Superconducting Magnet Research Articles

Experimental investigation of axial tensile and fatigue behaviors of HTS round strands

For the development of high-temperature superconducting (HTS) magnet systems of future fusion devices, a novel HTS round strand based on a stacking structure was designed and manufactured using second generation (2G) HTS tapes. Different mechanical loads during operation can result in irreversible degradation of the strand. The axial tension and fatigue loads need particular attention. Therefore, it is important to investigate the electromechanical behavior of the round strand under various axial tension and cyclic loads. In this paper, the axial tensile and fatigue tests were conducted at 77 K, self-field. Taking 95% critical current (Ic) retention as the criterion, the results of the tensile tests revealed that the average tensile stress and strain were as high as 344 MPa and 0.47%, respectively. Fatigue characteristics were also investigated as a function of axial tensile stress. No significant performance degradation was observed up to 100,000 loading cycles with stress amplitudes ranging from 20 MPa to 200 MPa. Ic degradation occurs after 16,000 loading cycles with 380 MPa as the maximum stress. Furthermore, the microscopic defects of the round strand samples due to fabrication imperfections and mechanical loading were investigated using metallographic microscope and scanning electron microscope. These results presented in this paper are useful for comprehending and improving the mechanical behaviors of the strand in high-field and large-scale fusion magnet systems.

Cryogenic systems of SuperKEKB final focusing superconducting magnets

SuperKEKB is an accelerator to collide electron and positron particles with a nano-beam collision scheme and equipped with two cryostats to accommodate 55 superconducting (SC) magnets for final focusing. Each cryostat is cooled by an independent cryogenic system employing a refrigerator of 250 W at 4.41 K and a subcooler to supply subcooled liquid helium (LHe), which absorbs heat leak by the relatively small sensible heat (∼2.4 J/g) with an acceptable temperature rise of 0.35 K from 4.41 K to 4.76 K (saturated temperature at 0.161 MPa). The associated limitation of the permissible heat leaks necessitated a careful cryostat design with comprehensive thermal considerations to reduce heat conduction and radiation and maintain stable cryogenic conditions of SC magnets. This paper presents the designs of the cryostats and cryogenic systems to address the technical challenges to cool SC magnets with the compact spatial constraints. Cryogenic systems were designed to supply a subcooled LHe flow of ∼20 g/s and to provide a mixed capacity of refrigeration and liquefaction loads. The liquefaction component amounts to 40 L/h (∼1.3 g/s), approximately half of the total refrigerator capacity to cool the vapor-cooled current leads of SC magnets, which is also found to significantly influence the total LHe flow (∼20 g/s) to cool the cryostats. Dedicated studies were performed to investigate the SC magnet temperature variations with the LHe flow, the system operation modes such as cooldown and quench of SC magnets, and to assess the heat leaks with local and global methods, which facilitate the practical operation of cryogenic systems for the SuperKEKB accelerator and are detailed in this paper.

A simple study for an optimized operation of the ITER Cryodistribution cold rotating machines

The ITER superconducting (SC) magnet system is composed of 4 distinct units: the Central Solenoid (CS) coils, the Toroidal Field (TF) coils, the coil encasing and supporting Structures (ST), and the Poloidal Field and Correction Coils (PF&CC or simply PF). Including another cold component unit, which is the Cryopump (CP) system, each unit is assigned its own Auxiliary Cold Box (ACB) with cold rotating machines (CRMs) to ensure efficient operation from a cryogenic perspective. The CRMs, which are the cold circulator (CCr) and cold compressor (CCp), are respectively in charge of generating the high mass flow rate (2.0–2.7 kg/s) and low temperature (3.7–4.2 K) of supercritical helium (SHe) that is supplied to each magnet unit and CP system. To maintain the proper cryogenic conditions of the magnets and CPs during fusion experiments, the ITER Cryogenic System can produce an average cooling power of 75 kW at 4.5 K, including the need for the 50 K gaseous helium (GHe) cooled high-Tc SC current leads. Depending on the operation mode of the ITER Tokamak, up to 28 kW of that power can be consumed by the CRMs due to compression of helium.In this study, we will explore possibilities for optimized operation of the CRMs by adjusting the mass flow rate and temperature of the SHe to minimize the heat of compression while maintaining the thermal stability of the ITER SC magnets and CP. By doing so, the CRM-heat-of-compression at 4.5 K can be reduced by more than 3 kW, providing additional “cryogenic” margins for higher-heat-load plasma experiments.

The final design of the ITER Interlock Discharge Loop Interface Boxes (DLIB) and its compliance with the IEC 61508 standard

The Central Interlock System (CIS) is in charge of implementing the ITER Central Investment Protection Functions. A dedicated architecture based on hardwired loops will be responsible for the protection of the superconductive magnet system. These loops act in a transversal way connecting all systems directly involved in the protection of the magnets at the plant level. The base of the coordination between the hardwired loop and the different users is a common interface called DLIB (Discharge Loop Interface Box).The IEC 61508 standard, which defines the ‘Functional Safety’ provisions for I&C systems, has been used as the guideline to define the lifecycle of the device, starting from the specification up to operation and maintenance of the Investment Protection Function performed by the DLIB.The whole dependability of the DLIB has been improved and demonstrated though a detailed verification and validation process, including the Safety Integrity Level (SIL). The general approach to test plans including the Manufacturing tests, to identify any issue related to the series production of the component, Early Stage Screening, to identify latent defects, Qualification testing to identify any externally induced defects, and Accelerated life testing, to emulate the end of life behaviour of the DLIB.The paper provides a summary for the whole process from the design up to the final validation of the Discharge Loop Interface Boxes (DLIBs) that will be used to coordinate the Fast Energy Discharge protection for the ITER magnet protection.

Unraveling quench dynamics and real-time continuous detection in HTS tapes through distributed fiber optic sensing

High-temperature superconducting (HTS) tapes, coils, and magnets often experiences intricate quench instabilities and failures during high current-carrying operations, posing challenges to their practical applications. This study addresses the need for a measurement approach capable of monitoring multi-field signals in superconducting structures within cryogenic and extreme electromagnetic environments. We explore the application of distributed fiber optic sensing (DFOS) technology, specifically employing the optical frequency domain reflectometry scheme, which offers distinct advantages over traditional point-type electrical testing methods, particularly for superconducting materials and magnets. In this experimental study, we continuously track the quench evolution process in superconducting tapes using both bonded and stress-free fibers for real-time monitoring. A comprehensive analysis of the acquired temperature and thermoelastic strain profiles provides essential insights into the dynamic behavior of quench events. The findings demonstrate the effectiveness of DFOS in identifying and characterizing the onset and propagation of quenches. By arranging bonded and stress-free fibers in parallel on the HTS tape’s surface, we successfully decouple the effects of strain-temperature cross-sensitivity, enabling the extraction of temperature and train profiles. The bonded fiber optic sensor demonstrates rapid sensitivity to the thermally quenched events. Temporal derivatives of voltage and thermal strain exhibit characteristic plateaus and slope changes during quenches, respectively. The voltage rate displays two plateaus corresponding to superconducting-to-normal transitions, while strain rates exhibit potential as criteria for identifying quench events in HTS materials. Moreover, DFOS outperforms traditional terminal average voltage measurement, capturing quench evolution tails from the initial point of quench until the complete transition of the segment into the normal state. This experiment provides a solid foundation for further exploration of the underlying quench mechanism.

Design and Numerical Study of Magnetic Energy Storage in Toroidal Superconducting Magnets Made of YBCO and BSCCO

The superconducting magnet energy storage (SMES) has become an increasingly popular device with the development of renewable energy sources. The power fluctuations they produce in energy systems must be compensated with the help of storage devices. A toroidal SMES magnet with large capacity is a tendency for storage energy because it has great energy density and low stray field. A key component in the creation of these superconducting magnets is the material from which they are made. The present work describes a comparative numerical analysis with finite element method, of energy storage in a toroidal modular superconducting coil using two types of superconducting material with different properties bismuth strontium calcium copper oxide (BSCCO) and yttrium barium copper oxide (YBCO). Regarding the design of the modular torus, it was obtained that for a 1.25 times increase of the critical current for the BSCCO superconducting material compared with YBCO, the dimensions of the BSCCO torus were reduced by 7% considering the same stored energy. Also, following a numerical parametric analysis, it resulted that, in order to maximize the amount of energy stored, the thickness of the torus modules must be as small as possible, without exceeding the critical current. Another numerical analysis showed that the energy stored is maximum when the major radius of the torus is minimum, i.e., for a torus as compact as possible.

BELFEM: a special purpose finite element code for the magnetodynamic modeling of high-temperature superconducting tapes

Predicting the performance and reliability of high-temperature superconducting (HTS) cables and magnets is a critical component of their research and development process. Novel mixed finite element formulations, particularly the h -φ-formulation with thin-shell simplification, present promising opportunities for more efficient simulations of larger geometries. To make these new methods accessible in a flexible tool, we are developing the Berkeley Lab Finite Element Framework (BELFEM). This paper provides an overview of the relevant formulations, discusses the current state of the art, and discusses the main aspects of the BELFEM code structure. We validate a first 2D thin-shell implementation in BELFEM against selected benchmarks computed in COMSOL Multiphysics and compare the performance of our code with a comparable formulation in GetDP. We also outline the next steps in the development process, paving the way for more advanced and robust modeling capabilities.

Design optimization of a stepped HTS magnet for electrodynamic suspension train

High-temperature superconducting (HTS) magnets are promising candidates for transportation and power systems, such as the electrodynamic suspension (EDS) train, ultra-high field magnet and magnetic resonance imaging, because of their large current-carrying capacity and low power loss. The critical current depending on magnetic flux density is an essential factor in assessing the application performance of HTS magnets. As usual, the existing HTS magnet is wound with rectangular cross-section, which results in magnetic field concentration inside winding. In this paper, we propose a novel HTS magnet structure with stepped cross-section to alleviate the magnetic field concentration, and resultantly improve the critical current. From this point, this paper aims to design and optimize a stepped HTS magnet with the critical current maximized. Firstly, the structure design of the stepped HTS magnet is performed with a consideration of the application scenario of EDS train. Then, the critical current of the HTS magnet is estimated with a homogenized self-consistent model. Afterwards, an HTS magnet with stepped cross-section is optimized, fabricated and finally tested. The critical current was experimentally measured to verify the simulation results, followed by the electromagnetic investigations of the stepped HTS magnet in the EDS train.

Fabrication of YBa2Cu3O7-y Bulk Superconductor Through Melt Growth

A YBa2Cu3O7-y bulk with the crystal orientation identical with that of single seed was grown from the seed by the application of melt growth, wherein YBa2Cu3O7-y superconductive green compact was placed on the upper part of Sm 123 superconductive seed single crystal. Field cooling was employed as the condition for the measuring magnetic levitation; it was measured at the temperature of liquid nitrogen, utilizing a Nd–Fe-B permanent magnet with the force of 3.86 kG. The magnetic levitation measurements yielded values of 23.537 N on the surface of the upper part and 30.562 N at the bottom of superconductive bulk magnet. The trapped magnetic forces on the surfaces of superconducting specimens were measured using a permanent magnet with a surface magnetic force of 5.25 kG. The measurements yielded values of 1.74 kG on the upper surface and 1.82 kG at the bottom of the superconducting bulk magnet. Superconducting single crystals were grown from specimens obtained from the upper and bottom parts of the bulk magnet. These specimens were prepared using the melt reaction process. The superconducting properties, such as magnetic levitation and trapped magnetic force, were found to be higher in the region that came into contact with the seed crystal.

Massive parametric study for prospective design space of a compact tokamak fusion reactor

Using a novel computational algorithm that integrates tokamak systems analysis with neutron transport calculations, the minimum major radius (R0) and resulting maximum magnetic field at the plasma center (BT) can be determined uniquely for a given plasma performance. Plasma performance was varied extensively using a supercomputer (KAIROS) and scanning over a wide range of physics, technology, and system parameters to derive the optimal design space for a compact tokamak fusion reactor. Given the design goals of fusion gain Q > 20.0, net electric power > 100 MW, neutron wall loading < 2.0 MW/m2, indicator of divertor power handling PSOL/R0 < 25 MW/m, direct capital cost < $4.0 billion, and steady-state operation, a prospective design space was determined depending on the levels of physics and technology. Advanced engineering features, such as maximum allowable magnetic field at the toroidal field coil (Bmax = 23 T), were implemented by adopting high-temperature superconducting magnet technology, using an advanced shield material such as tungsten carbide (WC), and a plug-bucked magnet support structure. It was found that by exceeding the energy confinement scaling laws of IPB98y2, a design space of R0 < 4.0 m could be accessed under the following conditions for a conventional tokamak: confinement enhancement factor H > 1.7, bootstrap current fraction fBS > 0.55, magnetic field at the magnetic axis BT > 4.0 T, fusion power Pfusion > 600 MW, and thermodynamic efficiency ηth > 0.33. For a spherical tokamak, these conditions were H > 1.3, fBS > 0.6, BT 〈 6.0 T, and Pfusion 〉 500 MW. Sensitivity studies on the energy confinement scaling laws showed that stricter conditions on the physics parameters were required with the ITPA20 scaling law, whereas the conditions were mitigated with the β-independent scaling law.

Electromagnetic drag forces between HTS magnet and tube infrastructure for hyperloop

Maglevs are typically accelerated using electromagnetic propulsion and levitation. High-temperature superconducting (HTS) magnets along with electrodynamic suspension (EDS) and linear synchronous motors are one of the best options for Hyperloop. However, the strong magnetic fields generated by HTS magnets on the pods inevitably interact with the magnetic and conductive structures in the vacuum tubes, along with the tube itself, while the pods move through the tubes. This interaction is observed as a drag force on the pods, significantly reducing the propulsion efficiency. This study comprehensively analyzes the electromagnetic drag force (EDF) generated by HTS magnets on pods, which accounts for most of the drag forces faced by Hyperloop. Theoretical analysis and 3D FEA simulations are performed to analyze the propulsion forces with HTS magnets and all the drag forces on the pods. The EDF generated by AISI 1010 steel rebars in concrete guideways is even greater than the designed propulsion forces of 40 kN. Consequently, high-manganese (Hi-Mn) steel and insulated steel rebars are adopted and analyzed using 3D FEA simulations. The EDFs generated by the AISI 1010 steel and Hi-Mn steel vacuum tubes are determined by varying the distance between the HTS magnets and tubes at 50 and 1200 km/h, respectively; a minimum distance of 0.75 m is determined by a drag force below 8 kN within their operating velocities. Lastly, the total EDFs of the AISI 1010 steel and Hi-Mn steel tubes with EDS rails are obtained through the optimal design of rebars and tubes. The simulation results show that the total EDFs can be significantly reduced to below 10 kN (approximately 25% of the designed propulsion force after the levitation of pods).

Power Supply for Superconducting Magnets

The self-heating caused by the heat entering through the current supply conductor from the outside to the inside of the cryostat and by the energizing current in the power supply unit of the high-temperature superconducting coil magnet (hereafter referred to as HTS magnet) causes an increase in the cooling load of the cryocooler. As a countermeasure, a technique was devised to increase the superposition of the incoming current on HTS magnet current by multiple control of the time-division drive of a multi-parallel chopper installation inside the cryostat in order to reduce the energizing current of the conductors. The circuit simulation results confirmed that the circuit is valid for stable increase of magnet current up to the target value, including at startup, and for low ripple current values. In addition, the size of the main circuit board for magnet current 300A was also confirmed by a schematic design in combination with the short circuit of the HTS magnet using a parallel connection of low ON resistance MOSFETs. Excitation power supply for HTS magnet under study is intended for use with high-temperature superconducting non-insulated (NI) coil, but it may also be used for metallic superconducting magnets.

Design and Manufacture of a Test Cryostat With MgB2 Superconducting Magnet Significantly Reducing Helium Consumption

A test cryostat equipped with a conduction-cooled MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> high-temperature superconducting bias magnet has been designed, manufactured, and demonstrated. This cryostat is more environmental-friendly and user-friendly than the conventional cryostat with an immersion-cooled NbTi superconducting magnet. Liquid helium consumption is reduced by almost half of that of a previous conventional cryostat because the MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> magnet does not need liquid helium for cooling itself. A ramp-up rate of the bias magnet has also been improved to 1 A/s, which is almost 10 times faster than that of the previous NbTi magnet, due to the high stability of the superconducting MgB <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> magnet. A maximum bias magnetic field is 1 T at the center of the magnet. A helium vessel of the cryostat is 212 mm in inner diameter so that various superconducting samples such as bulks, short samples of wires, and small coils, could be immersion-cooled by cryogen and tested in it. A performance demonstration was also successfully conducted.

High Fault-Tolerance Dual-Rotor Synchronous Machine With Hybrid Excitation Field Generated by Halbach Permanent Magnets and High Temperature Superconducting Magnets

Superconductors can help electric machines achieve extremely high torque and power densities. However, operational instability is still a severe challenge for most superconducting electric machines. This paper proposes a novel dual-rotor hybrid excitation variable flux synchronous motor (DRHE-VFPM) suitable for applications with high safety requirements. The primary characteristic is using series-connected permanent magnet (PM) and High-temperature superconducting (HTS) magnet as the rotor's flux sources. These two magnets are on different rotors and steadily excited by each other through armature slots, improving several critical characteristics of the superconducting motor. First, the new electromagnetic topology can avoid severe quench accidents. At normal operation, the PM and HTS rotors can produce a large total electromagnetic torque. When the HTS magnets are damaged by the overtemperature or the mechanical shocks, the motor can still use the PM rotor to generate continuous torque instead of sudden interruption. Secondly, because an excessively strong rotor field will limit the speed, the variable flux density of the HTS magnet allows fuller use of voltage to extend the speed range and improve the power density. Moreover, the main magnetic circuit of the motor changes significantly under different operating states, which is analyzed theoretically in detail.

Quenching of a no-insulation high-temperature superconducting magnet

We report details on the quenching incident of an 18 T high-temperature superconducting (HTS) magnet, which occurred in December 2020. It has been received that the no-insulation (NI) design of an HTS magnet is relatively safe in quenching. However, the NI design could not completely prevent the magnet from quenching and damaging the associated system. Due to significant vibrations and fast energy dissipation during quenching, the magnet and the detector components are seriously damaged. The manufacturer inspected the magnet after the incident and repaired it in the spring of 2021. The magnet showed stable and consistent performance after the repair. It is evident that the NI-HTS magnet still requires quench protection circuits to secure the magnet and associated system.

Thermal Design of the Conduction-Cooled High Temperature Superconducting Magnet for Pulsating Magnetic Field

A conduction-cooled high temperature super-conducting (HTS) solenoid is fabricated to produce pulsat-ing 4 T magnetic field for magnetic refrigeration. HTS con-ductor of 245 m is wound as a solenoid with 112 layers each of which consists of 12 turns of the conductor. For a stable ramping operation of the magnet, AC loss inside the wind-ing pack should be effectively rejected to the heat sink. The thermal drains are inserted between the layers and con-nected to the magnet flange which is cooled by a commercial two-stage Gifford-McMahon cooler. They are made of Ox-ygen-Free High Conductivity (OFHC) copper, and has a width of 6 mm and a thickness of 50 μm. Thermal analysis is conducted to confirm the effect of the thermal drain on the AC loss heat rejection. AC loss of the magnet is esti-mated by using T-A formulation (current vector potential T and magnetic vector potential A). The ramp rate of the mag-net and the heat sink temperature are set to 0.1 T/s and 4.5 K, respectively. In the thermal diffusion calculation, each winding layer is considered as the lumped capacitance. In addition, thermal conduction between the adjacent lay-ers and that through the thermal drains are considered. The thermal analysis confirms that the conductor temperature of the winding is maintained lower than the operating limit of 20 K. Based on the results of the thermal analysis, the stable pulsating operation of the conduction-cooled super-conducting magnet is predicted to be feasible and is also confirmed by the experiment.

Through-Wall Excitation of a Conduction Cooling HTS Magnets by a Linear-Motor Type Flux Pump

The flux pumps can realize non-contact excitation of high-temperature superconducting (HTS) magnet and eliminate heat leakage caused by current leads. However, earlier proposed flux pump needs to be installed inside the cryostat, which leads to additional heat load to the cryogenic system. Here, we report the successful demonstration of a through-wall linear-motor type flux pump, with its heating parts such as the copper windings completely outside the cryostat, while its generated heat is dissipated in the air and insulated from the cryostat. With an insulation HTS double pancake coil installed in the cryostat and cooled by a G-M cryogenic cooler at 32.6 K, we have demonstrated the injection of direct currents of 42 A into the closed-loop, without the current leads. Thus, the feasibility and reliability of using the linear-motor type flux pump as the power source to excite the HTS through the cryostat is verified, making it possible for future applications such as in MRI and superconducting motors, etc.

Bending and Twisting Characteristics of REBCO Lap Joint With Indium

Joint-winding of high-temperature superconducting (HTS) magnets has been proposed for an advanced helical reactor, the FFHR-d1 reactor. In the fabrication method, stacked tapes assembled in rigid structure (STARS) conductors—in which a simple stack of rare-earth barium copper oxide (REBCO) tapes within a metal casing—are connected using a bridge-type mechanical lap joint with indium foil inserted. Previous studies showed sufficiently low joint resistances in straight conductors, although curved joints would be necessary for the fabrication of helically-wound HTS coils. In this study, we devised joint, twist, and bend (JTB) and twist, bend and joint (TBJ) procedures for the curved-joint fabrication. We evaluated the joint resistance as a function of bending and torsional strain for the JTB procedure as well as the joint resistance as a function of bending radius or twist pitch for the TBJ procedure. Based on the experimental results and discussion, the JTB procedure could be applied to large-scale helical coils—such as those used in the FFHR-d1 reactor—whereas the TBJ procedure could be adopted for smaller-scale helical coils such as those used in the FFHR-a1 reactor.

Analysis of the Experimental Test Results of Laboratory-Scale HTS DC Motor for High-Torque Electric Propulsion

A laboratory-scale direct current (DC) rotating machine with a high-temperature superconducting (HTS) magnets was conceptually designed to obtain 500 W output power under a liquid nitrogen cooling method to verify the operation characteristics of the HTS DC motor. The experimental test results of the HTS motor for high-torque electric propulsion are analyzed and presented in this paper. The two HTS magnets were fabricated and assembled using a dynamo system to demonstrate the operation. The excitation current for the two HTS magnets connected in series was kept within 0–40 A using the DC power supplier. The rotational speed of the motor could be kept at 100–600 rpm, and the torque was measured during real-time operation. These test results can be used to design a large HTS DC rotating machine for an electric propulsion system.

Sudden-Discharging Quench Dynamics in a No-Insulation Superconducting Coil.

It is generally agreed that no-insulation (NI) high-temperature superconducting (HTS) magnets do not quench because of the turn-to-turn energy-releasing bypass unique to NI. However, these magnets, especially with high operating current and low ambient thermal capacity, still occur unexpected quenches when the current through the magnets suddenly drops to zero (i.e., the sudden-discharging quench). Here, we report this kind of quench, which is different from that widely-reported quench happening during charging (i.e., the energizing quench). Here, a demonstrative coil with 655-turns, 350 A operating current, and 4 K conduction cooling, is used to prove this sudden-discharging quench, and a simulation model is built to reveal the quench dynamics. Results show the turn-to-turn heat triggers the initial partial quench in the inner coil turns and then the induced overcurrent spreads out the quench like an avalanche to the outer coil turns.