Bi-2223 Coil Research Articles

Quench and self-protecting behaviour of an intra-layer no-insulation (LNI) REBCO coil at 31.4 T

This paper presents experimental results on a quench of an intra-layer no-insulation (LNI) (RE: rare earth)Ba2Cu3O7−δ (REBCO) coil in a 31.4 T central magnetic field and simulated results on the quench. We have been designing a persistent-mode 1.3 GHz (30.5 T) nuclear magnetic resonance (NMR) magnet with a layer-wound REBCO inner coil. Protection of the REBCO coil from quench is a significant issue and the coil employs the LNI method to obtain self-protecting characteristics. We conducted high-field generation and quench experiments on an LNI-REBCO coil connected to an insulated Bi2Sr2Ca2Cu3O x (Bi-2223) coil under a background magnetic field of 17.2 T as a model of the 1.3 GHz NMR magnet. The coils successfully generated a central magnetic field of 31.4 T. Although the LNI-REBCO coil quenched at 31.4 T, this quench did not cause any degradation to the coil. A numerical simulation showed the current distribution during the quench was non-uniform and changed rapidly over time due to current bypassing through copper sheets between layers, resulting in faster quench propagation than in an insulated REBCO coil. During the quench propagation, the peak temperature (T peak) and the peak hoop stress BzJR (σθ, peak) were calculated to be 330 K and 718 MPa, respectively. These are below critical values that cause degradation. The simulation also showed that the high electrical contact resistivity (ρ ct) of 10 000 µΩ cm2, between REBCO conductors and copper sheets in the LNI-REBCO coil winding, played an important role in protection. When ρ ct was as low as 70 µΩ cm2, the quench propagation became too fast and large additional currents were induced, resulting in an extremely high σθ, peak of 1398 MPa, while the T peak was as low as 75 K. In short, the high ρ ct in the present coil caused a high T peak, but succeeded in suppressing σθ, peak and protecting the coil from the quench.

Cooling Unit for the 500 kV Saturated Iron Core Fault Current Limiter

The high temperature superconducting dc bias winding of the single-phase 500 kV saturated iron core is designed to be cooled with liquid nitrogen down to a temperature of 72 K. To achieve this temperature level, the coolant has to be vaporized below atmospheric pressure. An open cryogenic system was designed and fabricated, which composed of a cryostat, heat insulation pipelines, a liquid nitrogen tank, a control circuit, and a vacuum pump. Since the dc bias winding is composed of two YBCO windings and one Bi-2223 coil, three pairs of current leads are assembled on the cryostat. A control system is used to maintain the liquid nitrogen level and the operation temperature by regulating the pumping rate. According to the testing results, the steady-state heat loss including the cryostat, the conduction heat loss of current lead is about 135 W, and the total heat loss including the Joule heat loss is about 187 W at 77 K.

Enhancement of the Critical Current of Bi-2223 Superconducting Coil by Using of Soft Ferromagnetic Material

The effects of soft ferromagnetic material placed nearby the high-Tc Bi-2223 coil is analyzed. Electro-magnetic simulation have been carried out to find the distributions of the magnetic field of the Bi-2223 coil. The used soft ferromagnetic material could change the distribution of the Bi-2223 coil’s magnetic field, and the transport capability of the Bi-2223 coil changes consequently. According to the simulation results, the thickness of the soft ferromagnetic material plate shows a notable effect on the coil, and the position and width of the plate could present negative effects. Experimental result of a Bi-2223 coil with B30P105 silicon steel plate is consistent with the simulation conclusions, and the critical current of the developed Bi-2223 coil could achieve an improvement of 15% by using of a piece of silicon steel plate with a thickness of 0.3 mm.

Superconducting joint between multi-filamentary Bi2Sr2Ca2Cu3O10+δ tapes based on incongruent melting for NMR and MRI applications

Recently, we proposed a method of forming superconducting joints between high-temperature superconducting wires based on the incongruent melting of an intermedium, called crystalline joint by melted bulk. Using this technique with GdBa2Cu3O7−δ-coated conductors, a superconducting joint with a critical current of 7 A at 77 K was obtained. In the present study, a joint between multi-filamentary Bi2Sr2Ca2Cu3O10+δ (Bi2223) tapes was prepared without an intermedium, employing incongruent melting of Bi2223 filaments in the tapes. A superconducting joint was successfully fabricated, with critical currents of 12.2 A and 177 A at 77 K and 4.2 K, respectively. A Bi2223 coil terminated with the present joint method was operated in persistent current mode and showed an ultra-low field decay rate with a characteristic resistance of 10−12 Ω at 77 and 4.2 K.

First performance test of a 25 T cryogen-free superconducting magnet

A 25 T cryogen-free superconducting magnet (25T-CSM) was developed and installed at the High Field Laboratory for Superconducting Materials (HFLSM), IMR, Tohoku University. The 25T-CSM consists of a high-temperature superconducting (HTS) coil and a low-temperature superconducting (LTS) coil. A high-strength CuNb/Nb3Sn Rutherford cable with a reinforcing stabilizer CuNb composite is adopted for the middle LTS section coil. All the coils were impregnated using an epoxy resin for conduction cooling. Initially, a GdBa2Cu3Oy (Gd123) coil was designed as the HTS insert coil, and then a Bi2Sr2Ca2Cu3Oy (Bi2223) coil was also developed. The HTS insert and the LTS (CuNb/Nb3Sn and NbTi) outsert coils are cooled by two 4K GM and two GM/JT cryocoolers, respectively. The LTS coils successfully generated a central magnetic field of 14 T at an operating current of 854 A without any training quench. The Gd123 coil generated 10.15 T at an operating current of 132.6 A in the absence of a background field. Subsequently, the operating current of the Gd123 insert was increased in a step-by-step manner under a background field of 14 T. The Gd123 coil could be operated up to 124.0 A stably, which corresponds to 23.55 T, but quenched at around 124.6 A (23.61 T). The Bi2223 insert coil using a Ni-alloy reinforced Bi2223 tape successfully generated 11.48 T at an operation current of 204.7A in a stand-alone test and 24.57 T in a background field of 14 T. The differences between the calculated and the measured values of the central magnetic fields are about 0.4 T for the Gd123 insert and 0.1 T for the Bi2223 insert around 24 T.

Study on the Excitation of a Bi-2223 Small Superconducting Coil by a Pulse-Type Magnetic Flux Pump

This paper presents further research on the pulse-type magnetic flux pump (MFP). The pulse-type MFP is used to excite a Bi-2223 double-pancake superconducting coil cooled by a cryocooler. The inductance of the coil is 76 μH. The cryogenic structure of the MFP is optimized by adding a cold frame outside. The MFP is powered by a dc voltage source. The excitation effects on the Bi-2223 coil are investigated by changing the input voltage. The pumping current curves have been obtained with various input voltage from 2.0 to 5.5 V. The fitting calculations show that the saturated current in Bi-2223 high-temperature superconductor (HTS) coil is 200 A when the input voltage is 5 V. The joint resistance is also numerically analyzed, and it is 0.18 μΩ. By measuring the current ripple of the Bi-2223 HTS coil, it is found that the excitation current fluctuation is ±4 A when the input voltage is 5 V. Consequently, it is expected that the pulse-type MFP could compensate the decaying current caused by the low n-index and joint resistance and enable the HTS magnet operating in persistent current mode. Further optimization on the structure and pumping process is still needed to improve the excitation properties of the pulse-type MFP.

AC losses in Bi-2223 Single-Pancake Coils From 72 to 1152 Hz—Modeling and Measurements

Studies of ac losses in superconducting materials, particularly in pancake coils, are of great importance for a better understanding of the superconductivity phenomena and its applications in power systems. Due to different technical difficulties, these studies are usually performed considering one of two approaches: considering superconducting coils of few turns and studying ac losses in a large frequency range versus superconducting coils with a large number of turns but measuring ac losses only in low frequencies. In this paper, ac losses are simulated and verified experimentally in a 128 turn Bi-2223 coil in a range of frequencies from 72 Hz till 1152 Hz. Obtained results show a frequency-dependent behavior. Experimental results are compared to those obtained from two different numerical modeling methods: one using commercial software based on finite-element methods and another using a variational method approach. All obtained results agree, validating the simulations and serving as basis for simulations with larger high temperature superconducting coils.

Degradation of a REBCO Coil Due to Cleavage and Peeling Originating From an Electromagnetic Force

This paper presents the results of a high field generation test for layer-wound HTS coils in a background magnetic field. The test revealed new types of degradation for a REBCO coil caused by an electromagnetic force. A series-connected REBCO coil and a Bi-2223 coil were charged at 4.2 K in a background magnetic field of 17.2 T to generate a central magnetic field of 28.2 T (nuclear-magnetic-resonace frequency of 1.2 GHz). However, a premature normal voltage appeared on the REBCO coil, and the charging was stopped at 25 T. Unwinding the REBCO coil after the experiment found: 1) delamination, due to a cleavage and peeling, of the conductor in a soldered joint inside the winding; and 2) axial movement and edgewise bend deformation of the REBCO conductor at the outermost layer. The keyword of the present degradation is stress concentration, which is also the origin of the more widely known thermal stress-induced degradation.

Shimming for the 1020 MHz LTS/Bi-2223 NMR Magnet

Using a Bi-2223 innermost coil, the world's first NMR magnet with a frequency beyond 1 GHz has been developed and operated at the National Institute for Materials Science during 2014-2015. The existing 920 MHz (21.6 T) NMR magnet was successfully upgraded to a 1030 MHz (24.2 T) magnet by replacing the Nb3Sn innermost coil with a Bi-2223 coil. After charging the magnet to 1020 MHz (24.0 T), a shimming operation was started to obtain the homogeneous magnetic field required for NMR measurements. However, a large magnetic field inhomogeneity appeared, which could not be compensated using conventional shimming methods, i.e., superconducting and room temperature shim coils. Therefore, a new ferromagnetic shimming technology was applied, which achieved powerful and fast-acting field compensation and performed comparably to active shimming. This enabled effective compensation of the magnetic field inhomogeneity, leading to a subsequently excellent NMR resolution test result of 0.7 ppb. This NMR resolution enables NMR measurements for a membrane protein sample.

Successful Upgrading of 920-MHz NMR Superconducting Magnet to 1020 MHz Using Bi-2223 Innermost Coil

We succeeded in upgrading the 920-MHz nuclear magnetic resonance (NMR) superconducting magnet (21.6 T) to 1020 MHz (24.0 T) by replacing the innermost Nb3Sn coil with a (Bi,Pb)2Sr2Ca2Cu3O10 (Bi-2223) coil. The 920-MHz NMR spectrometer had been installed in the National Institute for Materials Science, Tsukuba, Japan, in 2001. It has been operated in the persistent mode for six years. The upgrading project started in 2006. A Bi-2223 coil was developed as the innermost coil instead of the Nb3Sn one. The newly installed Bi-2223 innermost coil is connected to Nb3Sn and NbTi coils in series. The upgraded NMR magnet was seriously damaged by the Great East Japan Earthquake in March 2011. After more than two years of restoration and additional improvements of current leads and the power supply system, the magnet was cooled down to below 1.8 K in August 2014. The magnet successfully generated 24.0 T, corresponding to 1020 MHz, in October 2014. To achieve the required homogeneity and stability of the magnetic field, not only superconducting and room-temperature shim coils but also ferromagnetic shims were used. The 1020-MHz superconducting NMR magnet has been operated in a power-supply-driven mode for six months.

Performance Test of Cryogen-Free Bi-2223 HTS Dipole Magnet for Beam Line Switching

We have developed a Bi-2223 high-temperature superconducting dipole magnet for beam line switching at the cyclotron facility of the Research Center for Nuclear Physics, Osaka University. Exit beam lines are periodically switched by increasing and decreasing the magnetic field between 0 and 1.6 T with a switching time of 10 s. The magnet is equipped with two sets of Bi-2223 coil assemblies, which are conduction cooled by two 10-K Gifford-McMahon cryocoolers. We evaluated the superconducting property of the Bi-2223 coil assemblies in liquid nitrogen before installation in the cryostat. There were no degradation in wire performance during the assembly process. Magnetic field, strains, and temperatures of the coil assembly were investigated for performance verification after the magnet fabrication. The magnetic field and the temperatures meet specifications, and the deformation of the coil assembly is considered to be successfully suppressed by reinforcing structure. Furthermore, from a viewpoint of temperature, it is indicated that the magnet can make switching time and cycle time faster than that of the specified operation.

A Superconducting Magnetic Energy Exchange Model Based on Circuit-Field-Superconductor-Coupled Method

A superconducting magnetic energy exchange (SMEE) model based on a circuit-field-superconductor-coupled method is presented. The objective power system problems to be solved, e.g., power and load fluctuations, are transformed into equivalent energy exchange demanding values for the superconducting coil used in a superconducting magnetic energy storage (SMES) device. With the model, the buffering effects for the external power and load fluctuations and the superconducting properties of the superconducting coil itself can be obtained equivalently for optimizing and evaluating the designed SMES device. Four coil-current-dependent formulas are derived to estimate the hysteresis loss, coupling current loss, eddy current loss, and flux flow loss of the superconducting coil under different energy exchange operation conditions. A case study has been carried out using a Bi-2223 coil. The obtained results verify the effectiveness of the SMEE model and its feasibility for practical applications.

Transport Characteristics of a Bi-2223 Solenoid Coil in High Magnetic Fields

Transport characteristic of a (Bi, Pb) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Sr <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Ca <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">10</sub> (Bi-2223) solenoid coil was investigated in liquid helium in high magnetic fields up to 17 T. The solenoid coil was fabricated using the silver sheathed Bi-2223 superconducting tape, which has 2.8 mm in width and 0.32 mm in thickness, including a 0.05-mm-thick copper alloy lamination on both sides. The coil (winding pack) inner diameter, outer diameter, height, and number of turns were 65 mm, 83 mm, 60 mm, and 456 turns, respectively. The coil critical current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) was 28 A at 77 K in self-field. Two noise-canceling coils were set inside and outside of the Bi-2223 coil to reduce the inductive noise. The measurement setup of the noise-canceling coils is similar to that of the magnetization measurement using pick-up coils. Transport measurements were performed at 4.2 K (liquid helium) in the 20-T resistive magnet with 170-mm bore in LNCMI-Grenoble. The coil I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> was 151 A at 17 T, 4.2 K. It was also demonstrated that the noise-canceling coil noticeably reduced the inductive noise by 1/10, which made possible higher resolution voltage measurements.

Non-destructive Detection of Normal Transitions in High Temperature Superconducting Coil

High temperature superconducting (HTS) coils require reliable quench monitoring systems for safety operations. Also, locating of normal transition areas is important for developing reliable HTS coils. Resistive voltage measurement method is universally used for the quench detection and the location. The method needs many voltage taps soldered to superconducting windings and therefore insulations of superconducting wires have to be removed for the soldering and then it has high risks in terms of high voltage sparks. In this paper, the authors present a new detection method of normal transitions by using capacitor taps which can detect normal transitions regardless of the insulations (by no destruction) and has no risks about the high voltage sparks. In this method, two electro-conductive sheets are attached on a surface of a superconducting coil and are connected with an outside capacitor. Then two capacitors are created on the surface of the superconducting coil by sand iching the insulation between each electro-conductive sheet and the superconducting wire. The normal transitions can be detected as a voltage of the outside capacitor by voltage dividing with the three capacitors. A demonstration test was carried out for a Bi2223 coil and the quench detection and the location were successfully achieved.

高温超伝導コイルで発生する常伝導領域の非接触型検出法

In our previous paper, we proposed a new indirect method of detecting normal transitions occurring in hightemperature superconducting coils. In this method, sets of pick-up coil pairs arranged outside the cryostat are used in order to electrically measure the occurrence of normal transitions during coil operations. This indirect method has benefits such as high reliability, safe detection and easy maintenance. The principle of locally detecting a normal transition is theoretically explained and the validity of this method is clarified experimentally using a Bi-2223 coil. In this paper, it is experimentally confirmed that this measuring method can be used to detect normal transitions in windings of a high-temperature superconducting transformer with 500 VA around which there are small leakage magnetic fields due to an iron core. In addition, a method for detecting normal transitions in windings of the transformer during transformer load changes is proposed and the validity of the method is demonstrated experimentally.

Bi-2223 Innermost Coil for 1.03 GHz NMR Magnet

Because of their high critical fields, high- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconductors (HTS) are considered to be the only solution to dramatically increase the highest fields of NMR magnets. We have successfully demonstrated that a 500 MHz HTS/LTS NMR system with a Bi-2223 innermost coil could be used for solution NMR in a driven-mode operation. As the next step, the upgrade of the 920 MHz NMR system installed at the Tsukuba Magnet Laboratory is underway. The innermost Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coil has been replaced by a Bi-2223 coil. The coil was fabricated as a layer-wound coil using five Bi-2223 conductors reinforced with bronze tapes. It was connected in series with the outer Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn and NbTi coils. The magnet is expected to generate a field of 24.2 T (1.03 GHz of <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> H resonance frequency) at an operating current of 244.4 A. The test using the Bi-2223 coil and the outer Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coils in combination was successfully carried out. The coil has been installed in the 1.03 GHz NMR magnet. Its cooling and operation are scheduled to take place within Fiscal Year 2010.

HTS-NMR: Present Status and Future Plan

Using high-T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> superconductors (HTS) is considered to be the only solution to dramatically increase the highest fields of NMR magnets because of their high critical fields. However, it is not easy to apply HTS to an NMR spectrometer (HTS-NMR) because a persistent-mode operation with HTS cannot satisfy the field stability of 0.01 ppm/h at present. To overcome this problem, we are now developing an HTS-NMR spectrometer in a driven-mode operation. As the first step, a layer-wound coil was fabricated with bronze-reinforced Bi-2223 conductors. Instead of the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn coil, the Bi-2223 coil was installed as the innermost part of an existing NMR magnet. The magnet operated at a field of 11.7 T with a highly stabilized power supply. NMR measurements were carried out, and it was demonstrated that the quality of the multi-dimensional NMR spectra on the protein was equivalent to that obtained with a persistent-mode system. The next step will be to demonstrate its usefulness as a high-field NMR system. The upgrade of the 920 MHz NMR system installed at the Tsukuba Magnet Laboratory is underway. Its innermost coil is scheduled to be replaced by a Bi-2223 layer-wound coil for 2010. Its target field is 24.2 T (1.03 GHz).

Signal-to-Noise Ratio Improvement by Bi-2223 Surface RF Coil in 0.3T MRI System

A Bi-2223 surface RF coil was developed for a 0.3T permanent MRI system to achieve better signal-to-noise ratio (SNR) of images. Based on the finite element method (FEM), the SNR of a copper-made surface coil is analyzed, and the emphasis is put on the coil self-resistance that accounts for much higher percentage of the equivalent noise resistance in low field MRI than that in high field MRI. In order to improve SNR, a surface coil that is made of a Bi-2223 tape is fabricated for a 0.3T permanent MRI system. Phantom images are acquired, and the image SNR by the Bi-2223 coil in liquid nitrogen is improved by up to 35% compared with that by a copper coil at room temperature.

The contactless detection of local normal transitions in superconducting coils by using Poynting’s vector method

We have developed a new monitoring system to detect an unusual event in the superconducting coils without direct contact on the coils, using Poynting’s vector method. In this system, the potential leads and pickup coils are set around the superconducting coils to measure local electric and magnetic fields, respectively. By measuring the sets of magnetic and electric fields, the Poynting’s vectors around the coil can be obtained. An unusual event in the coil can be detected as the result of the change of the Poynting’s vector. This system has no risk of the voltage breakdown which may happen with the balance voltage method, because there is no need of direct contacts on the coil windings. In a previous paper, we have demonstrated that our system can detect the normal transitions in the Bi-2223 coil without direct contact on the coil windings by using a small test system. For our system to be applied to practical devices, it is necessary for the early detection of an unusual event in the coils to be able to detect local normal transitions in the coils. The signal voltages of the small sensors to measure local magnetic and electric fields are small. Although the increase in signals of the pickup coils is attained easily by an increase in the number of turns of the pickup coils, an increase in the signals of the potential lead is not easily attained. In this paper, a new method to amplify the signal of local electric fields around the coil is proposed. The validity of the method has been confirmed by measuring local electric fields around the Bi-2223 coil.

A Quench Monitoring System of Superconducting Coils by Using the Poynting Vector Method

A new quench monitoring system by using the Poynting vector method was proposed. This new system can indirectly detect normal transitions which are suddenly induced during an operation of superconducting coils. In this system, the superconducting coil is surrounded by the thin metallic cylinder composed of several pieces, where the cylinder is electrically insulated from the coil; potential leads to measure local electric fields are directly attached to this cylinder surface and pickup coils to measure local magnetic fields are put on the surface. The Poynting vector distribution around the coil can be obtained by using many sets of both the potential lead and the pickup coil. Since sudden changes of energy flow profiles around the superconducting coil are induced in the event of local normal transitions of the coil winding, such an event of coils can be detected by the change of the Poynting vector. This system has no risk of the destructive voltage breakdown which may happen in case of the existing balance voltage method, because there is no need of direct contacts on the coil windings. In addition, this method is expected to be higher reliability and sensitivity than the acoustic emission method. A demonstration test was carried out of monitoring the quench for the Bi-2223 coil partially immersed in liquid nitrogen, and a sign of normal transitions before the quench was successfully detected by this method.