Jinggang Qin

A novel numerical mechanical model for the stress–strain distribution in superconducting cable-in-conduit conductors

Besides the temperature and magnetic field, the strain and stress state of the superconducting Nb3Sn wires in multi-stage twisted cable-in-conduit conductors (CICCs), as applied in ITER or high field magnets, strongly influence their transport properties. For an accurate quantitative prediction of the performance and a proper understanding of the underlying phenomena, a detailed analysis of the strain distribution along all individual wires is required. For this, the thermal contraction of the different components and the huge electromagnetic forces imposing bending and contact deformation must be taken into account, following the complex strand pattern and mutual interaction by contacts from surrounding strands. In this paper, we describe a numerical model for a superconducting cable, which can simulate the strain and stress states of all single wires including interstrand contact force and associated deformation. The strands in the cable can be all similar (Nb3Sn/Cu) or with the inclusion of different strand materials for protection (Cu, Glidcop). The simulation results are essential for the analysis and conductor design optimization from cabling to final magnet operation conditions. Comparisons are presented concerning the influence of the sequential cable twist pitches and the inclusion of copper strands on the mechanical properties and thus on the eventual strain distribution in the Nb3Sn filaments when subjected to electromagnetic forces, axial force and twist moment. Recommendations are given for conductor design improvements.

Manufacturing of the ITER TF Conductors in China

The toroidal field (TF) coil conductor is a cable-in-conduit conductor made up of superconducting, Nb3Sn-based strands mixed with pure copper strands. The strands are assembled in a multistage cable around an open central spiral. The cable and its spiral are inserted into a stainless steel jacket. The jacket provides the helium confinement and must conform to the leak tightness standard defined herein. A technological complex has been developed at the Institute of Plasma Physics Chinese Academy of Sciences for serial production of ITER TF conductors is described. This technology was developed as a result of performing some R&D programs on selecting cabling parameters, welding technology, elaboration of cable insertion, compaction, and winding process. One qualification dummy TF conductor (760 m) and superconducting conductor (110 m) were successfully made. A new manufacturing complex was developed within the Institute of Plasma Physics Chinese Academy of Sciences for realization of this technology. The details of manufacturing procedures are described in this paper. It consists of a cabling workshop, and jacketing line equipped with full production machinery and test devices.

The Axial Tensile Stress–Strain Characterization of Ag -Sheathed Bi 2212 Round Wire

The stress distribution generated by the differences in the thermal expansion and the electromagnetic load is the driving factor for the transport properties of Bi2212 superconducting round wire (RW). The effort on studying the impact of strain on the transport properties is increasing, in terms of the axial and transverse stiffness of the RW. Consequently, the experimental stress-strain data are required at the RW level for accurate modeling, analysis and eventually for optimizing cable design and manufacture. In this paper, the axial tensile measurements on Bi2212 RWs and component materials (Ag and Ag/Mg alloy) have been performed at room temperature, 77 K and 4.2 K, respectively. Comparing with LTS strand (e.g., Nb3Sn and NbTi) , the stiffness of Bi2212 RW is less, which has become the primary problem in its application. A simple model was used to simulate the stress-strain characteristic, and compared to experimental results.

Cabling technology of Nb3Sn conductor for CFETR Central Solenoid Model Coil

The China Fusion Engineering Test Reactor (CFETR) is a new tokamak device, whose magnet system includes the toroidal field, central solenoid (CS), and poloidal field coils. To develop the manufacturing technique for the full-size CS coil, the Central Solenoid Model Coil (CSMC) project was launched first. The cable-in-conduit conductor used for CFETR CSMC refers to the International Thermonuclear Experimental Reactor CS conductor with the same short twist pitch cable pattern. As the short twist pitch and low void fraction, a higher compaction ratio during cabling is required, and the possibility of damages occurring on strands is increased. After numerical analysis and experimental trials on cabling technology, one short cable sample was manufactured, which shows good results compared with former experimental cables.

Manufacturing of ITER PF5 and CC Sample Conductors

The design of poloidal field (PF) coils and correction coils (CCs) for the International Thermonuclear Experimental Reactor (ITER) relies on the use of 45-kA NbTi cable-in-conduit conductors. All PF5 and CC conductors are produced in China. Research and development programs are needed to acquire knowledge on the behavior of such conductors. Since the conductors are new, full-size copper dummy conductors have been produced in advance for testing the cabling parameters, the definition of the automatic tungsten-inert-gas welding of a seamless jacket section, the elaboration of cable insertion, compaction, etc. Then, two short qualification conductor samples (the PF5 and the CC) are manufactured at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP), Hefei, China, with the NbTi advanced strands produced by the Western Superconducting Technologies Company, Ltd. In this paper, the manufacturing procedures for the two PF5 and CC conductor samples are described in detail.

Status of the ITER Conductors in China

The ITER magnet system is made up of four main subsystems: the 18 toroidal field (TF) coils, the central solenoid, the six poloidal field (PF) coils, and the correction coils (CCs). The feeder system, with its main busbar (MB) and CC busbar (CB), represents one of the main magnet components as well. All coils and busbars with different dimensions used cable-in-conduit conductors. China has signed three conductor packages, which are the so-called procurement arrangements, between ITER and the Chinese Domestic Agency (CN DA): a TF conductor package, a PF conductor package, and a CC and feeder conductor package. They include 7.5% of the TF conductors (11); all the PF2 (12), PF3 (16), PF4 (16), and PF5 (16) conductors; all the CC (18) conductors; and the MB (3) and CB (2) conductors for the feeders. Complex technologies have been developed by ASIPP for the serial production of all ITER conductors, in terms of cabling parameter design, welding, and elaboration of cable insertion, compaction, and winding processes. China has finished all qualification phases and is well into the main series production. All conductor samples required for quality control have successfully passed the SULTAN tests with good performance. The status of the production of ITER conductors in China is described in this paper.

Conductor Performance of TFCN4 and TFCN5 Samples for ITER TF Coils

The fourth China TF conductor sample (TFCN4) for Phase II, the left leg of the fifth China TF conductor sample (TFCN5) for Phase III, as well as the right leg of the TFCN5 conductor for Phase IV, were made of the WST strands, cabled by Baosheng, jacketed at ASIPP, and prepared by CRPP following the IO-approved procedure and tested in the SULTAN facility. The Tcs test results show that both legs of the TFCN4 and TFCN5 have high Tcs performance. Using the electrical method, the Tcs was 6.56 K for the left leg of TFCN4 and 6.58 K for the right leg of TFCN4 at 68 kA/10.78 T in the first test and 6.36 K for both legs after 1000 electromagnetic load cycles. In the case of TFCN5, Tcs was 6.56 K for the left leg and 6.30 K for the right leg at 68 kA/10.78 T in the first test and 6.33 K for the left leg and 6.02 K for the right leg after 1000 electromagnetic load cycles. According to the SULTAN test result, Tcs values of TFCN4 and TFCN5 conductor samples meet the ITER acceptance criteria.

CORD, A Novel Numerical Mechanical Model for Nb3Sn CICCs

The strain state of the superconducting Nb3Sn strands in multi-stage twisted ITER Cable-In-Conduit Conductors (CICCs) strongly determines the transport properties. For an accurate prediction of the performance and a proper understanding of the underlying phenomena, a detailed analysis of the stress and strain distribution along all individual strands is imperative. Also during the cabling process, the axial stress of the individual strands must be well controlled to avoid kinks, in particular when mixing different strands, e.g., Nb3Sn and copper strands. A mechanical model for a superconducting cable (CORD) was developed, which can predict the strain and stress states of all single strands including interstrand contact force and the associated deformation. The simulation results are not only important for analysis but can be used for optimization of cable manufacturing and conductor design optimization. We discuss the influence of the sequential cable twist pitches and the inclusion of copper strands on the mechanical properties.

Research on the Mechanical Properties of Jacket Used for Bi-2212 Cable-In-Conduit Conductor

The China fusion engineering test reactor is a new tokamak device. It is a commercial reactor, which demands a superconducting magnet with higher magnetic field. The maximum field of CS and TF will get around 15 T, which is much higher than that of present reactors. In order to meet the requirements, the new conductor with Bi2Sr2 CaCu2Ox is considered as one potential material for the superconducting magnets. Because Bi2212 wire needs to endure special heat treatment with oxygen, the jacket material is one key issue. As one new material, Ni80Cr has an excellent performance, which cannot react with Bi-2212 wire. It could be one potential material as Bi-2212 cable-in-conduit conductor jacket. In order to understand the mechanical properties of Ni80Cr, the samples with different conditions were prepared, and tested at high, room, and low temperature (4.2 K). The results are analyzed in this paper.

Impact of Indentation on the Performance of MgB 2 Round Wire

MgB2 round wire is a promising material for future fusion Cable in Conduit Conductor applied in low magnetic field, due to its larger T-margin compare with NbTi. Its a competitive candidate for feeders, poloidal field or correction coils. Since MgB2 is sensitive to strain, a short twist pitch cable pattern will be taken into consideration. During cabling and conductor manufacturing, the compression on the strand is inevitable, for a cable with short twist pitch can make more severe indentation on the strands. Previous studies have shown that indentation can caused degradation of the critical current of Nb3Sn and Bi-2212 round wires. So the same research is performed on MgB2 round wire, in this paper, the critical current of artificially indented samples is measured, and the results are compared with artificially indented Nb3Sn, NbTi and Bi-2212 round wires.