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With the rapid development of machine learning in the medical cloud system, cloud-assisted medical computing provides a concrete platform for remote rapid medical diagnosis services. Support vector machine (SVM), as one of the important algorithms of machine learning, has been widely used in the field of medical diagnosis for its high classification accuracy and efficiency. In some existing schemes, healthcare providers train diagnostic models with SVM algorithms and provide online diagnostic services to doctors. Doctors send the patient's case report to the diagnostic models to obtain the results and assist in clinical diagnosis. However, case report involves patients' privacy, and patients do not want their sensitive information to be leaked. Therefore, the protection of patient's privacy has become an important research direction in the field of online medical diagnosis. In this paper, we propose a privacy-preserving medical diagnosis scheme based on multi-class SVMs. The scheme is based on the distributed two trapdoors public key cryptosystem (DT-PKC) and Boneh-Goh-Nissim (BGN) cryptosystem. We design a secure computing protocol to compute the core process of the SVM classification algorithm. Our scheme can deal with both linearly separable data and nonlinear data while protecting the privacy of user data and support vectors. The results show that our scheme is secure, reliable, scalable with high accuracy.

High temperature superconducting (HTS) conductors or cables can be considered as a significant candidate in various large-current HTS applications. To develop large-current HTS conductors or cables, HTS wires of small square dimension have been suggested and studied in our group, and the wire is named Soldered-Stacked-Square (3S) wires. Based on 3S (2+10c)-wires composed of two HTS tapes and ten copper tapes, a novel HTS twisted cable structure named “2mm-7-cable” including seven (2+10c)-wires was firstly proposed in this study. This structure is most similar to the cable in conduit conductor (CICC) structure of low temperature superconductor (LTS). To begin with, we introduced the fabrication process of the 2mm-7-cables, including preparation of soldered-stacked-square (3S) wire, winding copper wire process, twisting and cabling process. Then, their critical current performance was evaluated experimentally. The test results of critical current show that when the voltage-lead is directly connected to the surface of the (2+10c)-wire after removing the copper wires of both ends, the measured resistance of 2mm-7-cable will be greatly reduced. And the summation of the critical currents of seven (2+10c)-wires are almost identical to the one of the 2mm-7-cable. Thus, the feasibility of the fabrication method for the novel HTS twisted cable is partially verified in this study.

On the basis of 24 T magnet, the Institute of Plasma Physics, Chinese Academy of Sciences are developing 35 T all superconducting magnet. The magnet was designed as a combination of 20 T REBCO insert coil and a 15 T 150 mm low temperature superconductor background magnet. The 20 T insert coil consists of two coils nested together. NI winding technology is applied to coil 1 to increase its current density. The 50 μm thick Hastelloy tapes will be used to co-wind with REBCO tapes of the coil 2 to further improve its over-all mechanical strength. Electromagnetic analysis shows that the maximum ratio of operating current to the critical current of coil 1 and coil 2 is 0.63 and 0.37 respectively. A more accurate model for calculating strain considering screening current is under study. The inner joint is applied to the coil 1 to avoid in-plane bending of REBCO tape, and the detailed preparation method is given in the paper. The winding and preliminary testing of all double pancakes of coil 1 have been completed, and the winding of coil 2 is in progress. This magnet is planned to be assembled and tested in 2021.

Stacking structure is the widely-accepted method to improve the current carrying capacity of second-generation high temperature superconducting (2G HTS) tapes. Meanwhile, due to the influence of high aspect ratio, narrowing concept becomes an effective way to improve the electromagnetic characteristics of HTS tapes. Solder-Stacked-Square (3S) HTS wire is novel HTS conductor with 1 mm width, which is derived from the normal HTS tapes and combined the meaning of both stacking and narrowing. As is known, the external magnetic field affects the critical current of HTS tapes. In order to get the reliable critical current of 3S HTS wire for the real application, it is important to evaluate the magnetic field dependence of 3S HTS wire in detail. However, the stacking process makes the 3S wire structure flexible and changeable. To be clear about the stacking effect of 3S wire on the critical current, we prepared 3S wire samples, which are respectively stacked by three different structures. In this paper, we measured the magnetic field dependence of critical current for these 3S wire samples. The experimental results are the necessary support for the design of 3S wire applications.

Combining several high temperature superconducting (HTS) tapes into an assembled HTS conductor can enhance the current-carrying capacity, mechanical strength and improve the large width-to-thickness ratio of HTS tapes. And in practical applications, joints can extend the assembled HTS conductors to required lengths. Therefore, encapsulated joints for Soldered-stacked (SS) wire has been proposed as a preliminary study to achieve long length Soldered-Stacked-Square (3S) wire. And in this paper, we numerically study the performances of different types of encapsulated joints. Firstly, encapsulated joints are divided into two categories according to their positions, and a nonlinear circuit model is presented to demonstrate the numerical study process and experiments are conducted to determine the parameters in this model. Then, resistance performances of the encapsulated joints are discussed such as the “multilinear resistance” phenomenon where the encapsulated joints show different resistance under different current. And HTS tape arrangements in the encapsulated joint are also discussed to achieve minimum resistances.

The high-temperature superconducting (HTS) null-flux electrodynamic suspension (EDS) system is one of the most promising solutions for high-speed transportation. In this article, a 3-D model based on the finite-element method (FEM) is established to solve the electromagnetic forces in the HTS null-flux EDS system with the commercial software COMSOL Multiphysics. The current induced in the null-flux coils and the forces on the HTS magnets were studied and compared with the results from our dynamic circuit model. The two models agree with each other with a difference of less than 6%. The FEM model can deal with 3-D geometries, allowing the analysis of complex situations, such as the eddy current in the cryostat walls. Furthermore, the model can be readily extended to solve coupled problems, including the heat and mechanical behaviors of the HTS magnets and/or the null-flux coils, which are engineering concerns in real applications.

Titanium andits alloyscan be heated using heavy oils, gases, resistance furnaces, liquid media, and conventional alternating current (AC) induction heaters. However, the novel superconducting direct current (DC) induction heater has several advantages over its AC counterpart in terms of heating time, efficiency, and heating quality. Therefore, it has gained remarkable success in the aluminum extrusion industry. The superconducting DC induction heaters can achieve efficiency by up to 90%, which is significantly higher than the AC induction heater (40–50%). In this study, experiments on the heating of Ti-6AL-4V (TC4) billets (Φ = 220 mm × L = 1,250 mm) using a high-temperature superconductor (HTS) induction heater were performed. A numerical 3D finite element method (FEM) model was developed to analyze the heating behavior of TC4 billets. The results from the simulation and experiments show a good agreement. Further, the radial temperature profile, heating power, and heating time were studied using the developed model. The results show that the penetration of the heating power decreases with the rotation speed. Hence a low radial temperature difference was achieved by applying an exceptionally low rotational speed on the DC induction heater, 100–500 rpm (corresponding to 1.7–8.3 Hz). We also found that heating power increased rapidly with the diameter of the TC4 billet. Therefore, the heating time could be substantially minimized by increasing the magnetic flux density and rotational speed, especially for a large billet with a diameter exceeding 300 mm. The results obtained in this study will be useful for industrial applications of Ti alloy pre-heating.

Pulsed lasers were used to drill small holes distributed homogeneously on buffer layers of coated conductors, aiming at improving their mechanical properties. Each hole should be engraved by only one pulse of laser for practicality. The effect of pulsed laser engraving on buffer layers, CeO2 in this study, were investigated. The size and depth of the hole can be controlled by adjusting laser parameters. ${\rm{REB}}{{\rm{a}}_{2}}{\rm{C}}{{\rm{u}}_{3}}{{\rm{O}}_{\rm{y}}}$ (REBCO) layers were then grown on the laser patterned buffer. The growth mechanism of REBCO above the drilled holes was studied. In addition, influence of drilled holes on the critical currents of coated conductors were studied. Finally, the effect on mechanical strength of as grown samples were studied by measuring their delamination strength under transverse tension.

With excellent mechanical properties and electromagnetic properties, compact superconducting cables are considered as potential candidates for large capacity power applications and fusion magnets. In this paper, the concept of a novel compact cable, conductors in tube (CIT) cable is introduced from the aspect of design, fabrication, and performance evaluation, respectively. The principal advantages of CIT cables are that the cable can release the residual stress created from the fabrication process, and the outer former of the cable can be used as both the supporter as well as the container of refrigerant liquid. Firstly, the topological structure and the detailed fabrication process of a CIT cable are presented. Then, a typical CIT cable is manufactured. The feasibility of this new structure is verified by its critical current test. Moreover, bending characteristics of CIT cables are evaluated and compared with Conductors on Round Core (CORC) cables from the aspect of both simulation and experiments. The superior bending performance enables it to be potential in the future compact magnet application.

Effective quench detection technologies are extremely important for protecting high temperature superconducting (HTS) applications. In recent years, an optical fiber encapsulated HTS tape (OF-tape) is proposed for quench detection by using distributed temperature sensing (DTS) technology. In this paper, an HTS coil (OF-coil) wound with a 39 m long OF-tape was prepared and energized by an AC overcurrent of 700 A to verify the feasibility of temperature observation during quench events. Furthermore, anti-stokes intensity was also measured to understand dependence of temporal resolutions on response times of coil quenches. The main results show that temperature variations of the OF-coil can be correctly measured after quench and the maximum temperature is shown at the middle turn of the OF-coil. Particularly, for the method of detecting anti-stokes intensity, quench response time of temporal resolution (TR.) with 30 ms is about 500 ms faster than that with 300 ms. However, when the TR. is 30 ms, more noise signal is observed. Accordingly, a trade-off should be considered between response time and noise signal for efficient quench detection.

A novel high-temperature superconducting (HTS) direct current induction heater exhibits great advantages such as high efficiency and superior heating quality for non-magnetic metal billet heating process in industrial applications. An HTS induction heater of 1 MW is used in the Al press industry. The HTS induction heater injects high resistive heating power through the driving motor. The higher the resistive heating power, the shorter the heating time. The heating power is a significant parameter used to analyze the heating time and improve the production efficiency. In this study, three types of non-magnetic metal billets were analyzed, including Al, TC4 (Ti-6AL-4V), and Cu, which are widely used in the heating process in industrial applications. The experimental results of the heating of TC4 and Al billets (Φ = 220 mm × L = 1,250 mm) using an HTS induction heater were obtained. The changing trend of the heating power during heating was different. The heating power of TC4 was different from those of Al and Cu. The heating power of TC4 was much lower than those of Al and Cu. A numerical 3-D finite element method model was built to analyze the heating power. The results of the simulation and experiments were consistent with each other. The electrical conductivity of the billet is a significant material property affecting the heating power. Finally, the influence of the billet diameter on heating behavior was analyzed for the three different metal billets. Understanding the effect of electrical conductivity of important industrial metals such as Al, Ti, and Cu alloys on heating power is useful for improving metal processing technologies.

5G and beyond (B5G) networks are leading a digital revolution in telecommunication in both academia and industry. It brings new paradigms in many aspects of people's daily lives due to its advantages. However, it still leaves some issues in terms of security and privacy as challenges. Blockchain, the public database, is an alternative to the traditional centralized systems, serving as the backbone technique in many systems, including manufacturing, economics, and industry. Blockchain is promising in solving the security issues in the sense that it provides desirable properties including decentralization, transparency, immutability, and so on. In this article, we investigate typical security and privacy issues in edge intelligence in B5G networks and devise a framework to integrate blockchain with such systems, which can provide guaranteed security as well as privacy. We also illustrate several possible solutions to these security and privacy issues in edge intelligence in B5G systems based on blockchain and Ethereum to show how blockchain contributes to the coming B5G networks. This work is supported by the National Natural Science Foundation of China under Grant 61872229 and Grant U19B2021, the Key Research and Development Program of Shaanxi under Program 2020ZDLGY09-06, and the Block-chain Core Technology Strategic Research Program of the Ministry of Education of China under Grant 2020KJ01030, and Hongkong Macao Joint Research Grant 2019WGALH21.. Yong Yu is the corresponding author.

Magnetic field monitoring devices are essential for studying and operating high-temperature superconducting (HTS) magnets. Apart from conventional Hall sensor, passive optical fiber sensor is also an option for magnetic field monitoring. In this article, we proposed a novel access based on yttrium iron garnet magneto-optic (YM) sensor, which is one kind of optical fiber sensors, to measure magnetic fields generated by an HTS double pancake coil. Moreover, we used a magnetic field bypass tube made of silicon steel material to expand the measurement range of YM sensor. The test results of YM sensor in liquid nitrogen demonstrated that the YM sensor was benchmarked against Hall probe up to 24.7 mT at 77 K. Using a silicon steel tube to shield magnetic field could extend the field measurement range to 37.6 mT. YM sensor could be one of the promising methods for monitoring HTS magnet magnetic field.

Energy conservation and emission reduction is a critical task for China's aluminum industry. During the last decade, the high temperature superconducting (HTS) technology has been successfully demonstrated to have great potential for energy conservation in this field. Different from the present AC induction heater, this HTS direct current (DC) induction heater is expected to significantly improve the energy efficiency of preheating aluminium billets before extrusion process. This paper thoroughly presents the development of the world's first commercial megawatt HTS DC inducting heater based on 2nd generation HTS tapes. Firstly, the commercial design of this 1MW HTS DC induction heater including design specifications, energy efficiency and commercial potential are analyzed carefully. Subsequently, the manufacture and operation of three core parts including HTS magnet with extra-large diameter, iron core structure and electrical driving system, are introduced. A low cost and practical driving system consisting of motors and gearboxes is developed to solve the problem of peak electromagnetic torque at low rotation. Meanwhile, the heating quality of aluminium billets (i.e., temperature distribution along the radial and axial direction) can be accurately controlled by an adjustable air gap structure. Finally, the present technique challenges and corresponding solutions of this commercial HTS DC induction heaters are summarized and proposed. Compared with an energy efficiency of 45-60% in the MW-class conventional heater, the energy efficiency of this HTS DC induction heater can achieve up to 80.6% based on test results. This work can also provide guidance for the design and optimization of HTS DC induction heaters.

The novel superconducting direct-current (DC) induction heater shows great advantages of high efficiency and heating quality compared to traditional alternating-current (AC) induction heater in aluminum extrusion. The efficiency of the superconducting DC induction heater can be up to 80–90%, which is much higher than the 40–50% efficiency of the AC induction heater. The uniform and gradient temperature distributions of the aluminum billet are the most significant parameters to evaluate the heating quality of this induction heater, which has a strong influence on the product quality of extrusion processing. In this paper, the superconducting magnet with adjustable air gap structure is designed to achieve controllable axial temperature uniformity and axial temperature gradient distribution. Each adjustable air-gap structure consists of 10 identical iron pieces whose position can be changed independently. Therefore, the uniformity and gradient temperature of the aluminum billet in the axial direction are controllable. The thermal optimization of the uniform and gradient temperature distributions for typical aluminum billets (Φ = 446 mm) is measured. A numerical finite-element method (FEM) model is built to analyze the heating behavior of the aluminum billet. The simulation and experimental results match well. The result of uniform and gradient axial temperatures are ±3 °C and the controllable range from 10 °C/m to 40 °C/m, respectively. The result obtained from the temperature-distribution optimization is useful for industrial applications.

Conductor on Round Core (CORC) cables with scalability, flexibility, strong mechanical strength and high current density are of large potential for different power applications. In this paper, CORC Cables are proposed to be the secondary winding for high capacity superconducting transformer. In order to evaluate the working performance of CORC cable in HTS (high temperature superconductor) transformer, firstly, fundamental parameters of short CORC samples including self-field critical current and room temperature resistance were measured by experiments. Then, to represent the real working condition, both rated current tests and overcurrent tests were carried out to the CORC cable, respectively. In rated current test, magnetization and transport AC loss were discussed by both experimental and numerical method. It is found that AC loss of the CORC cable with double layers is much smaller than that of the CORC cable with single layer. In overcurrent tests, when the maximum value of current is 3588 A, 3.44 times of I c , resistance of CORC is 0.12 mΩ. The recovery time is used to evaluate the recovery-under-load capability, the peak-to-peak value of voltage becomes stable after 6 cycles. Conclusions obtained in this paper can verify the feasibility of this technique and also provide useful information for future design of HTS transformers.

In high temperature superconducting (HTS) applications, effective and fast quench detection techniques are quite essential. Recently, an attractive combination of distributed optical fiber sensor and optical fiber encapsulated HTS tape (OF-tape) is suggested for quench detection. Before OF-tapes are applied in actual applications, their performance should be tested previously. In this paper, three types of OF-tapes, whose optical fiber diameters are 95, 170, and 250 μm, respectively, were prepared for our superconducting fault current limiter (SFCL) project, and these tapes are named tape-95, tape-170, and tape-250 in sequence. The SFCL requirements show that for the used HTS tapes, the critical current, the room temperature resistance, and the bearable AC impulse current must be within 500 ± 10% A, 60 mΩ/m, and 1500 A/100 ms, respectively. The test results indicate that the critical currents of tape-95, tape-170, and tape-250 are approximately 500 A, and the room temperature resistances of these tapes are 70, 65, and 57 mΩ/m, respectively. Moreover, the AC 1500 A/100 ms short circuit current will not damage these tapes seriously. According to the SFCL project requirements and these test results, tape-170 will be used in limiter coils.

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.

The efficiency of novel superconducting direct current (DC) induction heaters can be up to 80-90%. This is much higher than the 40-50% of conventional alternating current induction heaters. Induction heaters are used to preheat aluminum billets in extrusion pressing. Such devices use a driving motor to rotate the billet. A peak in the electromagnetic torque appears at low rotation speeds (of the motor). Therefore, this peak electromagnetic torque is of particular interest when designing the driving system, which is one of the main challenges for megawatt-class superconducting DC induction heaters. In this paper, the peak electromagnetic torque characteristics of different sized billets were analyzed. A numerical 3D finite element method model was built to analyze the peak electromagnetic torque and resistive heating power behavior of aluminum billets. The results obtained from simulations and experiments match well. The effect of the following five relative parameters, on electromagnetic torque and resistive heating power, were studied: billet diameter, billet length, magnetic flux density, billet material, and electrical conductivity. The peak electromagnetic torque increases nonlinearly with increasing billet diameter, up to 640 mm. The time elapsed between the peak to stable torque decreases nonlinearly with increasing magnetic flux density. The effects of billet material (copper, titanium alloy and stainless steel) on torque were analyzed and compared. These results are likely to guide more cost-effective and practical designs for the billet driving system of superconducting DC induction heaters.

REBa 2 Ca 3 O 7-x (REBCO, RE = rare earth) coated conductors are promising conductors for high magnetic field magnet applications. For REBCO magnets, the tape structures and their superconductive characteristic cause a persistent screening current flowing in the tapes. Due to the existence of the screening current induced field (SCIF), the overall magnetic field becomes unstable and shows a temporal drift. This problem limits the application of REBCO in high-precision magnets, such as MRI and NMR. In this paper, the soldered-stacked-square (3S) REBCO wire is proposed to reduce SCIF. To confirm the reduction effect, a solenoid coil and a benchmark coil were wound by the 3S wire and the commercial 5-mm-wide REBCO tape, respectively. The screening current distributions of 3S wire and 5-mm tape were analyzed. Then, the temporal drift of the overall magnetic field and SCIF hysteresis loop were both experimentally measured for comparison. The test results showed that the 3S wire can significantly reduce the negative effects of SCIF.

Quench detection and protection technologies are extremely important for high-temperature superconducting (HTS) magnet applications. In a previous work, we have verified the feasibility of using a Raman-based distributed temperature sensor (RmDTS) system to measure a no insulation (NI) HTS coil temperature changes in an overcurrent induced quench event. However, rough and slow temperature measurement is unsuitable for actual application on quench detection and protection. Therefore, we designed a new RmDTS system consisting of an RmDTS device whose temporal resolution is 15 ms and a thinner polyimide-coated optical fiber with a diameter of 150 μ m. Then, the optimized RmDTS system and an original one were, respectively, applied to detect the NI coil temperature during quenches. The comparative results show that the highest heat was generated on the 22nd turn of the NI coil, and the temperature response time of the original DTS system is 2 s slower than that of the optimized one. By using an optimization method, the spatial and temporal resolutions of the RmDTS system have been enhanced triple and 66 times than before.

No-insulation (NI) coils made by second-generation (2G) high-temperature superconducting (HTS) tapes have a better thermal stability than traditional coils which wound with insulation. The NI coils have been developed and been employed in many superconducting applications such as superconducting motor, transform, and generator. In these cases, the NI coils will inevitably be affected by ac magnetic field. When a superconductor carrying a dc current is exposed to an ac field, a dynamic resistance R dyn will occur and cause significant ac loss. In this paper, R dyn is calculated by field-decay method, which has a wider measurement range and higher accuracy than four-probe method. The correction curve for R dyn with the ratio of operating current and critical current (I op /I c ) is presented. Combining the persistent-current mode (PCM) circuit model of closed NI coil and the correction curve, a predictable simulation model for central magnetic field characteristic of closed NI coil with partial tape exposed to an ac magnetic field is established and verified by experiments.

Screening current is a persistent current that flows in the wide area of second-generation high-temperature superconducting (2G HTS) coated conductor, which is induced when a 2G HTS magnet is energized. The screening current induced field (SCIF) will cause the reduction and the temporal drift of the center magnetic field in the HTS magnets. This paper reports a novel structure HTS tape with 1 mm width for high precise HTS magnet design. The principle of this tape structure, which is so-called 2s + 1c tape, is to stack two narrow HTS tapes and one copper tape through the soldering furnace. In this paper, we did the experiments of the SCIF hysteresis loop for the coil wound with the 2s + 1c tape and the benchmark coil wound with the commercial 5 mm wide HTS tape. Then, a numerical model for SCIF hysteresis loop was verified by experimental results. Based on the validated model, we analyzed the behavior of SCF to other operating conditions, such as temperature control and current sweep reversal. Finally, the 2s + 1c tape was confirmed as a new approach to reducing the SCIF significantly.

Low-resistance joint technology is crucial for high-temperature superconducting (HTS) closed-loop coils that work in persistent current mode. The non-uniform current distribution (NUCD) phenomenon in HTS joint has a significant influence on joint resistance, which may lead to a nonlinear relationship between joint resistance Rj and overlap length l (Rj-l curve) instead of following the laws of resistance. In this paper, an arched structure joint made by YBCO tapes has been designed to provide a direct evidence for the NUCD phenomenon inside solder joint. It is found that the current transports mainly at the end of the overlap area and is much different from the regular conductors. The V/I-I method is proposed to measure Rj of solder joints (less than 2 nΩ) with various overlap lengths in order to obtain more precise data. A complete Rj-l curve (l = 5-100 cm) has been successfully illustrated and a nonlinear saturation relationship between Rj and l is presented. The joint resistance tends to approach to a lower limit value about 1.5-1.6 nΩ when the overlap length exceeds 20 cm.

A Raman-based distributed temperature sensor (RmDTS) system is a promising method for quench detection in high-temperature superconducting (HTS) magnets. To sense the temperature distribution of HTS magnets, optical fibers need to be wound with HTS wires synchronously. Accordingly, the bend limitation of the sensing optical fibers at 77 K is very important for an HTS coil temperature measurement. In this study, a segment with a 2.5-m-long polyimide-coated multimode optical fiber was bent as the curvature radii of 3, 5, 10, 15 and 25 mm, respectively, and applied to sense a room and a liquid nitrogen temperature. The experimental results of bending and without bending optical fiber show that when the curvature radius of the optical fiber is smaller than 10 mm, the temperature measurement is unreliable. The recommended minimal curvature radius of the used optical fiber for applying the RmDTS system to monitor the HTS coil temperature is 10 mm.

High-temperature superconducting (HTS) soldered-stacked-square (3S) wire has been proposed as a novel potential structure for the second generation HTS wire. Twisted method is an available way to decrease the anisotropy and improve the performance of 3S wire in high field application. This paper presents a three-dimensional numerical model based on H-formulation for the 3S-twisted (3S-T) wire. AC loss is evaluated as an important parameter when 3S-T wire is applied in alternating current or magnetic field circumstances. Three different structures of 3S-T wires are considered, and the validity of the general model is verified by the corresponding experimental results. The results show that there is no significant eddy loss existing in the 3S-T wire. Moreover, if more HTS tapes are contained in 3S-T wires, the current distribution will be closer to elliptical shape and the current phase in different YBCO layers will be changed. The transport AC loss can be slightly reduced through the twisted method. The work in this paper will be helpful to understand the electromagnetic characteristics of 3S-T wire.

Closed coils made by second-generation high-temperature superconducting (HTS) coated conductors are promising for superconducting magnets, which operating in a persistent current mode such as MRI, NRM, and Maglev. The magnetic field stability for the HTS closed coils after local thermal quench is one of the key indicators to the design of superconducting magnet systems. It significantly affects the operational reliability of superconducting magnets. This paper presents quantitative experiments on the magnetic field stability of a jointless stacked pancake (JSP) coil after local thermal quench. The JSP coil is assembled by stacking double-slit YBCO tapes, which is a true sense of HTS closed coil without any joints. A JSP coil with inner diameter of 30 mm is impregnated by paraffin wax and its central magnetic field stability is measured after various local thermal quench energies. The relationship between quench energy and magnetic field stability time is presented. The magnetic field of the JSP coil will stay stable when quench energy is lower than a threshold value Q m. The magnetic field stability of JSP coils comes from its parallel-structure. Besides, the magnetic field stability of the JSP coil is better than the series-structure closed coil obviously under the same quench energy.

Mechanical stability of no-insulation (NI) high-temperature superconducting (HTS) coils will be greatly enhanced after using a solder impregnation (SIM) method. For superconducting high-field magnet applications, besides the mechanical stability, the thermal stability of SIM coils is also a crucial parameter which needs to be investigated. Due to its high insulation, convenient installation, distributed temperature measurement, and insensitive to stress, a Raman-based optical distributed temperature sensor (RmDTS) system is an ideal option for monitoring the temperature of SIM coils. In this study, an NI HTS coil and an optical fiber were successfully impregnated together by solder. The critical current of SIM coil determined by the central magnetic field is 69 A. To create quench process, a dc current of 240 A was injected into an SIM coil, and the temperature of the coil was monitored by the RmDTS simultaneously. The test results show that the temperature of the SIM coil was unchanged although a four times Ic was applied.

Striated YBCO-coated conductor is a variant structure of the second-generation high-temperature superconducting (HTS) tapes. The ac loss is reduced owing to the YBCO layer of the striated YBCO tape is divided into filaments by laser scribing. Hence, it is promising for practical HTS electrical application, such as HTS transformer and cable. However, when the superconducting apparatuses are operated in the power grid, it will possibly experience the lightning impulse current in some extreme working condition. Thus, the performance of striated YBCO tapes suffering from exposure to lightning current is important to be studied. In this paper, a numerical model based on the electromagnetic–thermal coupling is built to analyze the lightning current performance of the striated YBCO tapes. The influence of structural changes on the thermal stability is discussed. The standard lightning current waveform is chosen, i.e., 8/20 μs. The simulated results show that cutting the copper edges of striated YBCO tapes will significantly degrade the lightning current withstand capacity. Instead, the degradation effect of grooves is not so obvious.

At Shanghai Jiao Tong University (SJTU), REBa2Cu3O7‐σ (REBCO) coated conductor (CC) research program started in 2007. With the support of China's First National Key Project on CC Program, SJTU successfully developed hundred meters long class CC tapes with over 300 A/cm (at 77 K, self-field) based on PLD (pulsed laser deposition)/RABiTS processes in 2011. To develop high-performance CC tapes, in 2012, we switched from RABiTS to ion beam-assisted deposition (IBAD)-MgO tape and a pilot PLD/IBAD process CC fabrication line was set up at Shanghai Superconductor Technology Corporation in 2013, and deposition processes were successfully transferred from lab to fabrication line. Under optimized conditions, the IBAD-MgO layers showed pure (001) orientation and excellent in-plane texture. Atomic force microscopy observation showed MgO layer had very smooth surface. The rms is less than 1 nm. On the textured MgO layer, single CeO2 layer showed pure (001) orientation and excellent in-plane texture of ${\text{4}}^{\circ}$ – ${\text{5}}^\circ$ . Reel-to-reel PLD process was already scaled up to >100 m/h tape speed. Kilometer (Km) long CC tapes with over 300 A/cm have been routinely fabricated. Recently, our research work focused on introducing high-density nanoscale artificial flux pinning centers into REBCO superconducting layer to improve in-field superconducting properties and further expand application areas of REBCO tapes.

The high-temperature superconductor (HTS) dc induction heater shows great potential in the efficiency increase and heating quality improvement, which has been validated by previous prototypes. Now, Shanghai Jiao Tong University has been developing and fabricating an industrial scale 1-MW HTS dc induction heater in China. The heater is utilized to preheat aluminum billets 446 mm in diameter × 800–1500 mm in length. Two cryocoolers are applied for coil cooling with pluggable sleeve and providing an additional safety margin whose normal operating temperature is 30 K. The magnet consists of a double pancake coils wound by REBa2Cu3O7-δ -coated conductors, which are produced by Shanghai Superconductor Technology Company. More than 12-km HTS tapes are used and the inductance of coil with iron is 98 H. The HTS magnet's critical current is 213 A. The 0.5-T dc magnetic field is set up in the two air gaps of HTS magnet with an iron core. The adjustable iron yoke distribution along the axial direction of billets can meet the 100 °C adjustable temperature gradient design requirement. The results will be helpful to design for the commercialization of the HTS dc induction heater.

In the high-temperature superconducting (HTS) magnets applications, fast quench detection is very important to protect the magnets from burning out. Usually, the method of voltage measurement is widely applied to detect quench. However, this method is not satisfied for large HTS magnets intrinsically. Besides, a novel quench detection, which is measured by Raman-scattering-based distributed temperature sensors (DTS) using flexible optical fibers, is suggested to be a potential candidate. To verify the feasibility of this method, we prepared a DTS system based on interrogating Raman-scattering signals and a no-insulation (NI) HTS coil mounted by a 2-m length optical fiber as a test sample. The main results show that critical current of the NI coil is 111 A. In the continued overcurrent-induced quench state under coil current of 160 A, the temperature rise was observed obviously at the position of 1.2 m in the optical fiber, and the temperature was up to 94 K in the near middle region of top surface in the NI coil.

The superconducting linear synchronous motor is one of the main applications of the high-temperature superconducting (HTS) magnet made out of coated conductors. The high operation current density and temperature of HTS magnet can lower the requirement of the cryocooler system. Moreover, if the superconducting rotor magnet is designed to work in persistent-current mode, the heat load of the magnet system can be further reduced because there is no joule heat and heat leakage on the current leads. However, in real working conditions, the rotor magnet is exposed to an alternating magnetic field (AMF) caused by the vibration and harmonic magnetic field from the stator magnet. This AMF may accelerate the decay process of the magnetic field in rotor magnet. In this paper, the magnetic field decay property of the HTS coil in AMF is experimentally discussed. In order to eliminate the impact caused by the joint resistance, a kind of joint-less stack coil that can operate in real persistent-current mode is fabricated and tested in an AMF generated by an electromagnet. The tests were conducted with different AMF amplitudes and frequencies, as well as different initial magnetic field in HTS coil.

No-insulation (NI) coil made of 2G high-temperature superconductor tape is widely used in a dc magnet due to its excellent performance of engineering current density, thermal stability, and mechanical strength. While the charging/discharging delay of the NI coil has become the biggest obstacle to applying it into an ac power device. In this paper, as a preliminary study, we test the capability of the NI coil to shield alternating magnetic field (AMF). When shielding AMF, the NI coil works in inductive mode such that each turn in the NI coil is induced at the same time and there might no longer exist any delay caused by turn-to-turn current path. To verify this hypothesis, three closed-loop coils, two NI coils, and an insulated (INS) coil are fabricated. The result of the contrast experiments shows that the NI coil could shield all the alternating magnetic flux through it, independent of turn-to-turn resistance and frequency of the background AMF. This capability is promising for the NI coil to be applied in the inductive superconducting fault current limiter (iSFCL) as the secondary winding, which can shield all the magnetic flux generated by the primary winding to keep the impedance of the iSFCL at a very low level during normal the operation of the power system.

This paper focuses on the comparison of the characteristics among three different types of high temperature superconducting (HTS) transformers. The iron core structures of the three types of HTS transformers are air core, partial core, and full core. The iron cores are made of oriented silicon steel sheets. The primary windings are four layers connected in series, and the secondary windings are nine double pancake coils connected in parallel. Fundamental tests, such as critical test, no-load test and load test, are performed in the environment of liquid nitrogen. The current nonuniformity of HTS partial core and full core transformers in secondary windings are also confirmed by the load test. Finally, we also investigate the characteristic of fault current limiting by sudden short-circuit tests. The experiment results show that the structure of the iron core have significant effects on the characteristics of HTS transformers. The results of this paper could support with useful data for the design and selection of HTS transformers.

A 1-MW industrial-scale high-temperature superconductor (HTS) dc induction heater is being designed and manufactured in Shanghai Jiao Tong University, China. This paper is to present the design and engineering analysis of a HTS magnet with iron yoke for this heater. This magnet is to generate a 0.5-T dc magnetic field in the iron's air gap, where the billet is rotated. To enhance the thermal stability, the no-insulation (NI) coil and metallic insulation (MI) coil (co-wind with stainless steel strips) is introduced to the magnet. The influence of the NI&MI technique on the magnet's ramping process is analyzed. A significant ramping delay occurs on both the NI and MI magnet. Joules losses are generated by the current “bypassing” through turn-to-turn contacts, which may lead to significant temperature rise on the coils. Higher ramping rate often means faster ramping process and higher temperature rise induced by ramping loss. The MI magnet shows a much shorter ramping time and lower ramping loss than its NI counterpart, due to higher turn-to-turn resistivity. The MI coil is more suitable than the NI coil for this large-scale magnet.

This paper focuses on conceptual design and electromagnetic analysis of a single-phase high-temperature superconducting (HTS) power transformer with a capacity of 300 kVA by using GdBCO tapes, which is one-third model of a 1 MVA/10 kV/0.4 kV HTS transformer. The conceptual design consists four parts: iron core, HTS windings, current lead, and cryostat. In the electromagnetic optimization of the 330 kVA HTS transformer windings via finite element method, the influence of winding configurations on the radial magnetic field and the current nonuniformity are considered. The details about the design and optimization of radial magnetic field will be discussed in this study.

This study numerically and experimentally analyzed the quench propagation characteristics of nonuniform REBa2 Cu3O7-δ (REBCO, RE = rare earth) coated conductors (CCs) at the self-field and under a nearly adiabatic conditions. Four nonuniform REBCO CCs cases were studied: a copper-laminated sample with a point defect, a bare sample with a point defect, a copper-laminated sample with a regional defect, and a bare sample with a regional defect. The finite element method was used with the numerical computational models for these four nonuniform cases. The temperature and voltage profiles obtained from the numerical models were compared with experimental data. The quench parameters, including the peak temperatures in the defect region and normal zone propagation velocity, were obtained at various operation currents (from 36% I c to 112% I c). Of the four cases, the bare sample with the regional defect had the highest peak temperature at the same time and percentage of critical current ( I c). The results will be useful for the quench detection and protection design of REBCO CCs for use in applications.

This paper presents a study on the coil voltage of the no-insulation (NI) high-temperature superconductor (HTS) pancake coils during fast discharging operations. Fast discharging tests are performed on both a double pancake (DP) and an single pancake (SP) NI coil. The measured coil voltages are compared with that from an equivalent circuit model, which has been widely used in previous studies. A voltage pulse is observed on both the NI coils at the beginning of the fast-discharging operation. It is much higher than that from the modeling. The peak value of the voltage pulse increases linearly with the initial current. Comparison tests show that the coupling iron core has little influence on this coil voltage pulse though the inductance of the coils is enlarged seven times by coupling iron core. This coil voltage pulse shows that, during the fast discharging operation, the NI coil has a much higher turn-to-turn resistivity at the beginning stage than that in following process. Since the coil voltage pulse can be 600 times of the modeling estimation, it may be a potential risk to the coil protection system and measuring instruments. Therefore, special attentions are required on this aspect in the design of NI magnets.

High-temperature superconducting (HTS) coils made of coated conductors that can operate in persistent current mode (PCM) are regarded to be promising in MRI/nuclear magnetic resonance and Maglev system. The temporal stability of the magnetic field trapped by the PCM coil is a key issue, which significantly determines the imaging quality of MRI and dynamic behavior of Maglev. This study focuses on the temporal stability of the trapped magnetic field in the double-slit HTS PCM coil, which is magnetized by a field cooling (FC) method at 77 K. The magnetic field decay behavior under different initial fields and FC ramp rates are systematically studied. The experiment results indicate that higher initially trapped field will lead to faster decay, but if the initially trapped field is lower than a certain value, this trend is no longer obvious. The FC ramp rate has a little impact on the field decay. Finally, a numerical model based on R-L circuit and E-J equation is established to fit the decay process. It is found that if the operating current is lower than 60% of the coil critical current, good temporal stability could be achieved. Another long-term experiment is performed that a stability of 8.1 ppm/h is achieved after 19 days' decay.

Based on the pre-existing assembled, stacked, or twisted structures of high temperature superconducting (HTS) wires, a novel soldered-stacked-square (3S) HTS wire is suggested and developed through narrowing, stacking, and soldering processes in this study. For the fabrication processes of the 3S wire, a narrow 2G tape with 1 mm width is applied, and the stacking and soldering processes are completed simultaneously. To confirm the performance of the 3S HTS wire, several 3S samples are fabricated. For the 3S HTS samples, we evaluate typical electrical and mechanical properties, and investigated for the feasibility of the 3S HTS wire in the HTS applications. The test results show that measured critical currents of two 3S wire samples are 170 and 255 A, and their curves are typical ones of HTS tapes. The measured self-field ac loss is also reasonable, which is analyzed from both theoretical models of the elliptical and thin strip equations. Moreover, the minimum bending diameter for the 3S samples is approximately 60 mm, and the allowable axial tensile force is 350 N. These mechanical properties of 3S wire are satisfied for the HTS applications significantly. Finally, the smallest round structure of the HTS wire is conceptually suggested for the 3S HTS wire.

This paper presents the characteristics comparison between an HTS air-core transformer (ACT) and an HTS partial-core transformer (PCT) by experiment and simulation. Two types of transformers were fabricated and tested in liquid nitrogen at 77 K. Fundamental characteristics were obtained by no-load, load, and short-circuit tests. The parameters of the equivalent circuits of the two transformers were also obtained. A field-circuit finite-element method was proposed to analyze the electrical characteristics. In addition, the experimental results were compared with the numerically calculated ones. The results show that the measured critical currents of the transformer with and without the iro n core are nearly the same. The measured exciting current and percent voltage regulation of the PCT is smaller than that of the ACT. The PCT has a better short-circuit current limiting function. The numerical results show that the uneven degree of circulating current of ACT is larger than that of the PCT. The loss characteristic was also discussed in this paper.