Low-temperature Superconducting Coils Research Articles

Quench Analysis of an LTS Quadrupole Triplet Magnet System for the IBS RAON In-Flight Fragment Separator

In this paper we present quench analysis results of a Low-Temperature Superconducting (LTS) quadrupole triplet magnet system, a part of the In-flight Fragment (IF) separator of a heavy ion linear accelerator complex, named RAON, currently being constructed by the Institute of Basic Science (IBS). This magnet system is composed of three quadrupole magnets: a triplet, surrounded by iron yokes and embedding hexapole/octupole LTS coils for field correction. The magnet will be operated at 4.2 K in liquid helium. For reliable and safe operation of this complex superconducting system, quench and protection analysis with possible failure scenarios must be performed. In this paper, we first discuss probable quench scenarios and then present results of the quench propagation analysis on: 1) coil currents and voltages by multi-coil model circuit analysis; and 2) simulated temperature distribution inside each coil. Our quench analysis results show that the maximum voltage and temperature in each coil are below safety limits, 2000 V and 150 K, respectively, and confirm that this quadruple triplet magnet system is self-protecting.

Screening Current-Induced Magnetic Field in a Noninsulated GdBCO HTS Coil for a 24 T All-Superconducting Magnet

A 24 T all-superconducting magnet, consisting of a 9 T GdBCO high-temperature superconducting (HTS) insert and 15 T low-temperature superconducting (LTS) outer coils, has been manufactured and tested at the Institute of Electrical Engineering, Chinese Academy of Sciences. A central magnetic field reduction and drift in the superconducting magnet system were caused by the screening current induced in the GdBCO HTS coil. Therefore, it is crucial to estimate the screening current-induced magnetic field in the HTS coil. In this paper, the magnetic field generated by the screening current was numerically calculated based on magnetization analysis. Three related experiments were performed at 4.2 K to verify the simulations: First, charging the HTS coil at different ramping rates; second, charging the LTS coils with central field of 10.82 T first and the HTS coil subsequently; and third, charging the LTS coils with central field of 15 T first and the HTS coil subsequently. The calculation results show a good agreement with the measured data.

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.

AC loss evaluation of an HTS insert for high field magnet cooled by cryocoolers

AC losses in a high temperature superconducting (HTS) insert coil for 25-T cryogen-free superconducting magnet during its initial energization are numerically calculated under the assumption of slab approximation. The HTS insert consisting of 68 single pancakes wound using coated conductors generates a central magnetic field of 11.5T in addition to the contribution of 14.0T from a set of low temperature superconducting (LTS) outsert coils. Both the HTS insert and the LTS coils are cooled using cryocoolers, and energized simultaneously up to the central field of 25.5T with a constant ramp rate for 60min. The influences of the magnitudes and orientations of locally applied magnetic fields, magnetic interactions between turns and transport currents flowing in the windings are taken into account in the calculations of AC losses. The locally applied fields are separated into axial and radial components, and the individual contributions of these field components to the AC losses are simply summed up to obtain the total losses. The AC losses due to the axial fields become major in the beginning of energization, whereas the total losses monotonically increase with time after the AC losses due to the radial fields become major.

Design of a Vector Magnet Generating Up to 3 T With Three-Axis Orientation

The low-temperature superconducting (LTS) coils are designed to produce strong magnetic fields at the working temperature of liquid helium. Some kinds of them have been created to produce strong and homogeneous magnetic fields, as in magnetic resonance imaging systems. However, the critical and mechanical parameters of superconducting material limit the performances of the coils. Most of the time, LTS coils are built to produce a homogeneous magnetic flux density in one single direction. Consequently, it is necessary to rotate the subject or the sample, when a study is needed depending on the magnetic field orientation. This is not suitable for special kinds of experiments involving high-temperature superconducting systems or biomedical applications, in which the subject must not be rotated. In order to satisfy these requirements, we have designed a new coil setup. The design was made using a 3-D electromagnetic software in order to calculate the performances of the coils system, to optimize the magnetic field homogeneity and to find a cost effective solution. The first results reported in this paper concern the design of a magnet of three Helmholtz coils, each of which generates a unidirectional magnetic field ( $x$ , $y$ , and $z$ ). Each Helmholtz coil can generate 1.73 T; thus, the flux density magnitude in the center of system could reach 3 T. The price of the all system, including the cryostat, is around 45 k $\geneuro$ .

Testing of stacked pancake coils for a cryogen-free 25 T superconducting magnet

A cryogen-free 25-T superconducting magnet with a 52-mm room temperature bore consists of an 11-T REBa 2 Cu 3 O 7-δ (REBCO) insert coil and outer 14-T lowtemperature superconducting (LTS) coils. The REBCO insert coil is composed of a stack of 56 single pancake coils. The inner and outer diameters of the REBCO insert coil are 102 and 263 mm, respectively, and the total conductor length is about 14 km. The REBCO insert coil is cooled by circulating helium gas using both single-stage and two-stage Gifford-McMahon cryocoolers, and the LTS coils are cooled by two Gifford-McMahon/Joule-Thomson cryocoolers. The maximum hoop stress of the REBCO insert coil was estimated to be 387 MPa when the central magnetic field was 25 T. If thermal runaway occurs in a conduction-cooled system, the REBCO coils will almost certainly be burned out. Therefore, the coil should have no damaged area in the winding because a damaged area would generate heat locally, eventually resulting in thermal runaway. In order to avoid the degradation of the coil due to thermal stress, the coil was divided into all winding parts in the radial direction. In practice, the coil was divided by inserting a polyimide tape coated with a fluorine resin, which has low adhesive strength, into the epoxy resin at every turn. Before fabricating and testing the actual REBCO insert coil, two kinds of coils were fabricated and tested in order to evaluate the validity of the winding method. Two stacked pancake coils wound with REBCO-coated conductors were fabricated and tested in liquid helium with a 4-T background magnetic field in order to evaluate whether the coil could withstand the hoop stress of over 387 MPa. Moreover, we fabricated and tested a demonstration coil which was composed of a stack of four single pancake coils with the same size as the pancake coils used for the REBCO insert coil in order to evaluate whether the coil could withstand the thermal stress and the electromagnetic force.

A Long Charging Delay for a No-Insulation REBCO Layer-Wound Coil and its Influence on Operation with Outer LTS coils

This paper presents a long charging delay for a no-insulation (NI) REBCO layer-wound (LW) coil and an electromagnetic-induced thermal runaway, resulting in damages on the coil. A small NI REBCO LW coil with a self-inductance of 1.62 mH showed a charging delay time constant of ∼3000 s at 4.2 K. The coil was installed in the cold bore of a low-temperature superconducting (LTS) magnet, and the LTS outer coil was charged. Although the circuit of the NI REBCO LW coil was opened, the induced current inside the coil screened the magnetic field generated by the outer coils. Eventually, the NI REBCO LW coil showed a thermal runaway during charging the outer coils. After the experiment, we found burnouts of the REBCO conductor near the electrodes and buckling of the conductor for all the layers of the winding. The present results point out the technical problem and the worst case for a NI REBCO LW coil, which must be considered.

Design of a Cooling System for a REBCO Insert Coil in a Cryogen-Free 25 T Superconducting Magnet

A cryogen-free 20 T superconducting magnet with a 52 mm room-temperature bore was developed and installed in Tohoku University in 2013. This magnet consists of a Bi2223 insert coil, which generates 4.5 T, and outer low-temperature superconducting (LTS) coils. Both coils were cooled by a GM/JT cryocooler with 4.2 W-class cooling capacity at 4.3 K. To generate a higher magnetic field, a new cryogen-free 25 T superconducting magnet using a REBCO insert coil, which generates 11.5 T, and new outer LTS coils is now under development. The magnetic field contribution of this REBCO insert coil is considerably higher than that of the previous Bi2223 insert coil, and the ac-loss of the insert coil during field ramping rises to approximately 9.7 W. The LTS coils have to operate at about 4 K, but the REBCO coil can operate at various temperatures above 4 K. In addition, the cooling capacity of a GM cryocooler is greater than that of a GM/JT cryocooler around 10 K. Thus, the REBCO insert coil is cooled to about 10 K by using two GM cryocoolers, independently of the LTS coils, which are cooled by two GM/JT cryocoolers. To protect the cryocoolers from the leakage field of the magnet, the two GM cryocoolers cool circulating helium gas through heat exchangers, and the gas is transported over a long distance to another heat exchanger provided for the REBCO insert coil. The maximum temperature of the REBCO insert coil was calculated under the most severe condition where an insert coil ac-loss of 9.7 W was generated continuously. And it was confirmed to be less than the target maximum temperature of 12 K.

Design of a REBCO Insert Coil for a Cryogen-Free 25-T Superconducting Magnet

A cryogen-free 25-T superconducting magnet with a 52-mm room-temperature bore consisting of an 11.5-T REBa <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">7-δ</sub> (REBCO) insert coil and outer 14-T low-temperature superconducting (LTS) coils is currently under development. The REBCO insert coil is composed of a stack of 68 single pancake coils. The inner and outer diameters of the REBCO insert coil are 102 and 276 mm, respectively; and the total conductor length is 17.6 km. The maximum hoop stress of the REBCO insert coil was estimated to be 455 MPa when the central magnetic field was 25.5 T. The REBCO insert coil is cooled by circulating helium gas using both single- and two-stage GM cryocoolers, and the LTS coils are cooled by Gifford–McMahon/Joule–Thomson cryocoolers. If thermal runaway occurs in a conduction-cooled system, the REBCO coil will almost certainly be burned out. Therefore, it is important to avoid thermal runaway of the REBCO insert coil under all operating conditions and protected operations. As operating conditions of the REBCO insert coil, quenching of the LTS coils should be assumed in the severest situation, since the current of the REBCO insert coil will increase after the LTS coils quench. From the results of quench analysis of the LTS coils and circuit simulations, it was confirmed that thermal runaway can be avoided by setting the intermediate potential of the REBCO coil to the ground potential and by using an external dump resistance of 11 Ω.

High Temperature Superconducting YBCO Insert for 25 T Full Superconducting Magnet

A 25 T superconducting magnet with a 10 T YBCO high temperature superconducting (HTS) insert and a 15 T low temperature superconducting (LTS) ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">$\hbox{Nb}_{3}\hbox{Sn} $</tex-math></inline-formula> and NbTi) outsert is fabricated. To design the superconducting magnet system, an optimal design software which combined the advantages of global optimization and local optimization has been proposed to study the number and type of coils to affect the final optimal results, and some important characteristics, for example, coil hoop stress and operating margin and quench characteristics are studied. The high temperature superconductor YBCO inserts with the inner diameter of 41 mm, the height of 115 mm, and outer diameter of 124 mm have been designed and fabricated with the operating temperature at 4.2 K. The outserts of NbTi and <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="TeX">$\hbox{Nb}_{3}\hbox{Sn} $</tex-math></inline-formula> with the inner diameter, outer diameter and height of 180.22, 443, and 400.5 mm, respectively, have been fabricated and tested. It reaches to the operating magnetic field at 15 T. The combined HTS and LTS coils can generate a central field of 25 T at 4.2 K. In this paper, the 25 T superconducting magnet with LTS and HTS coils are reported.

Concept of a Cryogenic System for a Cryogen-Free 25 T Superconducting Magnet

A cryogen-free 25 T superconducting magnet using a ReBCO insert coil that generates 11.5 T in a 14 T background field of outer low-temperature superconducting (LTS) coils is currently under development. The AC loss of the insert coil during field ramping is approximately 8.8W, which is difficult to dissipate at the operating temperature of the LTS coils (4K). However, since a ReBCO coil can operate at a temperature above 4K, the ReBCO insert coil is cooled to about 10K by two GM cryocoolers, and the LTS coils are independently cooled by two GM/JT cryocoolers. Two GM cryocoolers cool a circulating helium gas through heat exchangers, and the gas is transported over a long distance to the cold stage located on the ReBCO insert coil, in order to protect the cryocoolers from the leakage field of high magnetic fields. The temperature difference of the 2nd cold stage of the GM cryocoolers and the insert coil can be reduced by increasing the gas flow rate. However, at the same time, the heat loss of the heat exchangers increases, and the temperature of the second cold stage is raised. Therefore, the gas flow rate is optimized to minimize the operating temperature of the ReBCO insert coil by using a flow controller and a bypass circuit connected to a buffer tank.

Partial-Insulation Winding Technique for NbTi Coils

In 2010, we reported, for the first time, the no-insulation (NI) winding technique for high-temperature superconducting (HTS) pancake coils. Based on our test results of small NbTi NI coils, reported in 2011, the key benefits, i.e., enhanced mechanical integrity, compactness, and thermal stability, of the NI winding technique, appear intact for low-temperature superconducting (LTS) windings. However, the intrinsic charging delay observed in NI coils, caused by the lack of insulation, is more pronounced in LTS coils of a bare round wire than in HTS pancakes of thin, wide tape. Thus, to significantly reduce the charging delay in LTS coils of a bare round wire, we proposed a partial-insulation (PI) winding technique, a variation of the NI technique. In the PI winding of a bare round wire, a thin insulation sheet is introduced every few layers-note that in the PI winding, there are no turn-to-turn insulations. This paper reports results, experimental and analytical, of the PI winding technique in which bare-round-wire NI and PI coils were prepared to quantify the effects of PI winding technique. Three LTS coils of the identical dimension and magnet constant were wound with 0.4 mm diameter NbTi mono-filament wire and tested in a bath of liquid helium at 4.2 K, respectively, with three winding techniques: insulated (INS); NI; and PI. We analyzed the experimental results by applying an equivalent circuit model that had earlier been successfully applied to another set of experimental results. A graph model of resistance matrix was applied to estimate characteristic resistance of both NI and PI coils.

Quench Detection Method for Cryocooler-Cooled YBCO Pancake Coil for SMES

In this study, we focus on a quench detection method for a cryocooler-cooled double pancake coil wound with an YBCO bundle conductor for superconducting magnetic energy storage. Because the local normal-zone propagation velocity is significantly slower in a high-temperature superconducting coil than that in a low-temperature superconducting coil, it is difficult to detect the local normal transition using a voltage between both coil ends. In the application of an YBCO coil for superconducting magnetic energy storage, quench detection is considered to be more difficult because of the noise of the converter or other equipment. Therefore, the normal transition and quench must be detected using some other method. A current detection method, which measures the current redistribution in the YBCO coil with hall probes, was proposed and verified to be able to determine the normal transition by the numerical simulations in the previous paper. In this study, on the basis of the current detection method, the experiments and numerical simulation were performed on a cryocooler-cooled small model double-pancake coil wound with an YBCO bundle conductor composed of four electrically insulated YBCO tapes. As the results, the current detection method was verified to be effective in detecting the local normal transition; Hall probes were used to measure the magnetic field and to estimate the current in individual YBCO tapes making up the bundle conductor.

Quench Protection Design of an 8-T Magnet Built With Low- and High-Temperature Superconducting Coils

A quench protection system has been proposed for an 8-T split superconducting magnet consisting of low-temperature superconducting (LTS) NbTi coils and high-temperature superconducting (HTS) Bi2223 coils. For the LTS coils, a code has been developed to adjust the circuit parameters and the heater geometry. The calculation method of the internal electric potential in the LTS coil is introduced. For the HTS coils, the temperature and current margins are studied. The voltage signal for quench detection is chosen, and quench simulations on the HTS coil with different heating fluxes and heating zones are performed. The simulation results show that the employment of only the dump resistors can bring down the current and the temperature rise rapidly in the event of a quench. The coupling effect of the LTS and HTS coils is simulated and analyzed.

Quench Behavior and Protection in Cryocooler-Cooled YBCO Pancake Coil for SMES

The thermal behavior of a high-temperature superconducting (HTS) coil is significantly different from that of low-temperature superconducting (LTS) coil because it has a greater volumetric heat capacity at the temperature required for practical use. Therefore, the possibility of quench in HTS coils is much lower than that in LTS coils. In the application of the YBCO coil to Superconducting Magnetic Energy Storage (SMES) system, electrical charging and discharging are repeated; therefore, the superconducting characteristics of the YBCO coated conductor may deteriorate as a result of cyclic subjection to tensile strain. To enhance the reliability and safety of HTS coil, protection scheme assuming a quench is also required for HTS coil. In this study, we focus on the coil wound with YBCO laminated bundle (parallel) conductor supposing SMES application and investigate the characteristics of normal-zone propagation and the thermal behavior within the coil during a quench by using a newly developed computer code based on the finite element method and an equivalent circuit. And we also propose a quench detection method using observation of nonuniform current in YBCO laminated bundle conductor and discuss the validity of the detection method comparing with conventional quench-voltage detection.

Stability and Protection of Coils Wound With YBCO Bundle Conductor

The thermal behavior of a high-temperature superconducting (HTS) coil is significantly different from that of a low-temperature superconducting (LTS) coil. A HTS conductor has a greater volumetric heat capacity at the operating temperature envisaged for practical applications. Therefore, a HTS coil is much less likely to be quenched than a LTS coil by mechanical disturbances such as the heat generated by the cracking of the impregnation material or by the friction resulting from wire movements. However, the HTS conductor is cyclically subjected to tensile strain because electrical charging and discharging are repeated in real applications involving the Superconducting Magnetic Energy Storage (SMES) system. The superconducting characteristics may locally deteriorate due to this cyclic strain. Therefore, to enhance the reliability and safety of the HTS coil, a quench protection scheme is needed. Because the normal-zone propagation velocity is quite low, detecting a non-recovering normal zone is difficult in HTS coils, and quenching produces excessive overheating that may cause the conductor to melt. In this study, we focus on a coil wound with a YBCO bundle conductor used in SMES applications and investigate the redistribution characteristics of the transport current in and the thermal behavior of the coil during a quench; we use a newly developed computer code based on the finite element method (FEM) and an equivalent circuit. We also discuss a protection scheme to dump the magnetic energy stored in the coils on an external resistance connected in parallel.

AC Losses in a Conduction-Cooled LTS Pulse Coil With Stored Energy of 1 MJ for UPS-SMES as Protection From Momentary Voltage Drops

AC losses in the conduction-cooled low temperature superconducting (LTS) pulse coil with stored energy of 1 MJ are estimated. The 1 MJ coil is a superconducting pulse coil for 1 MW, 1 sec UPS-SMES. UPS-SMES is an uninterruptible power supply (UPS) with superconducting magnetic energy storage (SMES) for protection of production lines of an industrial plant or large-scale experimental devices such as a fusion device, from a momentary voltage drop and an instant power failure. The winding conductor for the 1 MJ coil is a NbTi/Cu Rutherford cable, which is extruded with aluminum. The 1 MJ coil was wound by a new twist winding method. A 1 MJ coil was fabricated and cooling and excitation tests were carried out. In this paper, two methods to estimate ac losses in conduction-cooled LTS coils are proposed. One method estimates ac losses in the coil under steady-state conditions. The other method estimates ac losses in the coil under transient-state conditions. For estimation of ac losses in the 1 MJ coil, measurement of temperature in the coil during tests and thermal analysis using the two-dimensional finite element method are compared. These procedures clarified that the 1 MJ coil has low losses.

Validation of the High Performance Conduction-Cooled Prototype LTS Pulse Coil for UPS-SMES

A conduction-cooled low temperature superconducting (LTS) pulse coil has been developed as a key technology for UPS-SMES. We have been developing a 1 MW, 1 s UPS-SMES for a protection from a momentary voltage drop and an instant power failure. A conduction-cooled LTS pulse coil has excellent characteristics, which are adequate for a short-time uninterruptible power supply (UPS). The LTS coil has better cost performance over the HTS coil at present and the conduction cooling has higher reliability and easier operation than the conventional cooling schemes such as pool boiling with liquid helium or forced flow of supercritical helium. To demonstrate the high performances of the LTS pulse coil, we have fabricated a prototype coil with stored energy of 100 kJ and have conducted cooling and excitation tests. The successful performance test results including current shut-off test with a time constant of 1.3 s and repeated excitation of a triangular waveform with high ramp rate are reported

Utilization of bulk high-temperature superconductors for shorter-period synchrotronradiation sources

A new application of high-temperature superconducting (HTS) bulk magnets tosynchrotron radiation (SR) sources, i.e., magnetic devices that generate a periodic field toproduce highly brilliant SR, is presented. Utilization of HTS bulk magnets brings twoadvantages over the existing SR sources: much stronger field than that of conventionalSR sources with rare-earth permanent magnets (REPMs) and higher feasibilitythan existing superconducting SR sources with low-temperature superconductingcoils. Two schemes for the application of HTS bulk magnets to SR sources arepresented. In the first scheme, HTS bulk rings are mounted on REPMs to enhancethe magnetic field. In the second one, HTS bulk magnets are arranged in lineinstead of REPMs. Principles and recent demonstration experiments are described.

Development of UPS-SMES as a Protection From Momentary Voltage Drop

We have been developing the UPS-SMES as a protection from momentary voltage drop and power failure. The superconducting system is suitable as electric power storage for large energy extraction in a short time. The most important feature of superconducting coil system for the UPS-SMES is easy handling and maintenance-free operation. We have selected low temperature superconducting (LTS) coils instead of high temperature superconducting (HTS) coils from the viewpoint of cost and performance. However, it is difficult for the conventional LTS coils to fulfill maintenance-free operation since the cooling methods are either pool boiling with liquid helium or forced flow of supercritical helium. Thus, a conduction cooled LTS pulse coil has been designed as a key component of the UPS-SMES. The development program of 1 MW, 1 sec UPS-SMES is explained.