A “permanent” high-temperature superconducting magnet operated in thermal communication with a mass of solid nitrogen

A new design for a portable “permanent” superconducting magnet system is explored. The design involves a persistent-mode high-temperature superconducting (HTS) magnet that is cooled by a solid heat capacitor. The system is an alternative to permanent low-temperature superconducting (LTS) magnet systems where the magnet is cooled by a bath of liquid helium.An apparatus was constructed to demonstrate stable operation of a permanent magnet wound with Bi2223/Ag conductor while in thermal communication with a mass of solid nitrogen. The apparatus includes a room-temperature bore and can function while it stands alone, detached from its cooling source, power supply, and vacuum pump. The magnet is operated in the 20–40 K temperature range. This apparatus is the first to demonstrate the operation of a superconducting magnet with a permissible temperature variation exceeding a few degrees kelvin. Models are developed to predict the experimental system's warming trend and magnetic field decay. The models are validated with a good agreement between simulations based on these models and experimental results. The results indicate that present HTS conductor critical current and index are not yet sufficient to provide field strengths and field decay time constants that are required for typical persistent-mode applications.     

35 kJ高温超导磁储能(SMES)的热输运实验研究

研制了中国首台高温超导磁储能直接冷却系统,该系统不使用低温液体(液氦、液氮).在10-3Pa的真空度下,高温超导磁体线圈由1台单级GM制冷机从室温293 K冷却到19 K,Bi2223电流引线由另一台制冷机冷却到77 K以下.整个系统在通140 A直流电流的时候产生了4.5 T的磁场.系统连续运行480 h(20 d),磁体和低温系统各参数动态特性良好.实验研究表明,控制系统的漏热,优化磁体内部导冷结构,有效减少热传导部件的接触界面热阻是制冷机直接冷却高温超导磁体的关键技术.     

Superconducting bulk magnet for maglev vehicle: Stable levitation performance above permanent magnet guideway

High-temperature superconducting (HTS) maglev vehicle is well known as one of the most potential applications of bulk high-temperature superconductors (HTSCs) in transported levitation system. Many efforts have promoted the practice of the HTS maglev vehicle in people's life by enhancing the load capability and stability. Besides improving the material performance of bulk HTSC and optimizing permanent magnet guideway (PMG), magnetization method of bulk HTSC is also very effective for more stable levitation. Up to now, applied onboard bulk HTSCs are directly magnetized by field cooling above the PMG for the present HTS maglev test vehicles or prototypes in China, Germany, Russia, Brazil, and Japan. By the direct-field-cooling-magnetization (DFCM) over PMG, maglev performances of the bulk HTSCs are mainly depended on the PMG's magnetic field. However, introducing HTS bulk magnet into the HTS maglev system breaks this dependence, which is magnetized by other non-PMG magnetic field. The feasibility of this HTS bulk magnet for maglev vehicle is investigated in the paper. The HTS bulk magnet is field-cooling magnetized by a Field Control Electromagnets Workbench (FCEW), which produces a constant magnetic field up to 1 T. The levitation and guidance forces of the HTS bulk magnet over PMG with different trapped flux at 15 mm working height (WH) were measured and compared with that by DFCM in the same applied PMG magnetic field at optimal field-cooling height (FCH) 30 mm, WH 15 mm. It is found that HTS bulk magnet can also realize a stable levitation above PMG. The trapped flux of HTS bulk magnet is easily controllable by the charging current of FCEW, which implies the maglev performances of HTS bulk magnet above PMG will be adjustable according to the practical requirement. The more trapped flux HTS bulk magnet will lead to bigger guidance force and smaller repulsion levitation force above PMG. In the case of saturated trapped flux for experimental HTS bulk magnet, it is not effective to improve its maglev performances by increasing of charging magnetic field, when the guidance force at WH 15 mm is 5.7 times larger than that by DFCM of FCH 30 mm. So introducing HTS bulk magnet into the present maglev system is feasible and more controllable to realize stable levitation above applied PMG, which is an important alternative for the present HTS maglev vehicle.     

A magnetic levitation rotating plate model based on high-Tc superconducting technology

With the wide requirements of the training aids and display models of science, technology and even industrial products for the public like schools, museums and pleasure grounds, a simple-structure and long-term stable-levitation technology is needed for these exhibitions. Opportunely, high temperature superconducting (HTS) technology using bulk superconductors indeed has prominent advantages on magnetic levitation and suspension for its self-stable characteristic in an applied magnetic field without any external power or control. This paper explores the feasibility of designing a rotatable magnetic levitation (maglev) plate model with HTS bulks placed beneath a permanent magnet (PM) plate. The model is featured with HTS bulks together with their essential cryogenic equipment above and PMs below, therefore it eliminates the unclear visual effects by spray due to the low temperature coolant such as liquid nitrogen (LN₂) and additional levitation weight of the cryogenic equipment. Besides that, a matched LN₂ automation filling system is adopted to help achieving a long-term working state of the rotatable maglev plate. The key low-temperature working condition for HTS bulks is maintained by repeatedly opening a solenoid valve and automatically filling LN₂ under the monitoring of a temperature sensor inside the cryostat. With the support of the cryogenic devices, the HTS maglev system can meet all requirements of the levitating display model for exhibitions, and may enlighten the research work on HTS maglev applications.     

Heat transfer performance of cryogenic oscillating heat pipes for effective cooling of superconducting magnets

The cryogenic oscillating heat pipe (OHP) for conduction cooling of superconducting magnets was developed and the function was demonstrated successfully. OHP is a highly-efficient heat transfer device using oscillating flow of two-phase mixture. The working fluids that are employed in the present research are Nitrogen, Neon and Hydrogen, and the operating temperatures are 67–91 K, 26–34 K and 17–27 K, respectively. The estimated effective thermal conductivities from the measurement data of the OHP were higher than one of the solids such as copper at low temperature. These results revealed that the cryogenic OHP can enhance the performance of cooling system for magnets.     

Development of 3kA conduction cooled HTS current lead system

The research and development of superconducting magnet energy storage (SMES) system, a national project, began in 1999. One of the purposes of this project is investigation concerning the application of high-temperature superconducting (HTS) SMES. As a part of this project, the 3 kA class HTS small model coil was manufactured in order to verify the possibility of realizing conduction cooled HTS SMES. Therefore, it is important to develop the conduction cooled current lead system for applying this coil. We developed a kA class conduction cooled HTS current lead system. This current lead system consists of the copper current lead and the YBaCuO (YBCO) HTS current lead. The YBCO bulk manufactured by Nippon Steel Corporation was applied to the HTS current lead. The YBCO bulk keeps high critical current density (J_c > 10,000 A/cm²) in the magnetic field (1 T) at 77 K compared with Bi2223 superconductor. The experiment of this HTS current lead system was carried out, and rated current of 3000 A was achieved successfully.     

The cryogenic system for ITER CC superconducting conductor test facility

This paper describes the cryogenic system of the International Thermonuclear Experimental Reactor (ITER) Correction Coils (CC) test facility, which consists of a 500 W/4.5 K helium refrigerator, a 50 kA superconducting transformer cryostat (STC) and a background field magnet cryostat (BFMC). The 500 W/4.5 K helium refrigerator synchronously produces both the liquid helium (LHe) and supercritical helium (SHe). The background field magnet and the primary coil of the superconducting transformer (PCST) are cooled down by immersing into 4.2 K LHe. The secondary Cable-In-Conduit Conductor (CICC) coil of the superconducting transformer (SCST), superconducting joints and the testing sample of ITER CC are cooled down by forced-flow supercritical helium. During the commissioning experiment, all the superconducting coils were successfully translated into superconducting state. The background field magnet was fully cooled by immersing it into 4.2 K LHe and generated a maximal background magnetic field of 6.96 T; the temperature of transformer coils and current leads was reduced to 4.3 K; the inlet temperature of SHe loop was 5.6 K, which can meet the cooling requirements of CIC-Conductor and joint boxes. It is noted that a novel heat cut-off device for High Temperature Superconducting (HTS) binary current leads was introduced to reduce the heat losses of transformer cryostat.     

Investigation into the use of solid nitrogen to create a “Thermal Battery” for cooling a portable high-temperature superconducting magnet

The design of a portable, “stand-alone” cooling system, for use with a high-temperature superconducting (HTS) magnet, is discussed. The HTS magnet is used to propel a magnetohydrodynamically powered model boat (approximately 120 cm × 60 cm). The aim of this investigation was to establish the suitability of solid nitrogen for use in the stand-alone cooling system, and determine the optimum method for exploiting its cooling power. It was found that obtaining good thermal contact between solid nitrogen and its container is very difficult if the nitrogen is frozen under vacuum, due to the formation of a thermal barrier between the nitrogen and its container. This problem is overcome if the nitrogen is frozen via conduction cooling from cold helium gas (at ∼4.2 K); and the design for a near isothermal “thermal battery” based on this principle is presented. This thermal battery has been constructed and integrated into the HTS magnet system onboard the model boat, and the results from the first trials of this system are presented here.     

Cryogenic heat pipe for cooling high temperature superconductors

The research in this paper investigates a consumable-free method of operating a high temperature superconducting (HTS) coil in space. The HTS wire resides inside a cryogenic heat pipe which is used for isothermalization. This paper presents the design, implementation, and testing of a cryogenic heat pipe for cooling high temperature superconductors. As a proof-of-concept, an 86 cm long straight heat pipe was constructed and enclosed two straight lengths of HTS wire. The working fluid, at saturation condition, maintains a constant temperature below the HTS wire critical temperature. Testing of the heat pipe in a vacuum chamber was conducted to verify the drop in HTS resistance correlating to the wire operating in a superconducting state.     

Influence of the Trapped Field on the Levitation Performance of the Magnetized Bulk High-<Emphasis Type="Italic">T</Emphasis><Subscript><Emphasis Type="Italic">c</Emphasis></Subscript> Superconductor

A magnetized bulk high-T _c superconductor (HTSC) magnet is a good candidate to improve the levitation performance of the high-T _c superconducting (HTS) maglev system. Compared with the unmagnetized bulk HTSC, the magnetized bulk HTSC magnet can supply stronger levitation or guidance force above a permanent magnet guideway (PMG). Different from the permanent magnet, the magnetic field of a magnetized bulk HTSC magnet is sustained by the induced superconducting current produced during the magnetizing process. Given that the induced superconducting current within the magnetized bulk HTSC magnet is very sensitive to the magnetic field, the levitation performance of the magnetized bulk HTSC magnet is directly related to its own trapped field and the magnetic field of the PMG. This article discusses the influence of trapped and external magnetic fields on the levitation performance of a magnetized bulk HTSC magnet by experiments, and the Critical State Model is used to analyze the test results. The analyses and conclusions of this article are useful for the application of magnetized bulk HTSC magnet in practical HTS maglev systems.     

High-Performance Permanent Magnet Railway Design Consideration of Magnetic Stiffness Parameters of YBCO Bulk Arrays

The monopole permanent magnet railway (PMR) is used in the present high-T _c superconducting (HTS) maglev vehicle system. In order to improve the performance of the present maglev vehicle system, many studies have been done to optimize the maglev system. Stiffness is one of the important parameters for maglev vehicle system design. In this article, measurement of the vertical and lateral magnetic force stiffness of HTS bulk arrays above monopole PMR and double-pole PMR are measured. Comparing the experimental results, it is found that the magnetic force stiffness of bulk arrays above double-pole PMR is better than that of monopole PMR. The experimental results show that the running stability of HTS maglev vehicle system using double-pole PMR is much better than that of using monopole PMR.     

Design analysis of a solid nitrogen cooled “permanent” high-temperature superconducting magnet system

Potential performance advantages of a solid nitrogen cooled “permanent” high-temperature superconducting (SN2/HTS) magnet system over a liquid helium cooled low-temperature superconducting (LHe/LTS) system are explored. The SN2/HTS system design includes a second solid heat capacitor that cools a radiation shield. Recooling of the heat capacitors is performed with a demountable cryocooler. The SN2/HTS system offers both enhanced stability and improved portability over a LHe/LTS system.Design codes are constructed to compare the SN2/HTS system design with a LHe/LTS design for a general permanent superconducting magnet system employing a room temperature bore. The codes predict the system volume and mass that should be expected for a given set of design requirements, i.e. field strength and bore size, and a given set of conductor properties. The results indicate that present HTS conductor critical current and index are not yet sufficient for producing SN2/HTS systems of a size that is comparable with that expected for a LHe/LTS system; however, the conductor properties of Bi2223/Ag have been consistently improving, and new HTS conductors are expected to be developed in the near future. The codes are used to determine the minimum Bi2223/Ag conductor performance required for a SN2/HTS system to be competitive with a LHe/LTS system.     

Flux Concentrator Optimization of PMG for High-Temperature Superconducting Maglev Vehicle System

The permanent magnetic guideway (PMG) composed of permanent magnet (PM) and steel is developed under flux concentration principle, which is the crucial component of high-temperature superconducting (HTS) maglev vehicle system. Optimum PMG design is an effective way to increase levitation force and associated stiffness for improving the load capability of HTS maglev vehicle. In order to realize higher vertical field component B _z in upper surface, three PMG demonstrators with three different forms of flux concentrator are fabricated with same volume of magnet. The levitation performances of onboard HTS bulks array over them are studied. The experimental results indicate that the PMG with a permanent magnet as the flux concentrator would produce biggest levitation force, levitation stiffness and trapped flux when interacting with HTS superconductor.     

Performance of superconducting nanowire single photon detection system with different temperature variation

The performance of cryocooler-based superconducting single photon detection system suffers from the intrinsic temperature oscillation, which is typically ∼300 mK around 4.2 K originated from the periodic expansion of the cryocooler’s working fluid (He). By using a rare-earth alloy (ErNi) plate with a high heat capacity at cryogenic temperatures in between the cold head of the cryocooler and the detector block, the detector temperature variation is successfully damped to be less than 10 mK. The dark count rate is reduced and the maximum working bias current is increased. The quantum efficiency of SNSPD system is significantly improved by 40%.     

Levitation Force Relaxation of HTS YBCO Bulk under Load

The loading capacity of the high temperature superconducting (HTS) maglev vehicle is closely relative to the maglev characteristics of the HTS bulk under the load. The investigation of the characteristics is necessary for the application of the HTS maglev vehicle. In our previous work, it was found that the levitation force relaxation increased with the increase of the reloading. In this paper, in terms of a HTS maglev system composed of the permanent magnetic guideway (PMG) and a HTS YBCO bulk, the levitation force relaxation under different loads is investigated by the numerical calculation and the experiment. It is found that the normalized relaxation rate of the levitation force increased with the increase of the load. The influence of the load on the current density distribution is also discussed in this paper. This work might be helpful for the research of the loading capacity of the HTS maglev vehicle.      

基于液氮冷源超导磁体氦气循环预冷系统设计

为了提高超导磁体300-80 K预冷过程中的降温效率和安全性,开发了一种新的预冷方法.设计了一台以液氮为冷源、氦气为循环介质的可控温预冷装置,对其内部结构进行了优化设计,包括低温风机、板式换热器、气动调节阀、翅片换热器等主要组成部分,整个装置与磁体构成一个闭合循环系统.在预冷装置的作用下,该超导磁体从300 K到80 ...     

基于液化天然气低温冷藏车的相变蓄冷实验

针对以水为相变工质通过铜质圆管壁与低温氮气换热发生固液相变问题,通过测温和可视化测量手段模拟研究固液相变贮存LNG冷量过程,获得管内低温气体、管外液相区温度分布及冰层图相,分析了管内换热和液相区自然对流综合影响下的冰层变化和分布特性,结果表明:该换热问题具有典型的变壁温变热流密度的热边界条件;冰层厚度在有限时间内近似线性增长,且沿管长锥状分布、冰层锥度随时问呈对数增长趋势;由于液相区水的密度反转效应使自然对流主流向发生改变,导致上下壁面冰层厚度发生反转.     

液氮温区高温超导滤波器的漏热分析

为在液氮温区工作的高温超导滤波器的制冷机冷源选型,基于Sage 10软件对超导滤波器件的冷却装置进行了漏热分析,仿真计算了铜线和同轴线的几何参数对传导漏热量的影响,以及真空罩、冷盘的尺寸和发射率对辐射漏热量的影响,并综合上述分析计算了超导滤波系统的总漏热量。在仿真计算中发现,信号线的导热和真空罩中冷盘的辐射漏热在系统总漏热量中起主导作用。仿真计算结果表明,通过增大信号线长度、减小信号线直径的方式可将信号线漏热量降至0.72 W,约为初始导热量的1/3。与此同时,冷盘表面采用抛光镀金的方式减小表面发射率,使辐射漏热量降至原来的2/3。根据模拟计算的最优结果,选择制冷量为3 W@77 K的制冷机作为高温超导滤波器的冷源。     

Optimal cool-down time of a 4 K superconducting magnet cooled by a two-stage cryocooler

A cool-down time is one of the major factors in many cryocooler applications, especially for the design of conduction-cooled superconducting devices. Cool-down time means a time cooling a thermal mass from a room-temperature to cryogenic-temperature within a stipulated amount of time. The estimation of cool-down time seeks the elapsed time to cool the thermal object by a cryocooler during initial cool-down process. This procedure includes the dimension and properties of thermal object, heat transfer analysis for cryogenic load, thermal interface between cold mass and cryocooler, and available refrigeration capacity of cryocooler. The proposed method is applied to the specific cooling system for 3 T superconducting magnet cooled by a two-stage GM cryocooler. The result is compared with that of experiment, showing that proposed method has a good agreement with experiment. In addition, the initial cool-down time can be shortened by employing thermal link between the cold mass and first-stage of cryocooler. Through a rigorous modeling and analysis taking into account the effect of thermal link size, it is concluded that there exists an optimal cool-down time during initial cooling in conduction-cooled superconducting magnet system.     

3D Modeling Permanent Magnet Guideway for High Temperature Superconducting Maglev Vehicle Application

A three-dimensional analytical method is proposed to model the permanent magnet guideway (PMG) involved in the high-temperature superconducting (HTS) maglev vehicle system, and with this analytical method, the non-uniformity of magnetic field along the longitudinal direction of the PMG due to the existent gap between the adjacent permanent magnet (PM) can be taken into account in the simulation of the HTS maglev vehicle system. The analytical expressions for two rectangular PM with different magnetization directions are deduced from the Biot–Savart’s law. The 3D modeling of the PMG is validated from the four aspects, i.e., the comparison of the measurement and calculation value of the magnetic field; the magnetic field contour of the PMG; the comparison of the 3D method results with the 2D method; and the comparison of the results from the present 3D analytical model and previous finite element software. Currently, using the 3D analytical model, we have proposed a 3D method to numerically investigate the HTS magnetic levitation/suspension system with bulk high-temperature superconductor (HTSC).