China Fusion Engineering Test Reactor Central Solenoid Model Coil Research Articles

The Electromagnetic and Structural Analysis of CFETR CSMC Double Current Lead

Before the formal fabrication of central solenoid coil for China Fusion Engineering Test Reactor (CFETR), the R&D of a prototypical coil namely the Central Solenoid Model Coil (CSMC) is performed to accumulate the design and fabrication experiences in the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). As one of the most important auxiliary components, current lead plays the role of transmitting large current from power supply to CSMC winding packs. Of the total 8 sets of current lead, six of them are double current lead. Once CSMC is energized, large electromagnetic force is induced due to the interaction of current flowing in the current lead and back ground magnetic field. And the stress is generated subsequently under the action of electromagnetic force. If the maximum stress is beyond the material strength assessment threshold, the structural design of the double current lead must be redeployed. In order to check the feasibility of the current design of CSMC, the double current lead located in the high field region is selected and the electromagnetic calculation and electromagnetic-structural coupling analysis are carried out subsequently. In the end, the stress in evaluated based on analytical design. The study in the paper will provide reference for the following design optimization of double current lead.

Mechanical Analyses of Turn & Ground Insulation for CFETR CSMC

The composite insulation materials used in the fusion device superconductivity magnet must have excellent thermal behavior, sufficient dielectric strength, rigorous mechanical performance etc. to adapt the severe multi-field operation environment. Glass fiber reinforced plastics are extensively used for insulation in fusion magnet. In view of its outstanding fabrication and material performances, it is also adopted as the turn & ground insulation for China Fusion Engineering Test Reactor (CFETR) Central Solenoid Model Coil (CSMC). During the cooling down stage of CSMC, large normal stress will be generated in the & ground insulation components. While in the energization stage, besides the normal stress, the shear stress will be generated. If the insulation components are teared down, unanticipated damage may happen on CSMC. Thus, in order to check the current structural design of insulation components, the representative turn insulation and ground insulation components are built. The mechanical analyses are launched subsequently. During the analyses, the multi-load cases are considered. And the detailed shear stress and normal stress in the insulation components are calculated and evaluated based on the insulation material property requirements.

Electrical Insulation Testing for CFETR CS Model Coil

The Central Solenoid (CS) model coil, which is used to develop and verify the larger-scale superconducting magnet technology of China Fusion Engineering Test Reactor (CFETR), has been designed and is currently being manufactured at ASIPP. In case of an emergency shut-down like in succession of a quench, the voltage across the coil may rise to about 2.1 kV. Therefore, the coil has to be provided with a reliable electrical insulation. The electrical insulation systems of the CS model coil consist of turn, layer, and ground insulation. The insulation electrical performance tests are required to carry out at different stages of the manufacturing process to verify the insulation performance of the model coil. This paper gives the description of the insulation system, the applied and proposed test procedures. The test process of the electrical qualification test on mock-up coil is introduced. The test results are analyzed to verify the feasibility of insulation system design and manufacturing.

Final Design of the CFETR Central Solenoid Model Coil

The central solenoid model coil (CSMC) for China Fusion Engineering Test Reactor (CFETR) is being developed in the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). The maximum magnetic field is 12 T in the bore of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coils. It has a 1.5 T/s changing rate under 47.65 kA operating current. It was designed as a hybrid superconducting magnet with Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn and NbTi coils to reduce cost. The detailed structure of the CSMC has been designed, it mainly including Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coils, NbTi coils, preload structure, coil buffer zone, joints, supports, feeder throughs, helium pipes and pipe supports. The preload structure consisting of 15 preload beams, 30 preload rods and support plates. And a 75 MN preload force will be applied on the preload structure at room temperature. This paper describes the final design of the Central Solenoid Model Coil. And the mechanical analysis results show that the final design is reasonable and feasible.

Heat transfer analysis during heat treatment of Nb3Sn coils for the CFETR CSMC

The heat treatment of Nb3Sn coils of China Fusion Engineering Test Reactor (CFETR) central solenoid model coil (CSMC) has been completed at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). The brittle A15 phase material requires coil winding prior formation of the superconducting material, and the technology of “wind and react” has been used to manufacture Nb3Sn coil. The Nb3Sn coils were heat-treated under the argon atmosphere and the heat treatment was successful. Since the critical properties of Nb3Sn strongly depend on the heat treatment temperature, especially the uniformity of temperature, it is necessary to analyze the detailed temperature change process during the heat treatment. During the heat treatment, several thermocouples were arranged on the coil surface for temperature monitoring. At the same time, the heat transfer process of heat treatment and the distribution of the coil temperature field is theoretically analyzed and simulated. The uniformity of heat treatment temperature meets the requirements. This paper mainly presents the heat transfer analysis, heat treatment process, and numerical simulations during heat treatment.

Development of the Turn Releasing Technology for the Nb3Sn Pancake Coil of CFETR CSMC

A central solenoid model coil (CSMC) is being developed to verify the large-scalar superconducting coil manufacture technology for China Fusion Engineering Test Reactor (CFETR) in Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). The turn insulation wrapping of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn pancake coil is conducted after the coil heat treatment. The turn releasing technology is necessary to be developed and verified to insure there is no damage to Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductor during the turn insulation wrapping of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn pancake coil. This paper introduces the design, analysis and verification for the safe stretch distance, the design and debugging of the special equipment, and the process experiment with the 4 × 4 mockup coil.

R&amp;D Activities of Joint Manufacture for CFETR CSMC

The Central Solenoid Model Coil (CSMC) project was launched in the Institute of Plasma Physics Chinese Academy of Sciences for the China Fusion Engineering Test Reactor (CFETR). The CFETR CSMC is designed to generate 12 T peak magnetic field when the running current is 47.65 kA. The CSMC is composed of two Nb3Sn coils in the internal high field and three NbTi coils in the external low field. Electrical joints are one of the key technologies for the CSMC, because the superconducting coils are electrically connected into a single circuit by the joints. Lap type joints have been designed for the CSMC for its proven reliability and easy assembly. It is a great challenge to develop special equipment and technologies for the joint manufacture, including vacuum brazed terminal sleeve assembly, jacket removal, chrome coating removal for Nb3Sn cable, nickel coating removal and silver plating for NbTi cable, compact termination, etc. This paper mainly focuses on the research and development activities of the joint manufacture and the solution of the technical issues. The techniques and equipment can provide a basis for the electrical joint manufacture for the CSMC and other large-scale superconducting magnets.

Seismic analysis of the CFETR CS Model Coil

The China Fusion Engineering Test Reactor (CFETR) Central Solenoid (CS) Model Coil is being fabricated by the Institute of Plasma Physics Chinese Academy of Sciences, and will be cryogenically tested in 2021. In order to validate the structural integrity of the CS Model Coil under seismic loads, response spectrum (RS) and time history analyses have been carried out. This paper first introduces the manufacturing progress of the CS Model Coil and the structure of the CS Model Coil. Then, the RS analysis is perfomed with the complete-quadratic-combination (CQC) modal combination rule. Finally, time history analyses under the artificial Hefei wave and the modified El Centro wave are conducted. The seismic response of the CS Model Coil is described and the obtained maximum von-Mises stresses are respectively 30.9 MPa and 34.0 MPa for the RS and time history analysis. These stresses are mainly caused by the horizontal excitations, and are lower than the allowable stress limit.

AC loss and contact resistance of different CICC cable patterns: Experiments and numerical modeling

• The AC loss and interstrand contact resistance measured on three prototype Nb 3 Sn Cable-In-Conduit Conductors with significantly different cabling twist pitch patterns. • Simulations, done by using the JackPot model are in fair agreement, confirming the consistency of data and allowing reliable predictions with the JackPot model. • For CICC cables, a design petal wrap coverage of 70 % should be maintained for CS cables to reduce the AC loss. A critical threshold value of void fraction exists, which makes the presence of petal wraps ineffective in reducing coupling loss. • The JackPot simulations with derived corrections for inter-petal contact resistance representing 70 % petal wrap coverage show that the Twente cable design has the lowest coupling loss. • So far, the Twente cable design seems a suitable candidate for CFETR or DEMO conductors, made of strain-sensitive strands such as Nb 3 Sn or other materials, for minimization of coupling loss and strand indentation. For upcoming nuclear fusion energy reactors, like the China Fusion Engineering Test Reactor (CFETR) and EU-DEMO, the superconducting Cable-In-Conduit Conductors (CICC) are in the design phase, and the operating conditions like electromagnetic forces can be higher than in previous devices like ITER. The prototype conductors for the Central Solenoid (CS) coils in the CFETR, for example, are designed to produce a peak field of 19.9 T and are expected to be made of high current density Nb 3 Sn strands. Investigations are also ongoing on the application of bismuth strontium calcium copper oxide (BSCCO) and MgB 2 strands for CICCs in fusion reactors. The latter material, MgB 2, could be applied for superconductors subjected to lower magnetic fields, such as Poloidal Field coils, Correction Coils, and Feeders. The performance of all these strands is sensitive to strain, and the mechanical strength of the brittle filaments is relatively weak. This requires a thorough analysis of the cable pattern in terms of the mechanical support of the strands along their length in combination with the minimization of the interstrand coupling currents and strand indentation. As an initial step to finding the most appropriate cable pattern for CICCs, three prototype CICCs made of ITER type Nb 3 Sn strands with significantly different cable twist patterns are tested experimentally for AC coupling loss, interstrand contact resistance, and strand indentation. The three cabling patterns referred to as the Twente, CWS (copper wound superconducting strand), and CFETR-CSMC (CFETR Central Solenoid Model Coil) design. The numerical code JackPot ACDC developed at the University of Twente is used to analyze the interstrand coupling loss and contact resistance. The new ASIPP (Institute of Plasma Physics, Chinese Academy of Sciences) triplet modified CWS design is aimed at reducing strand pinching during cabling, which causes degradation of transport properties during compaction and cyclic loading. The Twente design has the same objective but also aims at reducing the coupling loss while maximizing the mechanical lateral support for the strands by making the twist pitch ratio of the sequential cabling stages close to one. The CFETR-CSMC, taken as a reference for comparison, has cable a pattern mostly similar to the ITER CS cable design.

Manufacturing Status of CFETR Central Solenoid Model Coil

A central solenoid model coil (CSMC) is to verify the large-scalar superconducting coil manufacture technology for China Fusion Engineering Test Reactor (CFETR) in Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). The CSMC composed of Nb3Sn and NbTi hybrid superconducting magnet can reach to 12 T maximum magnetic field. The main manufacturing processes of the coil are developed and verified by R&Ds and mockup coil fabrication. For now, all the five coils winding and 2/3 of terminals fabricating are finished, and the heat treatment for the Nb3Sn inner coil is completed. The turn releasing and insulation wrapping process and station for the heat treated Nb3Sn coil has been developed. The major remaining processes for the five coils are ground insulation wrapping and vacuum pressure impregnation (VPI). The mockup coil VPI will be performed firstly to validate the process. This article introduces the main manufacturing progress of the CFETR CSMC.

Development of the quench protection system for the CFETR CS model coil

The Central Solenoid (CS) model coil, which is responsible for developing and verifying the larger-scale superconducting magnet technology of China Fusion Engineering Test Reactor (CFETR), has been designed and is currently being manufactured at ASIPP. The energy storage of the CS model coil is 407MJ, so a reliable quench protection system is needed to ensure the safety of the coil when a quench occurs. This paper describes the progress of the quench protection system. The two-stage commutating current technique is used to solve the difficulties that the breaker cannot directly break the high DC current. For the key components in the circuit, the prototype of the breaker was manufactured and the performance test was carried out. The engineering design and manufacture of the discharge resistor were carried out. An experimental study of the quench protection circuit was carried out to verify the feasibility of the design. Besides, the reliability of the quench protection control system is qualitatively analyzed based on the fault tree.

Pressure drop study of protective gas for heat treatment in CFETR central solenoid model coil

The inner superconducting coil of the China Fusion Engineering Test Reactor (CFETR) central solenoid model coil (CSMC) is made of Nb3Sn conductor of the Cable-in-Conduit Conductor (CICC) structure. After winding the model coil into shape, it is heat-treated by gas protection. The flow resistance of shielding gas in the coil is studied by means of theoretical calculation and experiment. The pressure drop test system mainly relies on the CSMC heat treatment atmosphere protection subsystem as the experimental platform. High purity argon was used as the working medium in the heat treatment experiment, and the pressure drop at the inlet and outlet of the coil was measured at different flow rates and temperatures. The flow resistance of argon in Nb3Sn coil during heat treatment is estimated by the empirical formula of CICC conductor pressure drop. The experimental results are in good agreement with the theoretical calculation.

Heat Treatment for Nb3Sn Coils of CFETR CSMC

A heat treatment system for superconducting coil had been built at the Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP) for the central solenoid model coil (CSMC) for the China Fusion Engineering Test Reactor (CFETR). The CSMC is composed of Nb3Sn and NbTi hybrid superconducting coils. As Nb3Sn is a brittle material, the technology of “wind and react” has been used to manufacture Nb3Sn coil. During the heat treatment process, various measuring instruments have been used to monitor the heat treatment parameters (e.g., temperature, pressure, impurity gas content). The heat treatment of the inner coil and the outer coil of CSMC has been completed. The evolution temperature and temperature uniformity meet the technical requirements of CSMC heat treatment. The performance of the witness sample meets the recommended value. The measurement results of the coil deformation meet the requirements of the Nb3Sn coil design. Here, the manufacture, measurements, and test results are presented, providing a solid basis for the heat treatment of large size superconducting coils.

A Reliability Analysis of CFETR CSMC Heat Treatment System Based on RPN-HAZOP Method

The safe and reliable operation of the China Fusion Engineering Test Reactor (CFETR) Central Solenoid Model Coil (CSMC) heat treatment system is the basis for the establishment of the CFETR central solenoid, Nb3Sn superconducting coil. The purpose of safety analysis of a heat treatment system is to find out the unreliable factors of the system, analyze the weaknesses of the system, predict the possibility and severity of system failure, and then improve the reliability of the system. The method of combining risk priority number (RPN) with Hazard and operability study (HAZOP) can make up for the deficiency of quantitative information in traditional HAZOP analysis. It is concluded that the fan subsystem is the weakest part of the heat treatment system by means of RPN-HAZOP. The deviation caused by temperature, cleanliness, and impurity gas content will greatly affect the performance of the superconducting coil. The author tries to put forward scientific and reasonable measures according to the principle of giving priority to solving the principal contradiction. Guided by the measures and suggestions, the heat treatment system runs normally, safely, and reliably. The results of the experiment show that all the process indexes meet the requirements of coil heat treatment technology.

Experimental Evaluation of the NbTi Inter-Winding Joint for CFETR Central Solenoid Model Coil

The Central Solenoid Model Coil (CSMC) project of China Fusion Engineering Test Reactor (CFETR) was begun in 2014 to develop and verify the manufacture technology of large-scale superconducting magnets. The CFETR-CSMC coil is mainly composed of inner Nb3Sn windings and outer NbTi windings. There are 12 joints in total, including six interwinding joints and six transition joints. A simple test facility is required for joint experimental evaluation. In this paper, the test facility, sample assembly, and installation and calibration of Hall sensor are described in detail. The first test results of NbTi inter-winding joint are also given. The resistance of the measured NbTi inter-winding joint is about 0.55 nΩ and changes slightly in the current range from 10 to 20 kA.

AC loss test and analysis of the prototype CIC conductor joint for CFETR CSMC

The Central Solenoid Model Coil (CSMC) project has been launched at the Institute of Plasma and Physics Chinese Academy of Sciences (ASIPP) for China Fusion Engineering Test Reactor (CFETR). Electrical joints are important aspect for CSMC, because the local heating from resistive losses can drive the superconductor into a less stable regime and result in premature quench. So part of the project is to develop joints with low DC resistance and AC loss. AC loss power and local heating of the full-size prototype conductor joint in the preliminary test scenario of CSMC are evaluated experimentally and numerically in this paper. The AC loss of the joint sample is measured in SULTAN facility with an applied harmonic field and wider frequency range. Based on the experimental results, the AC loss of the joint is characterized by Multizones Partial Shielding (MPAS) model. Then, the AC loss power of the joint is calculated in the test scenario. The temperature rise of joint is investigated numerically due to the simulated AC loss in the test scenario. The simulation results show the temperature rise in the joint is relatively higher which affects the stability greatly not only in joints, but also the coils. So the joint structure and test scenario of CSMC need to be optimized to reduce the AC loss and ensure safe operation. The way to reduce the AC loss deposit in the joint is also discussed.

Conceptual Design of the Power Supply System for the CFETR CS Model Coil

The central solenoid (CS) model coil (CSMC), which has a high stored energy, is developed to validate the CS manufacturing methods for the China Fusion Engineering Test Reactor (CFETR). The designed operation current of the model coil is 47.65 kA. The maximum rate of the magnetic field is 1.5 T/s. The stored energy of the model coil is 407.6 MJ. A set of verification test of the model coil will be conducted after manufacture, so a CSMC power supply, which generates the current for coil operation, and a quench protection, which could quickly release the stored energy, are required. The main circuit topology of power supply was designed according to the requirements of the testing current waveform. The current control was adopted, which controlled the actual output current provided by the power supply system to track any specified current reference of the model coil within acceptable precision. The design of the quench protection system for model coil was also described in this paper.

Stability analysis on Nb3Sn sample conductor of CFETR central solenoid model coil

The magnet system of China fusion engineering test reactor (CFETR) includes toroidal field, central solenoid (CS) and poloidal field coils. The design and fabrication of CS coil are most significant and challenging for China due to the lack of experience. As the core technique of CS coil, cable-in-conduit-conductor (CICC) has been chosen to ITER CS conductor with the short twist pitch (STP) cable pattern. STP cable pattern has been demonstrated to avoid the performance degradation of conductor after electromagnetic cycling and warm-up-cool-down thermal cycles during operation. For qualifying the manufacturing process and performance of conductor, two short sample conductors were manufactured at ASIPP and tested at Swiss Plasma Center. Performance tests include DC test, Tcs, AC loss and MQE (Minimum Quench Energy) measurements. This paper is mainly about the stability analysis, and the energy margins of the conductor at various operation conditions are investigated with test data and model simulation.

Development of the Helium Inlet and Outlet for the CFETR Central Solenoid Model Coil

China fusion engineering test reactor (CFETR) central solenoid model coil (CSMC) made with cable in conduit conductor superconductor had been developed in the Institute of Plasma Physics, Chinese Academy of Sciences. The highest magnetic field of CSMC is 12 T when the running current is 47.65 kA at 4.5 K, and the largest magnetic field change rate is 1.5 T/s. CSMC mainly consists of one Nb 3 Sn inner coil, one Nb 3 Sn outer coil, and three NbTi coils cooled by supercritical He flow. There are totally ten helium inlets and outlets distributed in CSMC. Due to the geometrical discontinuity within the helium inlet and outlet, the helium inlet area is one of the regions, where high stresses can be expected in the CFETR CSMC. This paper describes the structure design, stress analyses, welding R&D of helium inlet and outlet. A local finite element model based on structure design is created to calculate the stress on the helium inlet under electromagnetic load. The welding R&D activities, including welding trials, destructive test, and NDE have been carried out to develop welding technology.

Status of Winding and Forming Technology Development for CFETR CSMC

The central solenoid model coil (CSMC) for China Fusion Engineering Test Reactor (CFETR) is being developed in ASIPP (Institute of Plasma Physics, Chinese Academy of Sciences). It is a hybrid superconducting magnet of 12 T maximum magnetic field and 1.5 T/s changing rate, which is composed of Nb3Sn coils and NbTi coils in high and low magnetic fields, respectively. The coil continuous winding and forming is one of the key manufacture technologies for CSMC. This paper describes the developing status of the coil winding and forming processes. Based on the short sample bending trials, the winding design and optimization have been completed. At present, the winding production line has been constructed and is subjected for debugging. The forming tools for joggles and leads are being tested. Several technical issues during trials have been solved, e.g., the vertical plastic deformation of the coil and the keystoning of bent conductor. As a result, the continuous winding and forming trial for quad-pancake (QP) of Nb3 Sn inner coil has been conducted successfully, and presented here.