High-Tc Superconducting Magnets Research Articles

A Long-Life, High-Capacity and High-Efficiency Cryogenic System Developed for High-Tc Superconducting Magnet Applications

Cryogenic system plays a vital role in the field of high-Tc superconducting (HTS) magnet applications. For a variety of HTS magnet applications in energy storage devices, current limiters, generators, and maglevs, the technology itself is relatively mature. However, the problems associated with the used cryocoolers have hampered the advancement of their practical applications. At present, China is developing a new generation of electrodynamic suspension prototype train with HTS magnets. Compared with the existing commercial electromagnetic suspension technology, it has better suspension stability, larger suspension gap, and lower energy consumption. To meet the aimed cooling requirement of the vehicle-mounted HTS magnet, a long-life, high-capacity and high-efficiency cryogenic system based on the Stirling-type pulse tube cryocooler (SPTC) is designed and developed in the authors’ laboratory. The SPTC driven by the linear compressor without any moving component in the cold head has the intrinsic merits of long life and high reliability. The cryogenic system can achieve a minimum no-load cooling temperature of 12 K. Multiple SPTCs will provide a cooling capacity of 100 W at 45 K with the relative Carnot efficiency of 10.8 % to cool the HTS coils inside the magnet. This paper will describe and analyze the overall design approach, integration with the HTS magnet and the arrangement in maglev of the developed cryogenic system, together with the performance characteristics of the SPTC during the laboratory test.

A kilo-ampere level HTS flux pump

This paper reports a newly developed high current transformer–rectifier high-Tc superconducting (HTS) flux pump switched by dynamic resistance. A quasi-persistent current of over 1.1 kA has been achieved at 77 K using the device, which is the highest reported operating current by any HTS flux pumps to date. The size of the device is much smaller than traditional current leads and power supplies at the same current level. Parallel YBCO coated conductors are used in the transformer secondary winding as well as in the superconducting load coil to achieve high current. The output current is limited by the critical current of the load rather than the flux pump itself. Moreover, at over 1 kA current level, the device can maintain high flux injection accuracy, and the overall flux ripple is less than 0.2 milli-Weber. The work has shown the potential of using the device to operate high field HTS magnets in ultra-high quasi-persistent current mode, thus substantially reducing the inductance, size, weight, and cost of high field magnets, making them more accessible. It also indicates that the device is promising for powering HTS NMR/MRI magnets, in which the requirement for magnetic field stability is demanding.

R&D of applied superconductivity by a small business: experiences and future perspective

The increasing commercial availability of High-Tc Superconducting (HTS) wire in the decade following the discovery of this amazing class of materials opened the door to a range of unique application opportunities. The early international focus was on the power industry with a promise of a significant transformation in efficiency and supply security; however the realities of funding large and technically demanding prototypes and the lengthy timelines involved in realistic commercialisation pathways, meant that for a small business nearer-term opportunities would need to be explored. From a background in materials research and wire development activities, HTS-110 was established over 13 years ago to design and manufacture HTS magnets for a wide range of scientific and industrial applications; over the last decade significant progress has been made in the commercialisation of HTS magnets, both by HTS-110 and other manufacturers, paralleling the improvements in wire performance and quality. In this paper we review the developments of HTS magnet technology and applications leading to a range of niche application areas, from sample environments for synchrotron and neutron beamlines and other materials analysis applications, through to developments for the demanding realm of magnetic resonance, all of which leverage the benefits of high current density relative to copper and a relatively high operating temperature compared to the low-temperature superconductors. New future application areas promise to extend these developments and greatly expand the role of HTS magnets in our industrial society.

An HTS flux pump operated by directly driving a superconductor into flux flow region in the E–J curve

High-Tc superconducting (HTS) flux pumps are capable of compensating the persistent current decay in HTS magnets without electrical contact. In this paper, following work on a low-Tc superconducting self-switching flux pump, we propose a new HTS flux pump by directly driving a high-Tc superconductor into the flux flow region in the E–J curve. The flux pump consists of a transformer which has a superconducting secondary winding shorted by an YBCO-coated conductor bridge. A high alternating current with a much higher positive peak value than the negative peak value is induced in the secondary winding. The current always drives the bridge superconductor into the flux flow region only at around its positive peak value, thus resulting in flux pumping. The proposed flux pump is much simpler than existing HTS flux pumps.

Operational research on a high-Tc rectifier-type superconducting flux pump

High-Tc superconducting (HTS) flux pumps are capable of injecting flux into a superconducting circuit, which can achieve persistent current operation for HTS magnets. In this paper, we studied the operation of a rectifier-type HTS flux pump. The flux pump employs a transformer to generate high alternating current in its secondary winding, which is connected to an HTS load shorted by an HTS bridge. A high frequency ac field is intermittently applied perpendicular to the bridge, thus, generating flux flow. The dynamic resistance caused by the flux flow ‘rectifies’ the secondary current, resulting in a direct current in the load. We have found that the final load current can easily be controlled by changing the phase difference between the secondary current and the bridge field. The bridge field of frequency ranging from 10 to 40 Hz and magnitude ranging from 0 to 0.66 T was tested. Flux pumping was observed for field magnitudes of 50 mT or above. We have found that both higher field magnitude and higher field frequency result in a faster pumping speed and a higher final load current. This can be attributed to the influence of dynamic resistance. The dynamic resistance measured in the flux pump is comparable with the theoretical calculation. The experimental results fully support a first order circuit model. The flux pump is much more controllable than the traveling wave flux pumps based on permanent magnets, which makes it promising for practical use.

An Overview of Flux Pumps for HTS Coils

High-Tc superconducting (HTS) flux pumps are capable of injecting flux into closed HTS magnets without electrical contact. It is becoming a promising alternative of current source in powering HTS coils. This paper reviews the recent progress in flux pumps for HTS coil magnets. Different types of HTS flux pumps are introduced. The physics of these flux pumps are explained and comparisons are made.

Development of Saddle-Shaped Coils for Accelerator Magnets Wound With YBCO-Coated Conductors

With the advances made in accelerator technologies, particle accelerators are being employed in a wide range of fields such as physics, biology, and medical treatment. However, the diameters of the main rings in most accelerators are large; for example, even the smallest carbon cancer therapy accelerator has a main ring diameter of approximately 20 m. Since a rotating gantry for a carbon-ion beam is also very large compared with the gantry for a proton beam, widespread adoption of this therapy system has been restricted. In order to promote widespread adoption of carbon cancer therapy accelerators, it is necessary to reduce the bending radius of the carbon-ion beam. One way to achieve this is to use superconducting magnets with higher magnetic fields. High-T c superconducting (HTS) magnets have higher thermal stability and need lower power consumption for cooling than low-T c superconducting (LTS) magnets. Although saddle-shaped coils are suitable for accelerator magnets, an HTS conductor has anisotropic bending flexibility because of its tape shape. To evaluate winding technologies for HTS saddle-shaped coils, a number of coils were fabricated and tested. The test coils were about 400 mm long and 160 mm wide. Each test coil was wound using an approximately 45-m-long 4-mm-wide YBa 2 Cu 3 O x (YBCO)-coated conductor. Some of the coils were excited to measure their voltage-current characteristics in liquid nitrogen. An index value over 20 was obtained throughout an electric field range down to 10 -7 V/m, which indicated that the superconducting properties would not be degraded. To evaluate the quality of the magnetic field distribution and the thermal stability of a conduction-cooled HTS magnet, we fabricated and tested an HTS magnet consisting of eight saddle-shaped coils.

Transverse thermoelectric conductivity and magnetization in high-Tc superconductors

We use the time-dependent Ginzburg-Landau (TDGL) equation with thermal noise to calculate the transverse thermoelectric conductivity αxy describing the Nernst effect and magnetization Mz in type-II superconductor in the vortex-liquid regime. The nonlinear interaction term in dynamics is treated within self-consistent Gaussian approximation. The expressions of the transverse thermoelectric conductivity and magnetization including all the Landau levels are presented in explicit form which are applicable essentially to the whole phase. Our results are compared to recent simulation data on high-Tc superconductor.

Remagnetization effects due to lateral displacement above a PMG on bulk HTS magnet

For a high-Tc superconducting (HTS) maglev system with large force requirements, the use of magnetized bulk high-Tc superconductor magnets (MBSCMs) is a good candidate because of its strong flux pinning ability and corresponding high trapped flux. Different from the rare-earth permanent magnet (PM), the trapped flux of a MBSCM is sustained by the supercurrent produced by a magnetizing process, so the trapped flux is sensitive to variations of the supercurrent. The lateral displacement of a MBSCM above a PM guideway (PMG) will provide disturbance of the applied field and then alter the supercurrent as a process of remagnetization. Different magnetization histories will bring different remagnetization characteristics and consequently diverse levitation performances for a MBSCM during the lateral displacements. When the MBSCMs are applied into the HTS maglev system, the influence of lateral displacements on levitation performance should be taken into consideration. This article investigates the remagnetization characteristics of a MBSCM when it is subject to the lateral displacements above a PMG with different trapped magnetic flux and opposite magnetization polarities. Relevant analyses about the internal supercurrent configuration based on the critical state model are also included to better understand the remagnetization characteristic of a MBSCM.

수소 재액화용 단열 탈자 냉동기의 설계

Adiabatic demagnetization refrigerator (ADR) for hydrogen re-liquefaction operating between 24 K and 20 K has been designed. <TEX>$Dy_{0.9}Gd_{0.1}Ni_2$</TEX>, whose Curie temperature is 24 K, is selected as a magnetic refrigerant. The magnetic refrigerant powder is sintered with oxygen-free high purity copper (OFHC) powder to enhance its effective thermal conductivity as well as to achieve relatively high frequency. A perforated plate heat exchanger (PPHE) operated with forced convection is utilized as a heat switch. The forced convection heat switch is expected to have fast response relative to a conventional gas-gap heat switch. A conduction-cooled high Tc superconducting (HTS) magnet is employed to apply external magnetic field variation on a magnetic refrigerant. <TEX>$2^{nd}$</TEX> generation GdBCO coated conductor HTS tape with Kapton<TEX>$^{(R)}$</TEX> insulation (SUNAM Inc.) will be utilized for the HTS magnet. The magnetization and demagnetization processes are to be achieved by the AC operation of the HTS magnet. The designed magnetic field and target ramp rate of the HTS magnet are over 4 T with 180 A and 0.4 T/s, respectively. AC loss distribution on HTS magnet is theoretically estimated.

Thermal Analysis on Cryocooler Cooled High-Tc SMES Magnet

A model high-Tc Super-conducting Magnet Energy Storage (SMES) magnet, which is winded with high-Tc super-conducting Bi-2223 trip, is cooled by GM cryo-cooler. The SMES cooling feature is investigated. It took about 17 hours to reach the minimum temperature for the SMES cooling, and its axial temperature difference is 2.8 K, the radial temperature difference is less than 1 K. The thermal contact resistance results in the resource of larger axial direction temperature difference. By improved, the cooling time is about 25 hours for the cryo-cooler cooled 35kJ SMES magnet, its axial direction temperature difference is 0.4 K. At loading 140 A direction current, the magnetic density is 4.5 T, no temperature rise is observed. At loading 60 A alternating current, the upside temperature rise of 35kJ SMES magnet is about 5 K. The 35kJ SMES magnet has been operated stably 480 hours.

Experimental Test and Numerical Analysis to Estimate Permissible Transport Current Considering Protection of High-Tc Superconducting Tapes in Adiabatic Condition

High-Tc superconducting (HTS) wires have showed good capabilities as an alternative conductor for superconducting magnet. The HTS wires have more current carrying capability within high external magnetic field and its stability against external disturbance is much higher than the low temperature superconducting (LTS) wires. But the HTS wire has slower normal zone propagation (NZP) velocity which makes HTS magnets hard to be protected from unexpected hot spot. This paper presents experimental test and numerical analysis about the relation between transport current and rate of temperature rise in a hot spot. A sample which contained a HTS wire, a heater, and temperature sensor was fabricated and an experimental test on measuring normal zone propagation and temperature of the HTS wire was conducted. A numerical model was also built to estimate transient temperature rise of a winding pack model which was representing practical winding of HTS magnets. The numerical model and its numerical estimation results are expected to be a guideline about designing proper protection technique and operating current for HTS magnets.

Levitation performance of the magnetized bulk high- Tc superconducting magnet with different trapped fields

To a high- T c superconducting (HTS) maglev system which needs large levitation force density, the magnetized bulk high- T c superconductor (HTSC) magnet is a good candidate because it can supply additional repulsive or attractive force above a permanent magnet guideway (PMG). Because the induced supercurrent within a magnetized bulk HTSC is the key parameter for the levitation performance, and it is sensitive to the magnetizing process and field, so the magnetized bulk HTSC magnets with different magnetizing processes had various levitation performances, not only the force magnitude, but also its force relaxation characteristics. Furthermore, the distribution and configuration of the induced supercurrent are also important factor to decide the levitation performance, especially the force relaxation characteristics. This article experimentally investigates the influences of different magnetizing processes and trapped fields on the levitation performance of a magnetized bulk HTSC magnet with smaller size than the magnetic inter-pole distance of PMG, and the obtained results are qualitatively analyzed by the Critical State Model. The test results and analyses of this article are useful for the suitable choice and optimal design of magnetized bulk HTSC magnets.

Influence of the Trapped Field on the Levitation Performance of the Magnetized Bulk High-T c Superconductor

A magnetized bulk high-Tc superconductor (HTSC) magnet is a good candidate to improve the levitation performance of the high-Tc 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.

Analysis on Ground Conductor Shape and Size Effect to Levitation Force in Static Type EDS Simulator

This paper presents the characteristic analysis on the high-Tc superconducting (HTS) electrodynamic suspension (EDS) simulator. In EDS maglev trains, the relative velocity between the moving high field magnet and the fixed ground conductor makes the magnetic reaction between two components. The magnetic reaction generates the levitation force of the EDS system. The distribution of the magnetic flux on the ground conductor affects the strength of the levitation force. A EDS simulator basically consists of levitation magnet and ground conductor and one of these component should be a moving part. In the static type EDS simulator, the levitation force is generated by the velocity equivalent AC current. Therefore, the static type EDS simulator does not need the high speed moving parts and can test the effect of the variations of the ground conductors easily by the change of the frequency of the applying AC current. To design the static type simulator optimally, the HTS levitation magnet and the characteristics of the EDS system were numerically analysed by 3-D finite element method (FEM). Based on the numerical analysis, the static type EDS simulator was designed manufactured and tested. The simulator consists of an HTS magnet, the fixed ground conductor(s), an AC power supply, resonating components, force measuring devices, and a supporting structure. From the calculation and test results on this paper, the consideration of the magnetic flux distribution according to the levitation height should be included in the process of the ground conductor design.

HTSC Levitation Experiment With AC Current Modeling After EDS Maglev

The eddy current induced in the ground conductor by moving high field on board superconducting magnet forms the inductive levitation system. In other words, the change of magnetic flux linked in ground conductor is the source of the levitation force of electrodynamic suspension (EDS) system. As a preliminary study on the HTSC levitation magnet to develop high-speed maglev in Korea, instead of moving the magnet, in this paper, AC current was applied to the fixed superconducting magnet. The HTSC levitation tester, which models after electrodynamic suspension maglev, consists of one high-Tc superconducting (HTSC) magnet, a reaction plate, and force measuring components. HTSC magnet was composed of series connected pancake coils wound with Bi-2223 wire. AC current was applied into the HTSC magnet with various frequencies and the levitation force was calculated and measured.

Design and analysis of a 2 T practical HTS magnet

A 2 T practical magnet is constructed using high- T c superconducting (HTS) Ag-clad (Bi,Pb) 2Sr 2Ca 2Cu 3O 10+ x multifilament tapes. Pancake units made by the HTS tapes are prepared for stacking assembly to form the HTS magnet having a closed cycle cooling system for its practical operation. The work presented is mainly focused to analyze the magnetic field of the magnet and to achieve high field uniformity as practically required. Design parameters, magnetic fields, and fabrication techniques of the HTS magnet will be introduced with its analysis result and performance, and its magnetic field characteristics will be mainly presented in the paper.

Characteristics Analysis of a High-Tc Superconducting Power Supply Considering Flux Creep Effect

This paper describes the characteristics analysis of a high-Tc superconducting (HTSC) power supply considering flux creep effect, and also presents its operational characteristics through experiments. The power supply can be employed to charge an HTSC magnet. An HTSC power supply consists of two heaters, an electromagnet, a Bi-2223 solenoid, and a series-connected Bi-2223 pancake load. The pancake load was fabricated by connecting four double pancake coils in series. In the experiments, two cases of the pumping period, which correspond to 8.5 and 17 s, were used with an electromagnet of 331 mH and a dc heater current of 0.8 A. A region of the superconducting tape with a buried heating coil can be switched by means of its temperature change. In order to measure the pumping-current with respect to the magnet flux changes, a hall sensor was installed at the center of the Bi-2223 pancake load. The experimental observations have been compared with the theoretical predictions. In this experiment, the pumping-current has reached about 22.9 A

Characteristics of a HTS Persistent Current Switch System Considering the n-Value

This paper deals with characteristics of a High-Tc superconductor (HTS) persistent current switch (PCS) system considering the n-value. I also present its operational characteristics through experiments. A HTS PCS system mainly consists of a PCS, a HTS magnet load, and a magnet power supply (MPS). The HTS magnet load was made-up of two double pancake coils in series. In order to measure the persistent current with respect to the magnet flux changes, a hall sensor was installed at the center of the magnet load. In the experiments, the initial current dropped rapidly in the persistent current mode. In order to analyze current decay, we introduced a new electrical equivalent model considering the n-value. Experimental results of the HTS PCS system have been compared with the theoretical results. It has been concluded that the characteristics of initial current decay were mainly determined by the voltages generated in the pancake load due to n-value and joint resistance

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).