HTS magnets: stability; protection; cryogenics; economics; current stability/protection activities at FBML

The unique features that distinguish a high-temperature superconducting (HTS) magnet from a low-temperature superconducting (LTS) magnet are: (1) operating temperature and (2) temperature span over which the magnet remains superconducting. Thus, for two magnets, one HTS and the other LTS, both satisfying the same set of field specifications, the operating temperature and the operating range of temperature of the HTS magnet are generally an order of magnitude greater than that of the LTS magnet. In this CHATS 2002 Workshop, four issues, i.e., stability, protection, cryogenics, and economic issues will be examined as they are impacted by the two unique features, first as a general introduction to this presentation, between HTS and LTS magnets. This is followed by discussion of economics of large-scale HTS devices, specifically of fusion; electric power devices; high-energy physics; research-purpose NMR; and high-B DC magnets. The paper concludes with a presentation of preliminary results of stability/protection research presently being performed at the FBML.     

Heat conduction and thermal stabilization in YBCO tape

Yttrium barium copper oxide (YBCO) coated conductors are widely used in the conduction-cooled superconducting magnets with rapid development in refrigeration technologies at present. ‘Quench’ is a state that refers to the irreversible and uncontrolled superconductor to resistive transitions in the superconductor. The propagation of ‘quench’ or ‘normal zone’ has different characteristics in these high temperature superconductors (HTS) compared to low temperature superconductors. The superconductor to normal index, known as ‘n’ is much flatter in HTS. The hot spot emerging in local region due to quench and non-uniform critical current may cause permanent damage to whole HTS tape and hence the magnet winding pack. Thus it is necessary to determine the temperature profile along the length of HTS tape under a given energy (joule heating) such that propagation of the hot spot developed locally can be prevented early. In this study, a one dimensional, time dependent heat diffusion equation with appropriate boundary conditions are used to describe the consequences of the normal zone propagation resulting in the temperature diffusion in a HTS tape. The results demonstrate the necessity of adequate cooling of the edges of the flat HTS tapes to prevent irreversible normal zone transitions.     

‘Electromaglev’-magnetic levitation of a superconducting disc with a DC field generated by electromagnets: Part 1. Theoretical and experimental results on operating modes, lift-to-weight ratio, and suspension stiffness

We present results of a comprehensive study, both theoretical and experimental, of an ‘electromaglev’ system, in which a high-temperature superconducting bulk sample, e.g. YBa₂Cu₃O_7−δ (YBCO), is levitated stably in a DC magnetic field generated by electromagnets placed underneath the floating object. Results of the zeroth-order theory agree quite well with experimental results on lift-to-weight ratio and suspension stiffness for three bulk samples: (1) a solid YBCO disc; (2) a YBCO annulus; and (3) a YBCO annulus with a neodymium-iron-boron (Nd-Fe-B) permanent magnet disc (PMD) filling the centre. The experiment has also verified the need to satisfy two requirements to achieve stable levitation with a DC magnetic field only: (1) the spatial flow of the supercurrent in the sample must have at least two degrees of freedom; and (2) the electromagnets must generate a magnetic field profile that satisfies spatial requirements for lateral and pitch stability. A permanent magnet disc has only one degree of freedom for its Amperian current, thus it cannot be levitated stably in this system; the experiment has also demonstrated that an HTS solenoid (wound with silver-sheathed BSCCO-2223 tape) cannot be levitated stably, because the solenoid supercurrent flow is also restricted to the azimuthal direction only. The zeroth-order theory together with the Bean model shows that the supercurrent induced in a YBCO sample is independent of the critical current density, J_c, of the material but is directly proportional to the axial component of the field and that the lift of the sample is directly proportional to the product of the axial and radial components of the magnetic field generated by the electromagnets.     

Performance improvement and optimization of a high-temperature superconducting coil having different transport current portions by employing persistent current mode

To improve the performance of a high-temperature superconducting (HTS) coil, it is important to improve its transport current performance. In general, the critical current and n-value of an HTS (Bi-2223/Ag) tape depend on the applied magnetic fields and the angle between the magnetic field and the tape under a constant temperature. The critical currents in the coil edge of the tapes are particularly low because of the distribution of the magnetic fields. However, the critical currents in the central portion remain high. A large amount of current can be supplied to the central portion and the coil performance will improve by supplying different currents between these areas. In this study, I propose an HTS coil in which the coil edge and central portion are isolated for each excitation. Namely, I employ the characteristics of the persistent current mode. The analysis of varying regions of current separation confirmed an optimum current separation. This optimized coil improves the central magnetic field by 21% and the stored energy by 50% compared to those of a normal rectangular coil with an HTS tape of the same length.     

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.     

Development of Bi-2223 HTS tape and its application to coil and current leads

We are developing Bi-2223/Ag tapes with a high engineering critical current density by optimizing the powder-in-tube process and are studying its application to coil and current leads. We have fabricated 250 m-long tape and investigated optimized processing conditions to enhance engineering critical current density. More bubbling was found when the tape was heat-treated with a higher heating rate. Different kinds of superconducting joints were fabricated with multi-filamentary Bi-2223/Ag tapes, and 58% of retained I_c was achieved using the insertion of Bi-2223 core between two exposed tapes. Current decay property of the persistent mode HTS coil was investigated. Rapid current decay was observed when the operating current is in a flux-flow range. We could successfully fabricate a low heat leak type HTS current lead with Bi-2223/Ag–Au tapes by employing a stepped geometry. Using this lead, safe operation of 2 kA current transport was confirmed.     

Normal zone propagation characteristics of the HTS wires by heat energy applied

With the successful commercialization of Bi-2223 powder-in-tube wire, various attempts in the R&D of the high-T_c superconducting (HTS) magnets for high magnetic field applications are being implemented actively. Operating temperature of HTS magnet has to be maintained at the designed level but the magnetic energy and mechanical disturbance can cause unstable operational temperature of HTS magnet. Especially, the generated heat energy of inner HTS winding is apt to be accumulated, so the normal region appears in HTS winding. This paper deals with the quenching characteristics of three kinds of selected Bi-2223 wires: the high current density wire (HC-A) and the high strength wire (HS-A) made by AMSC and HTS wire (HW-I) made by Innost. The Innost wire has the highest minimum quench energy (MQE). The high current density wire has the highest normal zone propagation velocity (NZPV).     

舰船电力系统用1 MJ高温超导储能磁体设计研究

介绍了舰船电力系统用的1MJ螺管型高温超导储能磁体的设计优化步骤，给出了用Bi-2223超导带进行1 MJ磁体线圈的设计和优化结果，分析了高温超导体的各向异性对磁体临界电流的影响，讨论了储能容量一定的多螺管磁体系统在漏磁、储能密度和所需超导线材方面的变化。     

Critical current of superconducting magnets wound from an anisotropic superconducting conductor

A superconducting magnet wound from an anisotropic superconductor, critical current calculation procedure is shown. For axisymmetric solenoids with rectangular cross-section and uniform current density the coefficient K_φ(α,β) = B_φ^max/B_φ was calulated and is presented in graphical form in the plane for the angles φ = 30°, 60°, 90°. Using the coefficient K_φ(α,β) it is possible to determine the critical current for a given angle ψ. In the case of the I_c-Bβ_ψ characterisctics of the superconductor expressed analytically, the term to determine I_cφ was derived. The critical current of the solenoid is determined by the lowest value of the critical currents I_ψ. Some results of our experimental investigations, showing typical examples of a critical current anisotropy are presented.     

Low resistance splices for HTS devices and applications

This paper discusses the preparation methodology and performance evaluation of low resistance splices made of the second generation (2G) high-temperature superconductor (HTS). These splices are required in a broad spectrum of HTS devices including a large aperture, high-field solenoid built in the laboratory to demonstrate a superconducting magnetic energy storage (SMES) device. Several pancake coils are assembled in the form of a nested solenoid, and each coil requires a hundred meters or more of 2G (RE)BCO tape. However, commercial availability of this superconductor with a very uniform physical properties is currently limited to shorter piece lengths. This necessitates us having splices to inter-connect the tape pieces within a pancake coil, between adjacent pancake coils, and to attach HTS current leads to the magnet assembly. As a part of the optimization and qualification of splicing process, a systematic study was undertaken to analyze the electrical performance of splices in two different configurations suitable for this magnet assembly: lap joint and spiral joint. The electrical performance is quantified in terms of the resistance of splices estimated from the current-voltage characteristics. It has been demonstrated that a careful application of this splicing technique can generate lap joints with resistance less than 1 nΩ at 77 K.     

Electromagnetic optimization design of a HTS magnet using the improved hybrid genetic algorithm

The genetic algorithm (GA) is an efficient method in the optimization of superconducting magnets, but there are some limitations of the GA applied to practice design of superconducting magnet, such as poor local search ability, premature convergence, etc. An improved hybrid genetic algorithm is developed by combination of the sequential quadratic programming (SQP). A high temperature superconducting (HTS) magnet by Bi-2223/Ag tape is designed through the improved hybrid GA. A new configuration of the HTS magnet which can reduce the winding volume and become more convenient to construct is suggested with consideration of the constraints, such as central magnetic filed, critical current characteristic, storage energy, and so on.     

Reduction of radial magnetic fields in HTS solenoids with different constraint conditions

The critical current of HTS solenoid is heavily dependent on the value of radial magnetic fields in the coil winding section: the higher radial magnetic field in the winding section, the lower critical current of HTS solenoid is. So, we need to reduce the radial magnetic field components of the solenoid as much as possible to get high performance HTS magnet. In this paper we investigate various HTS coil shapes, obtained from the optimum algorithm, in which the radial magnetic field components are minimized. Three different constraints were used during optimization calculation: (1) fixed magnet volume condition, (2) fixed center magnetic field condition, and (3) maximum magnetic energy storage condition. The calculated results using those three different constraints showed almost similar shapes to one another and turned out to be effective to reduce the radial magnetic fields in the winding section. Finally, it is pointed out that two or three sectioned multi-magnet shape, which can be deduced from the calculated shape, could be more optimal to reduce the radial magnetic field in the coil, and more practical to fabricate.     

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.     

Effect of Pre-magnetization on Quasi-static Force Characteristics in a Space Superconducting Interface Structure Adopting High T c Superconductors

Flux-pinned interaction between high T _c superconductors (HTSs) and an applied magnetic field provides a new, no-contact interface approach that can be used in docking and assembling process of space module systems. Unlike operations on the Earth, the magnetization of the HTS happens in orbit which differs from the traditional field cooling (FC) magnetization, and the additional field has to be used to magnetize the HTS in advance and make it produce a self-stable force in the interacting process with the interfacing magnet. This paper presents a type of superconducting interface structure configuration consisting of bulk HTSs, actuation electromagnets and interfacing magnets, and discusses the effects of different magnetization conditions on the quasi-static force interaction between the HTS and the interfacing magnet. Primary experiments show that the HTS after pre-magnetization can show self-stable force behavior, which often happens in the traditional FC magnetization, and the self-stable force is further enhanced with the increase of the pre-magnetizing current. Multi-pulse field magnetization after the pre-magnetization is also applied to raise the trapped field strength (B _T ) of the superconductor. The results show that B _T is added with the increasing number of the pulsed field, and the corresponding self-stable force properties are also improved. Therefore, the pre-magnetization combined with the pulsed field magnetization can enhance the flux trapping in the HTS and bring more stable force for the superconducting interface structure.     

Aging characteristics of cryogenic insulator for development of HTS transformer

In the response to the demand for electrical energy, much effort aimed to develop and commercialize high temperature superconducting (HTS) power equipments has been made around the world. Especially, HTS transformer is one of the most promising devices. For the development of HTS transformer, the cryogenic insulation technology should be established. In this paper V−t characteristics of polyimide (Kapton) tape and GFRP used as turn-to-turn and structural insulations, respectively were studied. Moreover, breakdown hole site of GFRP after breakdown was also discussed. The experimental results show that the time to breakdown is conditioned on applied electric stress and the lifetime indices n of Kapton tape decrease slightly as the number of tape increases while the lifetime indices n of GFRP decrease strongly with increasing thickness. Furthermore, the breakdown holes of GFRP were not at the contact point, at which the electric field is maximum value, between sphere electrode and GFRP sample and its location depends on applied voltage as well as sphere diameter.     

Detecting and describing the inhomogeneity of critical current in practical long HTS tapes using contact-free method

The nondestructive and contact-free apparatus for measuring local critical current of long high temperature superconducting (HTS) tapes is presented. The local critical current of tape is acquired by using Hall probe array sensor to measure the remanent field after exposed to dc external magnetic field since the critical current is proportional to remanent field based on Bean critical state model. A detailed experiment on multifilamentary Bi2223/Ag tape is made to validate reproducibility, reliability, resolution, nondestructiveness and usefulness for manufacturer and user of tapes. The parameter COV (coefficient of critical current variation) is suggested for quantitatively describing the inhomogeneity and quality of practical long HTS tape based on Gaussian statistical analysis. The developed apparatus can detect HTS tape at velocity of 100 m/h with resolution smaller than 3 mm in liquid nitrogen.     

Maximum Endurance Lightning Current of Different High-Temperature Superconducting Wires

The over-current performance of high-temperature superconductors (HTSs) has been well studied. However, the superconducting devices used in power grid might also suffer lightning current. In previous researches, we have investigated the influences of the substrate layer, encapsulation layer, and tape dimension on the thermal stability of yttrium barium copper oxide (YBCO) tapes after suffering a lightning current. However, in this paper, we will focus on the maximum endurance lightning currents (MELCs) of different HTS tapes including YBCO and BSCCO tapes. We prepared and tested seven HTS samples. The critical current and n value of each sample were measured and calculated before and after the 8/20-μs lightning current tests. A maximum endurable value of lightning current which might not cause the critical current degradation was obtained from the testing results. Finally, the mechanisms causing the degradation were analyzed.     

Wide Variety of Experiments Using a Cryogen-Free 27.5 T Hybrid Magnet and a Cryogen-Free 18.1 T Superconducting Magnet

A cryogen-free hybrid magnet without liquid helium for operation, generating 27.5 T in a 32 mm room temperature bore of an 8 MW water-cooled resistive insert magnet in an 8.5 T background field of a cryogen-free superconducting outsert magnet, is being operated for basic research at low temperatures down to 17 mK in combination with a dilution refrigerator. In addition, we are developing functional materials using a differential thermal analysis DTA at high temperatures up to 1473 K in high fields up to 27 T. This cryogen-free hybrid magnet will be upgraded to generate 29 T by improving the outer superconducting magnet. A cryogen-free 18.1 T superconducting magnet with a 52 mm room temperature experimental bore, consisting of a Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) insert coil, has been developed using a GM-JT cryocooler. Recently, bronze-tape-laminated Bi2223 has revealed excellent irreversible stress tolerance of 250 MPa at 77 K. In addition, the critical current properties for recent Bi2223 tapes are largely improved from 200 to 400 A/cm-width at 77 K in a self-field. Therefore, the stainless steel reinforcement tape incorporated for the previous Bi2223 insert coil is no longer needed for a new Bi2223 one. A new Bi2223 insert coil with almost the same size as the existing insert coil can generate two times higher fields at the elevated operation current from 162 to 191 A. An upgraded cryogen-free superconducting magnet can offer a long-term experiment at the constant magnetic field of 20 T for an in-field heat-treatment investigation.     

Development of stable high field Nb-Ti superconducting magnets

This Paper gives an analysis of the disturbance spectrum within an adiabatic superconducting magnet, and discusses the technical requirements for constructing a stable high field Nb-Ti magnet. Using Nb-Ti wires made in China we wound six superconducting magnets smeared with wax-Al₂O₃ or wax-Gd₂O₃ powder, except for magnet SM-3. All the magnets produced central fields higher than 8.6 T and one of them produced a central field up to 9.24 T at 4.2 K. The quenching current on the load lines was >95% as much as the critical current of short samples, and no training effect was found, except for magnet SM-3. These magnets could be energized to maximum central field at a high speed, typically 4 T min ¹, and operated stably up to 8.6 T after quenching 10 times and eight cycles between 4.2 K and room temperature. This indicates the effectiveness of the techniques developed in this Paper.     

Fabrication of a working Bi-2223 superconducting magnet cooled by liquid nitrogen

A practical Bi-2223 superconducting magnet, working in liquid nitrogen (L.N₂), was designed and fabricated. Bi-2223 tape with a critical current of 147 A was prepared by a controlled overpressure (CT-OP) process at 77.3 K in self-field. Ten double-pancake coils were resistively connected by copper terminals. The bore diameter was 54 mm?, the magnet outer diameter was 122 mm?, the height of the magnet was 124 mm, and the weight of the magnet was about 3 kg. The maximum magnetic field at the center of the bore was 0.48 T with an operating current of 50 A. The experimental results agree well with design predictions calculated by finite element method. AC operation was also performed, and no distortion of the voltage waveform was observed. Therefore, this Bi-2223 superconducting magnet is a suitable replacement for copper magnets designed for applications in science and technology.