GdBCO Coils Research Articles

Quench initiation and normal zone propagation characteristics of smart insulated high-temperature superconducting coil

A smart insulation (SI) winding technique utilizing vanadium trioxide (V2O3) as a turn-to-turn insulation material is an effective means of achieving a suitable time constant and thermal stability in high-temperature superconducting (HTS) coils. However, the low electrical conductivity of the V2O3 makes the SI coil vulnerable to damage during quenching. To address this problem, this study explored a mixture of V2O3 and molybdenum (Mo) particles to improve the thermal stability of the SI coil. Two GdBCO coils, insulated with V2O3 and V2O3:Mo, respectively, were fabricated, and quench tests were performed at various operating currents in liquid nitrogen at 77 K. The findings revealed that the V2O3:Mo-insulated coil demonstrated improvements of 82%, 25%, and 160% in the minimum quench energy, and the normal-zone propagation velocities in the longitudinal and transverse directions, respectively, at 60% of the current-carrying capacity of the coil, which are essential performance parameters in the operation of a superconducting magnet.

The Influence of Local Critical Current Degradation on the Quench Characteristics of GdBCO Tapes and Coils

We experimentally investigated the influence of local critical current degradation on the thermal and electromagnetic behavior around the local degradation region and the time-dependent characteristics of the voltage across it. The local critical current degradation was produced by cutting a part of the width of a piece of GdBCO tape. The current that caused the tape to burn out and the voltages across the degradation region were measured in short tape samples and pancake coils made using GdBCO tape. In a sample without copper stabilizer covered with Styrofoam, the voltage rapidly increased over a short time period and the sample burned out soon after the operating current exceeded the local critical current. On the other hand, the tape with the copper stabilizer cooled by an LN 2 was able to stably maintain the voltage even when the operating current largely exceeded the local critical current. The current sharing effect of the copper stabilizer greatly contributed to an improvement in stability because the current sharing effect was much larger than the LN2 cooling effect. The cooling effect from the top and bottom surfaces of the coil was also important and should be improved upon as much as possible to suppress the temperature rise during coil excitation. The heat transfer effect in the radial direction between coil layers was smaller than the LN2 cooling effect.

Thermal and electrical properties of thermal-grease-insulated REBCO superconducting coils with respect to winding tension

This study investigates the thermal and electrical characteristics of a silicon-based grease insulation (GI) GdBCO coil with respect to the winding tension through charge, sudden discharge, and over-current tests. Charge and sudden discharge test results demonstrate that the charging/discharging delay time increases as the winding tension increases; this is because the characteristic resistance of the coil decreases due to the reduced contact resistance. The over-current test results confirm that the thermal/electrical stabilities of the GI coil are considerably enhanced with an increased winding tension resulting from improved thermal contact and the decrease in the electrical contact resistance between the turn-to-turn layers of the coil. Thus, as the winding tension increases, the charging/discharging rates decrease whereas the thermal/electrical stabilities improve. Overall, selecting the appropriate winding tension for a GI coil is critical for achieving thermal/electrical stabilities, as well as ameliorating the charging/discharging delay phenomenon generally observed in a no-insulation coil.

Study on Thermal-Quench Behaviors of GdBCO Coils Wound With Silicon Grease as an Insulation Material

This paper investigated quench initiation and propagation characteristics of grease insulation coils by conducting thermal-quench tests for two GdBCO single-pancake coils, namely a coil co-wound with silicon grease (termed as the SiG coil) and the other coil co-wound with Kapton tape (termed as the INS coil), as turn-to-turn insulation. The test results confirmed that the SiG coil exhibited better thermal and electrical stabilities when compared with the INS coil because the operating current could be bypassed in the radial direction through the turn-to-turn contacts when a local hot spot was generated. The SiG coil had superior thermal and electrical stabilities. However, a nonrecovering resistive zone could be generated because excessive Joule heat energy could be induced by radial current flow due to the existence of radial resistance that was mainly generated by silicon grease and nonsuperconducting materials including the substrate, stabilizer, and buffer layers within the high-temperature superconductor (HTS). Therefore, it is essential to consider critical Joule heat energy that is influenced by operating current and stored magnetic energy as well as the radial resistance to achieve self-protective 2G HTS coils.

Performance of the Fast-Ramping High Temperature Superconducting Magnet System for an Active Magnetic Regenerator

Fast magnetic field alternation is indispensable for continuous magnetic refrigeration. An active magnetic regenerative refrigerator (AMRR) utilizes magnetocaloric effect of magnetic materials which occurs during magnetization and demagnetization processes. A conduction cooled high temperature superconducting (HTS) magnet can be one of the prospective candidates as an alternating magnetic field generator. This paper describes the development effort of the cryogen-free HTS magnet for the AMRR. The magnet consists of twelve double pancake GdBCO coils which are insulated with polyimide tape. A two-stage GM cryocooler was employed to cool down the magnet. The critical current of the magnet was measured at the operating temperature before alternating current (AC) operation. In order to remove heat produced by AC loss with small temperature difference, thermal links between the cryocooler and the magnet were carefully designed. This paper presents the test results, AC loss analysis of the HTS magnet in the AMRR system. Maximum central magnetic field of 3 T (150 A) was achieved with the maximum ramping rate of 1 T/s (50 A/s). The AC loss was measured as 11.2 W at the operating conditions and the generated heate was effectively removed by the cryocooler. The AC loss was predicted by the numerical simulation and the simulation results were compared with the experimental results.

AC Loss Characteristics in kHz frequency band of a High Temperature Superconductor Coil for a Wireless Power Transmission System

Improvement of transmission efficiency is an important issue in a large-capacity wireless power transmission (WPT) system using copper coils for automobiles and electric railways. We have investigated the application of a high-temperature superconducting (HTS) coil to a high-power WPT system operating in the frequency region of 10 kHz. We measured the transport current losses of GdBCO and Bi2223 tapes and the total ac losses of GdBCO and Bi2223 coils in the kilohertz frequency band. We analyzed the current distribution within the tape using a finite element method (FEM) program. When the current load factor was low and the frequency was above 1 kHz, the ac losses per cycle increased with the frequency. This is because the eddy current loss in stabilized layers was larger than the hysteresis loss at the frequency of above 1 kHz. One the other hand, the ac losses per cycle were constant regardless of the frequency at the high-current load factor. This is because the hysteresis loss was larger than the eddy current loss regardless of the frequency. We investigated the maximum transmission efficiency at a resonance frequency in an HTS WPT system and a Cu WPT system. Although the quality factor of the HTS coil gradually decreased with the frequency and the operating current, the maximum transmission efficiency of the HTS WPT system was much higher than that of the Cu WPT system. We believe that the HTS coil is suitable for a large-capacity and high-efficiency WPT system in the frequency region below 10 kHz.

Investigation on quench initiation and propagation characteristics of GdBCO coil co-wound with a stainless steel tape as turn-to-turn metallic insulation.

This paper investigates the quench initiation and propagation characteristics of a metallic insulation (MI) coil by conducting thermal quench tests for a GdBCO single-pancake coil co-wound with a stainless steel tape as the turn-to-turn MI. The test results confirmed that the MI coil exhibited superior thermal and electrical stabilities compared to the conventional coils co-wound with organic insulation material because the operating current could flow along the radial direction due to the existence of a turn-to-turn contact when a local hot spot was generated. The results of the quench test at a heater current (Ih) of 12, 13, and 14 A indicate that the MI coil possesses a self-protecting characteristic resulting from the "current bypass" through the turn-to-turn contact. However, the test coil was not self-protecting at Ih = 15 A because the Joule heat energy generated by the radial current flow was not completely dissipated due to the characteristic resistance of the metallic insulation tape and the non-superconducting materials, including the substrate, stabilizer, and buffer layers within the high-temperature superconductor (HTS) tape. Even though the MI coil possesses superior thermal and electrical stability relative to those of conventional HTS coils co-wound with an organic material as turn-to-turn insulation, it is essential to consider the critical role of the Joule heat energy resulting from the operating current and stored magnetic energy as well as the characteristic resistances in order to further develop self-protective 2G HTS magnets.

Charge–Discharge and Thermal–Electrical Characteristics of GdBCO Coils Wound With Various Types of Grease as an Insulation Material

This paper reports the charge-discharge and thermal-electrical characteristics of GdBCO single-pancake coils employing various types of grease, such as N-grease (N coil), Si-based grease (Si coil), and vacuum grease (V coil), as an insulation material to ameliorate the charging-discharging delay observed in no-insulation (NI) coils. The charge-discharge test results confirmed that the use of grease as an insulation material can effectively reduce leakage current because the turn-to-turn grease layers serve as an effective insulation material. In sudden discharge tests, the time constant values of coils wound with N-grease, Si-based grease, and vacuum grease were drastically lower than those of the NI coil. In the over-current test at 1.5 Ic, the maximum voltages of N and Si coils were 3.8 times lower than that of the V coil, indicating that the thermal and electrical stabilities of N and Si coils were considerably enhanced. Moreover, the heat dissipating test results indicated that the peak temperature of the Si coil was lower than those of other coils because the Si-based grease acted as the most effective heat dissipater in the event of local hot spot generation in the coil. In conclusion, among the various types of grease tested in this study, Si-based grease may be the most effective turn-to-turn insulator for the development of highly stable second-generation (2G) HTS magnets with ameliorated charging-discharging delay of NI coil.

Effect of micro-ceramic fillers in epoxy composites on thermal and electrical stabilities of GdBCO coils

Abstract This paper reports the effects of ceramic fillers for epoxy composites used as impregnation materials on the thermal and electrical stabilities of three GdBCO coils impregnated with epoxy alone, with an epoxy/AlN composite, and with an epoxy/BN composite. During cool-down to 77 K, due to the high thermal conductivity of the filler materials, the coils impregnated with the epoxy composites that included the AlN and BN fillers exhibited faster cooling times than the coil impregnated with epoxy resin alone. Moreover, the addition of the filler could facilitate quench heat dissipation as well as ameliorate the discrepancy of thermal contraction between the GdBCO CC tape and the epoxy. In particular, the coil impregnated with the epoxy/BN composite exhibited superior performance in cooling, over-current, and repetitive-cooling tests. Therefore, the epoxy/BN composite could be the most effective impregnating material for the development of highly-stable superconducting rotating machines, with considerably enhanced reliability.

Analytical and experimental investigation of electrical characteristics of a metallic insulation GdBCO coil.

This paper presents results, experimental and analytical, of the electrical characteristics of GdBCO single-pancake coils co-wound with a brass tape as metallic insulation (MI coil). The GdBCO pancakes were subjected to sudden discharge, charge-discharge, and over-current tests. The sudden discharge and charge-discharge test results of the MI coil demonstrated that MI coils can be charged and discharged significantly faster than non-insulated coils that are wound only with GdBCO tape. In over-current tests at 150 A (1.25I(c)), the MI coil exhibited better electrical behavior, i.e., self-protecting features, than its counterpart co-wound with Kapton tape, an insulator. Moreover, the experimental and analytical results are in agreement, validating the use of a concise equivalent parallel-RL circuit model for the MI coil to characterize its electrical behavior. Overall, the MI winding technique is highly promising to help build compact, mechanically robust, and self-protecting magnets composed of REBCO pancake coils. With no organic material in the winding, MI REBCO pancakes will be immune to neutron radiation damage, making the MI winding technique a viable option for fusion reactors, such as for toroidal field, poroidal field magnets, and central solenoid.

HTS Nested magnet wound with 12 mm GdBCO tape and 4.4 mm YBCO tape

The properties of High Temperature Superconducting (HTS) tapes are progressing, as HTS tapes evolve from 1st generation to 2nd generation. This paper presents design and construction of a 2nd generation HTS magnet consisting of two nested GdBCO and YBCO pancake coils. Two HTS tapes of different widths were used to wind the HTS nested magnet. Considering that a higher magnetic field is applied to the inner magnet than to the outer magnet, 12 mm GdBCO tape was used for winding the inner magnet, which consisted of four single pancake windings. Eight double pancake windings wound with 4.4 mm YBCO tapes were used for the outer magnet. The test results show that the central magnetic field of the HTS nested magnet was 920 mT. The measured critical currents of the inner and outer magnet at 77K were 80.8 A and 32.6 A, respectively.

A Study on Cooling Performances and Over-Current Behaviors of GdBCO Coils With Respect to Epoxy Impregnation Methods

The thermal and electrical stabilities of two epoxy-impregnated GdBCO coils, one wet-wound with epoxy resin every turn and the other impregnated with epoxy resin using vacuum pressure impregnation (VPI) after dry-winding, were evaluated through cool-down, over-current, and repetitive cooling tests. During the cool-down test, the VPI coil showed a more rapid cooling rate compared to the wet-wound coil. The VPI coil also exhibited better thermal and electrical stabilities in the over-current tests because the quench heat could easily be dissipated due to the absence of epoxy between turns. Moreover, in the repetitive cooling tests, the VPI coil exhibited reduced degradation in the superconducting property compared to the wet-wound coil due to lower vulnerability to the discrepancy of thermal contraction between the GdBCO tape and the epoxy resin.

A Study on Normal Zone Propagation Behavior of Partially Insulated GdBCO Coil

This paper reports the quench initiation and propagation characteristics of two GdBCO single-pancake coils, one wound with Kapton tape every two turns and the other completely wound with Kapton tape, to investigate the effect of a partial insulation (PI) winding technique. The thermal/electrical stabilities of the PI coil were improved by turn-to-turn contacts between the partially uninsulated turns, leading to an increased minimum quench energy. Furthermore, transverse normal zone propagation velocities of the PI coil increased because local heat was well transferred through uninsulated turns during quench events.

Effect of partial insulation winding scheme on discharge characteristics of GdBCO coils

In this study, sudden discharge tests were performed on partially-insulated (PI) GdBCO coated conductor single-pancake coils to investigate the effects of a PI winding scheme on the discharge characteristics. Three sets of PI coils were constructed with insulation at 3 different intervals: every 2nd, 4th, and 8th turn. Each set included coils with 4 different insulation-angles: 90°, 180°, 270°, and 360°. Discharge delay of the PI coils was alleviated by increasing the insulation-interval or insulation-angle due to increased insulated area, which partially hinders radial current flow. The results of the sudden discharge test were in fairly good agreement with simulations of the PI coils using an equivalent circuit model consisting of a single RL circuit in parallel. The characteristic resistance of the PI coils was reasonably estimated through the numerical approach with contact surface resistance and compensation for the current-bottleneck effect, which enables the PI coils to be utilized in practical superconducting magnet applications.

Investigation of thermal and electrical stabilities of a GdBCO coil using grease as an insulation material for practical superconducting applications.

This paper presents the effects of thermal grease on the electrical and thermal characteristics of GdBCO pancake coils, observed through charge-discharge, sudden discharge, over-current, and thermal quench testing. In charge-discharge and sudden discharge tests, a coil using thermal grease as an insulation material demonstrated faster charging/discharging rates compared to a coil without turn-to-turn insulation. In the case of over-current tests, the coil using thermal grease exhibited the highest electrical stability. Furthermore, thermal quench testing showed the coil employing thermal grease to possess superior thermal characteristics, with rapid cooling and low temperature rise. Overall, the use of thermal grease as an insulation material may be a potential solution to the problems observed with the existing insulation materials, possessing fast charging/discharging rates with superior thermal and electrical stabilities.

A superconducting joint for GdBa2Cu3O7−δ-coated conductors

Second-generation (2G) GdBCO-coated conductors (CCs) are promising for superconducting magnet applications because of their high critical current (Ic) density, low dependency of the Ic on the external magnetic field, good mechanical properties and reasonable cost, which offer opportunities to develop ultra-high-field magnets. However, they have not been used in high-temperature superconducting (HTS) applications with persistent current mode (PCM) operation such as nuclear magnetic resonance/magnetic resonance imaging magnets owing to unavailability of fabrication techniques for proper joining and contacts. Here we report a resistance-free joint, termed a ‘superconducting joint’, for 2G GdBCO CCs that forms a direct connection to establish a superconducting closed loop for PCM operation. The Ic value of the joined CCs is identical to that of the parent conductors in a liquid nitrogen bath (77 K). Moreover, the initially induced magnetic field of a model GdBCO coil containing a superconducting joint is maintained without decreasing, indicating the complete absence of electrical resistance. Thus, this fabrication method is a unique practical solution for lengthening the 2G HTS CCs and, more importantly, achieving PCM operation in 2G HTS magnet applications, including ultra-high-field nuclear magnetic resonance/magnetic resonance imaging magnets generating more than 1 GHz.

The Effects of Liquid Cryogen on the Thermal/Electrical Characteristics of a GdBCO Coil in a Mixed Cryogen Cooling System

The electrical and thermal properties of a mixed cryogen-cooled 2G HTS coil with respect to liquid cryogen amount were examined to investigate the influence of the solid/liquid ratio on the thermal/electrical stabilities of the GdBCO coil. Thermal contact between the GdBCO coil and solid nitrogen was considerably improved as the amount of liquid neon (LNe) increased. Minimum quench energy values of the GdBCO coil were saturated as the amount of liquid neon increased, which suggested that the excessive LNe was not effective for enhancing the thermal stability of the GdBCO coil. The temperature traces of the GdBCO coil during the quench/recovery tests confirmed that the thermal stability of the coil was enhanced with the aid of the LNe.

Superconducting and Mechanical Properties of Impregnated REBCO Pancake Coils Under Large Hoop Stress

We performed hoop stress tests of REBCO multilayer pancake coils impregnated by epoxy resin. The mechanical deformation and electric field-current properties were measured under the large hoop stress. The maximum hoop stress of about 530 MPa per Hastelloy substrate, calculated from the BJR relation, was applied in the background magnetic field of 8 T. Because the hoop stress level is much smaller than the mechanical tolerance of the GdBCO tape, the coil performance was limited by the angular dependence of critical current in this test. Furthermore, the hoop stress test under the large electromagnetic stresses was also carried out for the other (Y, Gd)BCO coil. The (Y, Gd)BCO epoxy impregnated coil was operated without any degradation even in the huge hoop stress over 1300 MPa. The mechanical deformation of the coil is analysed on the basis of the measured strains. We confirmed the large strain about 0.3%-0.5% because of the hoop stress over 1300 MPa at Iop = 460 A and B = 13.5 T but no degradation of the coils. However, it is suggested that the coil deformation is very complicated under the large electromagnetic stress.

GdBaCuO and YBaCuO Long Coated Conductors by IBAD-PLD Method—Enhancement of Production Speed and Critical Current in a Magnetic Field

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We have been developing long coated conductors with high critical current, <formula formulatype="inline"><tex>$I_{\rm c}$</tex></formula>, by multi-plume and multi-turn pulsed laser deposition (MPMT-PLD) method. In addition to <formula formulatype="inline"><tex>${\hbox{YBa}}_{2}{\hbox{Cu}}_{3}{\hbox{O}}_{7-{\rm X}}$</tex></formula> (YBCO) coated conductors, we have recently succeeded in fabricating long <formula formulatype="inline"><tex>${\hbox{GdBa}}_{2}{\hbox{Cu}}_{3}{\hbox{O}}_{7-{\rm X}}$</tex></formula> (GdBCO) coated conductors. The 30 m long and 60 m long GdBCO conductors showed higher <formula formulatype="inline"><tex>$J_{\rm c}$</tex></formula> values in a magnetic field and lower anisotropy than the conventional long YBCO conductor. Critical current, <formula formulatype="inline"> <tex>$I_{\rm c}$</tex></formula>, of the GdBCO at 3 T and 77 K was more than 20 A, which was three times larger than that of the YBCO long coated conductor. To demonstrate the high performance of GdBCO, we fabricated GdBCO coils and compared it with the YBCO coil. The GdBCO coil was successfully energized up to large <formula formulatype="inline"><tex>$I_{\rm c}$</tex></formula>'s of 74.2 A and 184.8 A at 77 K and 64 K, which generated magnetic field of 0.5 T and 1.2 T at 64 K and 77 K. We confirmed that the GdBCO conductor is more promising than the YBCO conductor for the application because its superior transport properties in a magnetic field. </para>

GdBCO and YBCO long coated conductors and coils

We developed long coated conductors and a coil of GdBa2Cu3O7−x (GdBCO). The GdBCO coated conductor exhibited higher critical current density (Jc) in a magnetic field property than that of the YBa2Cu3O7−x (YBCO) coated conductor. The GdBCO coil was wound using a 53.5m long GdBCO coated conductor. The coil was a 4 turn-83 layered solenoid with a bore diameter of 28.0mm, inner diameter of 30.0mm, outer diameter of 70.9mm, and height of 44.3mm. The coil was successfully energized to a current of 74.2A at 77K in a liquid nitrogen experiment, and 895A at 4.2K in a liquid helium experiment. Then, the generated central magnetic fields (B0) were 0.48T, and 5.68T, respectively. Thus, high current operation and high magnetic field generation were first occurred for a coated conductor coil. The maximum hoop stress of the coated conductor in the coil was 67MPa in the liquid helium experiment. In terms of 0.2% proof strength of coated conductors, the tolerable stress for GdBCO conductors can be increased. The GdBCO coated conductor is promising for magnetic field application.