Cryogen-Free 23 T Superconducting Magnet Employing an?YBa2Cu3O7 Coated Conductor Insert

We have successfully constructed an 18.1 T superconducting magnet conductively cooled by a GM/JT cryocooler. The double-pancake insert using Ag-sheathed Bi₂Sr₂Ca₂Cu₃O₁₀ (Bi2223) tape with stainless steel reinforcement tape generated 2.5 T in a 15.6 T background magnet. In order to develop a cryogen-free high-field superconducting magnet producing over 20 T, an YBa₂Cu₃O₇ (Y123) coated conductor insert is intended for upgrading the cryogen-free 18 T superconducting magnet. The Bi2223 insert, whose size is 176 mm outer diameter, 90 mm inner diameter, and 252 mm coil height, is now excited at 162 A operation current, and will be replaced by a new Y123 insert. We have already confirmed excellent mechanical properties of 1000 MPa hoop stress tolerance for Y123 coated conductor tape with Hastelloy substrate. This means that we no longer need stainless steel reinforcement for the insert. As a result, an Y123 insert with almost the same size as the Bi2223 insert is designed to generate 7.5 T at 187 A, because the number of turns can be improved extremely. A cryogen-free 23 T superconducting magnet can sufficiently be developed for a long-term experiment at a constant high magnetic field.     

Effective thermal conductivity of the conductor insulation of the ITER toroidal field model coil at operation temperature

During current transients in the toroidal field model coil (TFMC), the radial plates carry eddy currents that generate Joule heat. The heat sink is the forced flow helium cooling of the conductor. Vice versa, during accidents, the radial plates act as a heat sink during the heat up of the conductor. In both cases, the time constant of heat transfer is given by the thermal conductivity of the insulation of the conductor. The code system MAGS (magnet system) is used to recalculate fast discharge experiments of the TFMC at the TOSKA facility. The model takes into account the transient magnetic field, the current in the conductor circuit, in the radial plates and coil case, the ac-losses in the conductor and the helium flow. The results clearly indicate that the power distribution in the radial plate should be taken into account and the thermal conductivity of the insulation is considerably lower than assumed up to now.     

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 a Superconducting Magnet System with Zero Liquid Helium Boil-off

A homogeneous magnetic field superconducting magnet with a cold bore of 250 mm and a central field of 4.3 T has been designed, manufactured, and tested with zero liquid helium boil-off. As a result of magnetic field homogeneity considerations, the magnet is composed of three coaxial coils: one main coil and two compensation coils. All coils are connected in series and can be charged with a single power supply. The magnetic field homogeneity is about ±3.0 % from ?200 mm to 200 mm in axial direction with 86 mm in diameter. The magnet can be operated in persistent mode with a superconducting switch. A two-stage GM cryocooler with a capacity of 1.5 W at 4.2 K was used to cool the superconducting magnet. The cryocooler prevents the liquid helium from boiling off and leads to zero helium loss during static operation. The magnet can be operated in liquid helium circumstance by cooling the gas helium with the cryocooler without additional supply of liquid helium. Under this condition, the magnet is successfully operated up to 4 T without quench. The magnet system can be generating 0.25 L/h liquid helium with the cryocooler by supplying the gas helium without loading the magnet. In this paper, the magnet design, manufacture, mechanical behavior analysis, and the performance test results of the magnet are presented.     

Effect of self-field and current non-uniformity on the voltage-temperature characteristic of the ITER central solenoid insert coil by numerical calculations

For the optimisation of a magnet design with cable-in-conduit conductor (CICC) technology it is essential to comprehend the scaling of the critical current from the separate strand characteristics to the finally assembled cable performance in a coil. Several model coils have been tested in the framework of research for the International Thermonuclear Experimental Reactor (ITER). At present, the scaling of the critical current from the strand to the full cable performance and the apparent decrease of the n-index from strand to cable in the voltage-current curves is not understood. It is important to recognize the mechanisms behind this phenomenon in relation to the cost of the superconducting strand, which is significant in the manufacture of the magnets. Therefore, basic phenomena like the cable conductor self-field, the current unbalance introduced by the non-uniformity of the joints and a possible reversible or irreversible degradation of the voltage current characteristic of a strand during cable manufacture or electromagnetic loading of the magnet have to be considered. The voltage-current characteristic of the strand is extensively explored for the relevant range of magnetic field, temperature and axial strain space. Accordingly a numerical six-element network model is developed to simulate the conditions and behaviour of the last stage cable elements of a full-size ITER conductor. The experimental data, mainly in terms of voltage-current (VI) or -temperature (VT) characteristics, are obtained on the central solenoid insert coil (CSIC) experiment performed in Naka (Japan) in the framework of the research for ITER.The numerical model, which is briefly introduced, is used to study the cable performance by using experimentally obtained cable parameters like inter-strand (and bundle) contact resistance, strand critical current data as a function of magnetic field, temperature and applied axial strain, and external cable self-field measurements by Hall sensors for reconstruction of the current non-uniformity.The effect of a current redistribution due to the cable self-field on the voltage-temperature curve is calculated in correlation with the transverse resistance between the strands and last cabling stage bundles (petals). A realistic unbalanced current distribution is established by introducing non-uniform joints at the extremities of the CS-insert cable.It appears that the cable self-field effect hardly gives any change in the shape of the VT curve but merely a shift towards lower temperature giving a reduction of the current sharing temperature T_cs (10 μV/m) of <0.1 K. For typical CICCs with Cr-coated Nb₃Sn strands, there is practically no current redistribution due to the cable self-field, because of the high inter-strand contact resistance.An unbalanced current distribution also gives an earlier voltage rise in the VT curve, mainly at low levels of the electric field. At a 10 μV/m criterion practically no reduction of the T_cs (<0.1 K) is found by the numerical simulation. However, in the CSIC the experimentally obtained overall reduction in T_cs from strand to cable is 0.7 K for an operating current of 40 kA at 12.5 T background field.According to the results of the numerical simulation, the cable self-field effect and the non-uniform current distribution, which is unavoidably caused by the joints, cannot explain the early voltage rise and low n-index in the VT curve of the CS-insert coil. It is very likely that electromagnetic forces play a role in causing reversible degradation in critical current or even irreversible due to strand (filament) damage. Neither can it be excluded that strand deformation during cabling has an impact on the final conductor performance as well. Therefore additional effort is required in detailed 3D modeling of the possible strand deformations inside a cable and the impact it has on the strand performance by experimental verification on strand level.     

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.     

Design of Power Supplies for the Pulsed High Magnetic Field Facility at HUST

Two types of pulsed power supply, a modular 12 MJ/25 kV capacitor bank and a 100 MVA flywheel pulsed generator, are under construction for the pulsed high magnetic field facility at the Huazhong University of Science and Technology (HUST) in Wuhan, China. The capacitor bank consists of 11 independent 1 MJ modules with a short circuit current of 40 kA each and 2 independent 0.5 MJ modules for 50 kA each. The bank is used to energize coils for magnetic fields in the 50–80 T range with pulse duration from 15 to 200 ms. The pulsed flywheel-alternator is used to energize a 50 T/100 ms long-pulse magnet via two 12-pulse power converter modules. Each converter module is designed to operate in the 95 to 66 Hz frequency operation range of the generator and can provide a no-load voltage of 4.6 kV and a full-load voltage of 3.4 kV at the rated current of 20 kA. In this paper the design of these two types of power supply is presented.     

Design and Performance of the First Dual-Coil Magnet at the Wuhan National High Magnetic Field Center

The first 80 T dual-coil magnet was manufactured and tested at the Wuhan National High Magnetic Field Center (WHMFC). The inner coil consists of 8 layers of 2.8 mm × 4.3 mm CuNb microcomposite wire developed in China; the bore diameter is 14 mm and the outer diameter 135 mm. The outer coil was wound directly on the inner coil with 12 layers of 3 mm × 6 mm soft copper. Each conductor layer of both coils was reinforced by Zylon/epoxy composite. The inner and outer coil were driven by a 1.6 MJ/5.12 mF capacitor bank and by eight 1 MJ/3.2 mF modules, respectively. At the voltage of 14.3 kV for the inner coil and 22 kV for the outer coil, the inner and outer coils produced peak fields of 48.5 T and 34.5 T respectively, which gave a total field of 83 T. This was the first combined operation of the new capacitor banks installed at the WHMFC. We present details of the design, manufacture and test of the dual-coil magnet and discuss crucial material properties. Based on this experience, a second dual-coil magnet will be designed; the enhanced design will be discussed. With the total energy of 12.6 MJ, peak field up to 90 T is expected.     

Tohoku High Magnetic Field Research Activities Using Cryogen-Free Superconducting Magnets

In order to realize a hybrid magnet with no need of liquid helium for operation, we have designed a 28 T cryogen-free hybrid magnet with a 32 mm room temperature experimental bore, and actually tested to generate the peak field of 27.5 T. An easy-to-operate cryogen-free hybrid magnet is now operating for basic research in high fields up to 27 T at low temperatures down to 40 mK, using a dilution refrigerator. In addition, we are intending to develop a 100 mm wide bore water-cooled resistive insert magnet combined with a cryogen-free superconducting outsert magnet for X-ray diffraction measurements in steady magnetic fields up to 20 T. It found that YBa₂Cu₃O₇ (Y123) coated conductor tape with Hastelloy substrate has the excellent mechanical property of 1000 MPa hoop stress tolerance. We are carrying out the research and development of a 30 T all superconducting magnet immersed in liquid helium and a 23 T cryogen-free superconducting magnet, employing Y123 tape.     

Frequency effects in a pancake coil

An aluminum pancake coil is designed to amplify pulsed power in the range of 100-MW to a 100-mΩ electro-thermal load. For the tradeoff between the coil energy density limit and energy discharge efficiency, the frequency dependence of its current and magnetic field distributions is calculated in the range of DC to 1 kHz. Near DC magnetic field measurements are compared to the calculated results. Energy efficiency of about 85% has been measured for the discharge of 1.6 kA from the coil to a linear resistive load in the range of 40-400 mΩ     

First experience with the new Coupling Loss Induced Quench system

New-generation high-field superconducting magnets pose a challenge relating to the protection of the coil winding pack in the case of a quench. The high stored energy per unit volume calls for a very efficient quench detection and fast quench propagation in order to avoid damage due to overheating.A new protection system called Coupling-Loss Induced Quench (CLIQ) was recently developed and tested at CERN. This method provokes a fast change in the magnet transport current by means of a capacitive discharge. The resulting change in the local magnetic field induces inter-filament and inter-strand coupling losses which heat up the superconductor and eventually initiate a quench in a large fraction of the coil winding pack.The method is extensively tested on a Nb–Ti single-wire test solenoid magnet in the CERN Cryogenic Laboratory in order to assess its performance, optimize its operating parameters, and study new electrical configurations. Each parameter is thoroughly analyzed and its impact on the quench efficiency highlighted.Furthermore, an alternative method is also considered, based on a CLIQ discharge through a resistive coil magnetically coupled with the solenoid but external to it. Due to the strong coupling between the external coil and the magnet, the oscillating current in the external coil changes the magnetic field in the solenoid strands and thus generates coupling losses in the strands. Although for a given charging voltage this configuration usually yields poorer quench performance than a standard CLIQ discharge, it has the advantage of being electrically insulated from the solenoid coil, and thus it can work with much higher voltage.     

ITER CC导体接头测试装置设计与研制

根据国际热核实验反应堆(ITER)校正场线圈(CC)导体接头低温电阻的测试要求,设计并研制了一套用于超导导体接头的低温测试装置。该装置主要包括10 kA超导变压器、低温测试杜瓦、磁体失超保护系统和数据采集系统等。超导变压器的初级线圈及次级线圈采用LHe浸泡的方式进行冷却。超导变压器初级线圈电流引线采用常规铜电流引线,为增加铜的传热面积,采用编织铜引线代替铜棒引线。初级线圈外接磁体电源,利用电磁感应原理,在次级回路感应出超导导体接头测试所需的电流。已经成功进行了一次CC导体接头的低温实验,接头电阻的测试结果分别为8.4纳欧姆和9.3纳欧姆。     

Critical current test results of 13 T-46 kA Nb3Al cable-in-conduit conductor

In the framework of ITER-EDA, a 13 T-46 kA Nb₃Al conductor with stainless steel jacket has been developed in order to demonstrate applicability of an Nb₃Al conductor with react-and-wind technique to ITER-TF coils. Using a 3.5 m sample consisting of a pair of conductors with 0% and 0.4% bending strain, the critical current performances of the Nb₃Al conductors were studied to verify that the conductor achieves the expected performance and the bending strain of 0.4% does not originate degradation. The critical currents were measured at background magnetic fields of 7, 9, 10 and 11 T at temperatures from 6 to 9 K. The expected critical currents were evaluated taking into account the variation of the strain in the cross-section due to the bending strain as well as self-field and non-uniform current distribution as results of an imbalance in the joint resistance and inductances. The calculation results indicate that the current distribution is almost uniform and the experimental results showed good agreement with the expected critical currents. Accordingly, we can conclude that the fabrication process of this conductor is appropriate and the react-and-wind technique using the Nb₃Al conductor is applicable to ITER-TF coils. In addition, the critical current of the Nb₃Al conductor is expected to be 108 kA at 13 T and 4.5 K, resulting in a sufficient margin against the nominal current of 46 kA. Furthermore, it was found that the decrease in the critical current by thermal strain can be made small by applying the bending strain to the conductor so as to reduce the compressive strain at higher fields, i.e. inner side of the coil, in the conductor cross-section.     

The High Field Magnet Laboratory at Radboud University Nijmegen

The High Field Magnet Laboratory is dedicated to materials research in the highest continuous fields, up to 33 T, generated with resistive magnets. Circulating cold water evacuates the heat losses incurred when the coils are operating at a voltage drop of up to 500 V at the maximum current of 40 kA. There are three 20 MW resistive magnets with bore sizes of 32 and 50 mm, and a 50 mm bore hybrid magnet system allowing uninterrupted measurements at 30 T for many hours at a time. The infrastructure represents a major investment and has been in operation since 2003. At the moment HFML is extending its capabilities with the construction of a THz FEL. In this paper we will describe the facilities and the possibilities to perform a wide range of experiments in materials research, and show some highlights of the research performed. Even higher fields can be made with a background field provided by a large-bore superconducting magnet: in this paper we will also present the construction of a hybrid magnet system that will generate fields to 45 T.     

Design of a 60 T Quasi-continuous Magnetic Field System with a Hybrid Capacitor/Rectifier Power Supply

At the Wuhan National High Magnetic Field Center, a 135 MW rectifier power supply is being installed nearby a 11 MJ capacitor bank power supply. By combining the two power supplies, a 60 T / 100 ms quasi-continuous magnetic field can be achieved in a monolithic copper coil magnet with a 22 mm diameter bore. Comsol Multiphysics 3.5a and Matlab 7.11.0 were adopted to verify the performance of the magnet and the hybrid power supply system. Details of the designed magnet, the power supply and the simulation results are presented.     

Designing an efficient resistive magnet for magnetic resonance imaging

We present an alternative procedure to design a 0.1 T resistive magnet for magnetic resonance imaging. The procedure considers the conductor to be uniformly located over the cylindrical surface and treats it as coil elements. It applies the linear programming method with upper and lower bounds to constrain the current density to a fixed value in order to produce a desired magnetic field over a region of interest. The approach minimizes the power and preserves the predefined homogeneity, resulting in spatial clusters that define the coil's magnet. We demonstrate the method in a practical design situation.     

Experimental study on a Nb3Al insert coil under high magnetic field

Nb₃Al is one of the most promising superconductors to replace Nb₃Sn in large scale, high field superconducting magnet. Since the complicated conductor manufacturing process, long and stable Nb₃Al conductor is difficult to acquire in a commercial scale. Based on a 70 m length of Nb–Al precursor conductor, we designed and fabricated a Nb₃Al coil. The coil winding, low temperature diffusion heat treatment and epoxy impregnation are described in detail. The finished Nb₃Al coil is tested as an insert in a background magnet. The test is performed at the background field from 7 T to 15 T. The test results are analyzed and presented in this paper.     

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.     

Test Coils Made of Tl-1223 Tapes

Silver sheathed Tl-1223 tapes were prepared by a powder-in-tube process. The critical current density of short samples was 18 kA/cm² at 77 K. Longer tapes up to 1.2 m, prepared by sequential pressing, had a critical current density of 12 kA/cm². From these tapes we have wound two coils. A solenoid coil with 5 windings was made of 8 tapes with a total length of 4.5 m. At 77 K the critical current of the coil was 23 A in the self generated magnetic field (18 Gauss at the centre of the coil). Using an iron yoke the critical current remained at 22 A while the generated magnetic field increased to 120 Gauss. A pancake coil with 15 windings, made of 5 tapes with a total length of 5 m, generated a magnetic field of 149 Gauss at the critical current of 12 A. From measurements of the critical current density of our tapes in applied magnetic fields, we conclude that coils made of Tl-1223 tapes can be used to generate higher magnetic fields at 77 K.     

Ramp-rate limitation experiment using induced current method. Part 1: experimental results

This paper describes a methodology of performing ramp-rate limitation experiments using only a background magnet without a power supply for the tested cable. Three-strand U-shape cable-in-conduit conductor (CICC) samples were prepared with various parameters; three different surface contact resistance conditions and two hybrid cables with a pure copper wire or a stainless steel wire. Samples showed distinctive ramp-rate limitation phenomena with very sharp and sensitive transition between no-quench and quench results. Multiple quench-recovery processes during the continuous magnetic field ramp were also observed due to fast recovery of the sample after quench. The induced strand-to-strand loop current is believed to play the most important role in a quench of CICC during the ramped field.