet, which is currently under construction at the High Magnet Field Laboratory of the Chinese Academy of Sciences (CHMFL), uses both Florida-Bitter-type resistive coils and Nb3Sn-based cable-in-conduit conductor superconducting coils to produce 45-T central field in a 32-mm-diameter bore. As one of the main parts of the CHMFL hybrid magnet, the cryostat components of the superconducting outsert will provide the thermal barrier of the Nb3Sn coils. Manufacturing of the main cryostat components, particularly the cryostat vessel, the thermal shield, the vacuum pumping units, and the overpressure protection device, has been completed. In this paper, the design and construction of the main cryostat components are going to be reported.

Heat treatment of large-scale superconducting coils is limited by the power of the furnace and the weight of the superconducting coils. For the superconducting coils to be used in a 45-T hybrid magnet, the practical temperature curve will lag behind the setting temperature curve, particularly for the lowest temperature stage (210 °C). There is also a risk that the heat treatment might be interrupted due to a general power failure or equipment failure. The highest temperature stage (640 °C) is the key stage to form Nb3Sn crystals; thus, interruption of the heat treatment during the 640 °C stage is expected to cause a great impact on the performance of a Nb3Sn superconducting magnet. This paper compares the performance of Nb3Sn strand samples given the recommended heat treatment with samples subjected to simulated temperature lag or simulated power failure. All of the samples had Residual Resistance Ratios, critical currents, and calculated heat losses that were satisfactory for use in the superconducting part of the 45-T hybrid magnet.

In the last decades, various methods have been used to characterize superconducting behavior versus axial strain. In cooperation with the University of Twente (UT), the Institute of Plasma Physics Chinese Academy of Sciences (ASIPP) built a facility practically similar to the so-called “Pacman” spring device. The facility allows critical current measurements of an Nb3Sn strand at different temperature, magnetic field and applied axial strain. In the process of validating the new setup, an ITER Internal-Tin (IT) strand, which has also been measured at the UT, is tested with the ASIPP Pacman. The ITER IT wire has an irreversible strain limit around +0.3%. At ASIPP, the strand was characterized in a magnetic field from 10 to 13 T and applied axial strain from -0.7% to +0.5%. Similarities of the data obtained (IC(e), IC(B) and irreversible strain limit) from the two institutes were remarkable.

The superconducting magnet for a 40-T hybrid magnet is being developed at the High Magnetic Field Laboratory, Chinese Academy of Sciences. The hybrid magnet consists of a resistive insert and a superconducting outsert providing 29 and 11 T, respectively. The superconducting magnet was designed to be wound with four grades of cable-in-conduit conductors (CICCs). The superconducting coils contain over 5000 kg of $\hbox{Nb}_{3}\hbox{Sn/Cu}$ CICC in seven piece lengths. Three dummy conductors and seven prototype conductors have been successfully fabricated without incident. The conductor design, strand cabling, and conductor fabrication details will be introduced in this paper.

A 40-T hybrid magnet under construction at the High Magnetic Field Laboratory of the Chinese Academy of Sciences will use cable-in-conduit conductors for its superconducting outsert. The design necessitates the use of high current density (Jc) Nb3Sn strands (Jc ≥ 2100 A/mm2 at 12 T/4.2 K). The high Jc restacked-rod-processed Nb3Sn strand from Oxford Instruments Superconductor Technology was selected as one of the candidate strands. A series of tests was carried out to qualify the strand performance. Tests mainly included magnetization and critical current measurements as a function of axial strain, magnetic fields, and temperature. The test results are presented and discussed.

A model coil developed at the High Magnetic Field Laboratory, Chinese Academy of Sciences, will be wound with a Nb3Sn cable-in-conduit conductor (CICC). In this paper, we first introduce the model coil structure and its design requirements and then explicate the important components of the model coil structure, such as the preload structure, joint, cooling circuit, etc. Stress analyses of the preload structure, lead-in and lead-out CICC, joint, transitive CICC, and some supporting structures are performed, and the results are compared with the International Thermonuclear Experimental Reactor metallic and composite structural component design criteria.

A model coil for the 40-T hybrid magnet is being built at the Chinese High Magnetic Field Laboratory, Chinese Academy of Sciences. /restack-rod-process (RRP) strands adopted for the model coil were heat-treated according to the heat treatment (HT) schedule recommended by the manufacturer, which is Oxford Superconducting Technology. The microstructure of the reacted strand was analyzed. Measurements of the critical current, the residual resistivity ratio and the hysteresis losses were also carried out. The results of critical-characteristic measurements confirmed that the performance of the heat-treated wire was good. All characteristics indicated that the HT of the /RRP strands was successfully completed.

This paper is devoted to predict AC loss of cable in conduit conductor (CICC) which is of importance in the design of conductors. The consideration for the conductor's design and main parameters for the magnets are introduced. In order to attain a good accuracy in the calculation of AC losses, the field distribution within superconducting outsert should be considered. Calculation of the AC losses, including hysteresis losses and coupling losses, is conducted. An emphasis is put on the hysteresis loss during the ramp up of the current to the operational current (15.3 kA) and the coupling loss of the conductor in a power-down condition for insert. The results are obtained to be 74.9 kJ and 950 J for 40 T hybrid magnets, respectively. Based on the calculation, a brief analysis of losses effect on the conductor design and the operation of magnet is given for the purpose that the capacity of the cryogenertor can be evaluated and the stability regime can be improved in our future work on the hybrid magnets.

A 40 T hybrid magnet system will be designed and constructed at the High Magnetic Field Laboratory, Chinese Academy of Sciences. The superconducting outsert consists of two concentric superconducting coils. After the conceptual design of the superconducting magnet, the RD the first is a traditional method called global-local finite element technique, the second is a detailed 2-D FEA method.

A 40 T hybrid magnet system will be built at the Chinese High Magnet Field Laboratory (CHMFL), Chinese Academy of Sciences. It will include a 29 T resistive insert with a clear bore of 32 mm and a 11 T superconducting outsert with a room temperature bore of 580 mm. The superconducting outsert, which will be wound with cable-in-conduit conductors (CICC), is graded in function of the magnetic field. The high field section of the superconducting outsert will use superconductors and the low field section will be made of NbTi superconductors. At present, the preliminary design of the superconducting outsert has almost been completed. This paper describes features, such as conductor design, coil winding, heat treatment, support structure design, mechanical stress analysis.

The High Magnetic Field Laboratory of the Chinese Academy of Sciences will develop a 40-T hybrid magnet. The superconducting outsert, which consists of a NbTi coil and a Nb3 Sn coil, will provide more than 11 T of central field in a 580-mm bore. The superconducting coils will be wound of cable-in-conduit conductor (CICC) and cooled by flowing 4.5-K supercritical helium. A model coil of the Nb3Sn coil has been designed and will be manufactured to develop the technology of cabling, jacketing, winding, heat treatment, etc., and the model coil can also simulate the performance of the Nb3Sn superconducting coil under its operating conditions. This paper gives an overview of the conceptual design for the model coil and predicts its current-sharing temperature (Tcs).

A 40 T hybrid magnet is being developed at the High Magnetic Field Laboratory of the Chinese Academy of Sciences. The magnet consists of a resistive insert and a superconducting outsert providing nominally 29 T and 11 T, respectively. The super-conducting outsert with a room temperature bore diameter of 800 mm is composed of three Nb3Sn coils wound from four grades of cable-in-conduit conductor cooled with forced-flow supercritical helium at an inlet temperature of 4.5 K. This paper gives the final design of coils and support structure of the superconducting outsert. Finite element analyses has been performed for normal operational and fault loads. We also describe the cooling scheme, quench detection, and protection.

The main goal of the model coil is to demonstrate the feasibility of `Insulation-Wind-React' manufacturing process and to confirm the feasibility of the superconducting outsert design. The model coil was wound with Cable-in-Conduit Conductor (CICC) using restacked-rod process (RRP) Nb3Sn strand, and cooled with forced-flow supercritical helium with inlet temperature of 4.5 K. The test of the model coil in the 7.5 T background field has been completed, and the test program included cool-down, full DC characterization before and after cyclic loading, AC losses, and so on. The test results have confirmed the characterization of the Nb3Sn conductor and the mechanical performance of the model coil, and validated the 40 T hybrid magnet superconducting outsert design.