Xiongyi Huang

Superconducting joint is the key component for the superconducting magnet system, which determines the ohmic heat loading during high current carrying operation conditions. Tapered structure is usually used in CORC joint, which can realize the direct electrical contact between each REBCO layer and copper plate. However, when considering forced-flow cooling, traditional tapered structure needs to be optimized to realize cooling medium transmission and enhance current distribution uniformity. A new CORC joint with forced-flow cooling method is introduced in this paper. The relationship between tapered length of each layer and current distribution unevenness is also analyzed, it reveals that tapered length is greater than twisting pitch length of CORC cable, the current distribution uniformity is acceptable. For transporting cooling medium (liquid helium, supercritical helium and so on), there is two cooling channel, including central cooling channel and circumferential cooling channel, which can realize efficient transmission of central and annular cooling medium (The central core shaft with spiral holes can realize the exchange of cooling medium between the center and the annular direction). The forced-flow CORC joint has been prepared and 77 nΩ resistance has been achieved. 1.5 K temperature rise occurs with 4 kA current. In the updated design, the copper head size is further increased and cooling channels are added. The resistance achieved 46 nΩ and only 0.5 K temperature rise occurs with 4 kA current carrying. The developed CORC joint has been used on the HTS SMES system with maximum 10 kA current capacity.

The on-site construction of the International Thermonuclear Experimental Reactor (ITER) Tokamak Assembly Contract 1 (TAC1) project has been carried out for about three years. In past three years, multiple installation milestones were completed by Chinese Consortium (CNPE Consortium), such as Pit Ready, first feeder Coil Terminal Box (CTB) on final position, first onsite superconducting joint was completed, etc. In this paper, the lessons learned from the real installation on ITER site for the first of kind of Correction Coil (CC) and Magnet Feeder systems are introduced. It includes the following aspects: Installation process optimization for Bottom Correction Coil (BCC) on temporary position; Key installation technologies for components of Magnet Feeder system; Superconducting joint assembly and insulation between CTB and Cryostat Feedthrough (CFT); Application of reverse engineering technology in precision assembly process. Furthermore, the paper explains technology challenges as well as modifications due to the first experiences in onsite construction.

Accelerator Complex such as NICA built in JINR has superconducting magnets operating at 10 kA level, the stable operation of which needs high performance SMES system to achieve energy compensation. The SMES needs to be operated with several kA-level current carrying and kA/s current ramping rate. The AC loss heat deposition resulting from fast current ramping makes immersion cooling and conduction cooling method inapplicable. The forced flowing cooling high temperature superconducting (HTS) CORC cable used for the SMES system with high current capacity is introduced in this paper. For achieving better bending performance and control the ohmic loss in the mandrel of the CORC cable, we choose flexible metal conduit as the mandrel and casing externally. It can form an internal cooling channel and outer channel at the same time, which can realize satisfactory cooling effect. The bending performance of the CORC with new type mandrel is also studied in this paper and related testing experiments are conducted at liquid nitrogen temperature zone. The results show that the CORC cable with flexible metal conduit as mandrel has satisfied bending performance and there is nearly no critical current degradation after bending and straighten. The pressure drop of the forced flow cooling based on this kind of CORC cable is also analyzed. The pressure drop meets the operation requirements of forced flow cooling. The SMES system based on this kind of CORC cable is being developed and will be finished in 2024.

Accelerator Complex such as NICA built in JINR has superconducting magnets operating at 10 kA level, the stable operation of which needs high performance SMES system to achieve energy compensation. The SMES needs to be operated with several kA-level current carrying and kA/s current ramping rate. The AC loss heat deposition resulting from fast current ramping makes immersion cooling and conduction cooling method inapplicable. The world’s first forced flow cooling 0.5 MJ HTS SMES with 6 kA current capacity based on CORC cable is being developed at ASIPP. For achieving better bending performance and control the ohmic loss in the mandrel of the CORC cable, we choose flexible metal conduit as the mandrel and casing externally. The combined action of internal and outer cooling channels can realize satisfactory cooling effect. For reducing the AC loss heat deposition during fast current changing, the CORC cable is wound with variable winding angles at different layers. A special designed HTS joint is designed for realizing the kA-level current transferring and forced cooling. Now the engineering design of the HTS-SMES is finished, which consists of 8 sub-solenoid coils and the inductance is 28 mH. The coils are cooled by forced flow supercritical helium and operated at 6 K. The manufacture is undergoing and will be finished in 2023.

Comprehensive Research Facility for Fusion Technology (CRAFT) is a comprehensive research platform for the research and development of the key components of the fusion reactor in China, among them, the toroidal field (TF) superconducting magnet system is an important part of the facility. The design and analysis of high-temperature superconducting current leads (HTSCL) are carried out in this article. The main contents are as follows: 1) The design requirements and parameters of HTSCL are introduced, the heat transfer model of HTS module is established, and the temperature distribution and heat load under multiple working conditions are calculated; 2) Based on parameterized language, the algorithm of electromagnetic analysis of HTS module is realized, and the current-carrying capacity under multiple working conditions are studied; 3) 1-D steady-state heat transfer equation of gas cooled current lead is transformed by Runge–Kutta algorithm, and optimization analysis of heat exchanger is given; 4) The shielding of the HTS module is designed to maximize the current-carrying capacity; and 5) The loss of flow accident (LOFA) and Burnout performance are calculated, respectively. The calculation results show that the main performance parameters of HTSCL meet the design requirements of TF magnets.

A set of gas nitriding coating device suitable for the electrostatic deflector (ESD) of cyclotron is designed. Using this device, the TiN coating on small electrode of titanium alloy coupons was carried out in the mixture of nitrogen and ammonia. The effects of nitriding temperature and time on the reaction results were studied, the results of the coating experiment were analyzed, and the surface treatment parameters suitable for gas nitriding TiN coating on the surface of the titanium alloy electrode of the ESD were obtained. X-Ray Diffraction (XRD), scanning electron microscope (SEM) and X-ray photoelectron spectroscopy (XPS) were used to characterize and analyze the properties of nitrided coatings. And a high voltage discharge experiment was performed on the sample to measure its dark current. The results show that under atmospheric pressure, annealing temperature of 900 °C, and holding time of 8 h, the coating conditions of titanium alloy electrode materials has the best performance.

The China Fusion Engineering Test Reactor (CFETR) is a nuclear fusion reactor that China is about to develop and manufacture independently. The central solenoid (CS) is one of its key components for inducing and maintaining the plasma during the tokamak operation. In phase II of CFETR, the maximum magnetic field of the CS magnet must be 17.5 T. In such a high magnetic field, NbTi and Nb3Sn cannot carry very high current. Thus, the use of high-temperature superconductors is one of the options for the manufacture of CFETR CS coils. Research on high-temperature superconducting joints as the key components for the electrically connection of cables has already been launched in Institute of Plasma Physics, Chinese Academy of Science (ASIPP). In this paper, the design of the overlap joint for the CS coil is described: the Bi-2212 strands in the cable terminations are soldered together with the pure indium layers with a saddle-shaped copper sole. Moreover, the analysis of the joint resistance, the AC loss, and the temperature rise caused by the joining is discussed in detail.