High-temperature Superconducting Magnet Research Articles

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.

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).

Anisotropic NMR data acquisition with a prototype 400 MHz cryogen-free NMR spectrometer.

High-temperature superconducting (HTS) materials have recently been incorporated into the construction of HTS cryogen-free magnets for nuclear magnetic resonance (NMR) spectroscopy. These HTS NMR spectrometers do not require liquid cryogens, thereby providing significant cost savings and facilitating easy integration into chemistry laboratories. However, the optimal performance of these HTS magnets against standard cryogen NMR magnets must be evaluated, especially with demanding modern NMR applications such as NMR in anisotropic media. The stability of the HTS magnets over time and their performance with complex pulse sequence experiments are the main unknown factors of this new technology. In this study, we evaluate the utility of our prototype 400 MHz cryogen-free power-driven HTS NMR spectrometer, installed in the fumehood of a chemistry laboratory, for stereochemical analysis of three commercial natural products (artemisinin, artemether, and dihydroartemisinin) via measurement of anisotropic NMR data, in particular, residual dipolar couplings. The accuracy of measurement of the anisotropic NMR data with the HTS magnet spectrometer is evaluated through the CASE-3D fitting protocol, as implemented in the Mestrenova-StereoFitter software program.

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.

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.

Operation of Prototype Race-Track Closed-Loop HTS Coil With Metal-as-Insulation in Solid Nitrogen

The second generation (2G) high temperature superconductor (HTS) has excellent current-carrying density performance in strong magnetic field. Thus, the maglev system driven by 2G HTS magnets has been studied a lot. In the design of magnets, metal-as-insulation coil retains self-protection feature of the no-insulation coil, and it performs well in charging time. On the other hand, using solid nitrogen (SN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ) as a “thermal battery” may effectively reduce the volume of on-board HTS magnets. In order to design on-board magnets with fast charging, small volume and self-protection, firstly a prototype coil was manufactured. Then the experiment apparatus was set up and experiments in the persistent current mode were carried out. It was obtained by fitting that the magnetic field decay rate of the prototype coil was about 0.24% per day. This paper provides a foundation for the production of real-scale on-board magnets cooled by SN <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> .

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.

Preliminary Design Study on Non-Twisted HTS Conductor for Fusion Applications

Full transposition of superconducting wires within a conductor may not be a requirement for high temperature superconductor (HTS) magnet for fusion applications. Already, conduction-cooled 20 T model coil built with non-transposed HTS stacks has been demonstrated by Commonwealth Fusion Systems (CFS). Here, we discuss 3 possible stacked HTS cryogen-cooled conductor concepts for toroidal field (TF) magnet applications. The first one is somewhat like LTS conductors, the second, HTS stacks are capped by copper stabilizer, the last one, only stacks without copper. We simulate a simplified TF magnet model, 12 T, size of KSTAR, using the 3 conceptual HTS conductor designs. A comparative thermo-hydraulic analysis for a fast charging case has been carried out and its implications on HTS conductor design are further discussed.

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.

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.

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.

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.

Conceptual Design of a 20 T Hybrid Cos-Theta Dipole Superconducting Magnet for Future High-Energy Particle Accelerators

High energy physics research will need more and more powerful circular accelerators in the next decades. It is therefore desirable to have dipole magnets able to produce the largest possible magnetic field, in order to keep the machine diameter within a reasonable size. A 20 T dipole is considered a desired achievement since it would allow the construction of an 80 km machine, able to circulate 100 TeV proton beams. In order to reach 20 T, a hybrid Low-Temperature Superconductor (LTS) - High-Temperature Superconductor (HTS) magnet is needed, since LTS technology is presently limited to ∼16 T for accelerator magnet applications. In this paper, we present the design of a 6 layers 20 T hybrid dipole magnet using Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn (LTS) and Bi2212 (HTS). We show that it is possible to achieve this magnetic field with accelerator field quality, with sufficient margin on a realistic conductor, keeping the stresses within safe limit, avoiding conductor degradation.

Thermal Analysis of Leakage Currents of No-Insulation HTS Racetrack Coils Using a Three-Dimensional Turn-Distributed Equivalent Circuit FEA Model

In promoting novel products with superconducting magnets to transportation and science industries, the most important factor is reliability. Recently, it was reported that a no-insulation (NI) superconducting magnet could be damaged with quenches caused by a thermal runaway, significant temperature deviation, extremely high characteristic resistance, etc. To resolve this issue, numerical analysis of high-temperature superconducting (HTS) racetrack coils with NI winding technology should be conducted. The authors proposed a homogenization method to analyze operating characteristics of the NI HTS coils by simplifying the finite element analysis (FEA) model and decreasing the solution time. In order to estimate their thermal stabilities, the proposed FEA simulation is one of the efficient methods for solving the electrical and thermal factors simultaneously. In this paper, the thermal analysis of the leakage currents of NI HTS racetrack coils is presented using a three-dimensional (3D) turn-distributed equivalent circuit FEA model. The charging and discharging delay phenomena were modeled by considering the contact resistances by turns. With the equivalent circuit model of the NI HTS magnets, the 3D racetrack coils were designed by considering a single turn and stacked turns, including metal insulation layers. Their material properties according to temperature variation were applied in the transient analysis. The critical currents by turns were estimated by considering the magnetic field, its angle, and temperature with a given current simultaneously. The simulation results were compared with a fabricated NI HTS magnet and are presented in this paper.

Influence of Shielding Currents in Coated Conductors on Field Quality of Iron-Dominated HTS Magnets Estimated by Numerical Analyses

Additional magnetic fields induced by shielding currents (persistent eddy current induced in superconductor) are called shielding-current induced fields (SCIFs). SCIFs are serious problem in magnets using tape-shaped high-temperature superconductor (HTS) wires, because it would worsen the accuracy and stability of the generated magnetic field of the magnets. In iron-dominated HTS magnets, magnetic field distribution at air gap between iron magnetic poles is dominated by poles' shape, so the influence of SCIFs on the quality of generated magnetic field is usually considered negligible. However, quantitative estimation of SCIFs is required to support this assumption. Therefore, we developed a practical method to estimate SCIF distributions in iron-dominated HTS magnets and use it to estimate temporal change of SCIF distribution in an iron-dominated HTS magnet. According to the analysis results, we confirmed relative error of magnetic field caused by SCIFs is in the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-4</sup> , which is negligible in many magnet applications.

Investigation on nonuniform current density and shape deformation affecting the magnetic field performance of a saddle-shaped no-insulation HTS cosine–theta dipole magnet

High magnetic fields are desirable for discovering new particles in particle accelerators. Dipole magnets using superconductors have played a key role in creating the required field intensity and uniformity. In contrast, high temperature superconductor (HTS) dipole magnets have recently been spotlit because of their ability to generate higher magnetic fields compared to their low temperature superconductor counterpart. Similar needs have emerged in other fields using magnets, and no-insulation (NI) technology is considered a feasible option to reach high magnetic fields by overcoming the disadvantages of HTS magnets. However, research has rarely been carried out on the utilization of NI HTS magnet technology for dipole magnets in high-field accelerators. Here we show the design, fabrication, and test results of an NI HTS dipole magnet with numerical analysis results. This paper aims to investigate the effect of nonuniform current density and undesirable shape deformation on the magnetic field distribution of a saddle-shaped NI HTS dipole magnet. The magnet is designed and constructed considering the ‘constant perimeter winding’ technique and tested in liquid nitrogen. The field mapping process is also performed along a designated mapping trajectory to obtain the magnetic field distribution. A T-A formulation-based simulation model, named the ‘sequential simulation model,’ is suggested to reproduce the measurements and employed considering the current distribution and shape deformation. As a result of quantitative analysis of the transverse direction measurements, the magnetic field error decreased by 0.02 percent point (pp) when the nonuniform current density is considered. It decreased by 0.13 pp when the shape deformation is considered. Moreover, the critical current calculated through an additional numerical analysis shows an error of up to 10%. In conclusion, the saddle-shaped NI HTS dipole magnet can produce a sufficient magnetic field level for particle accelerator research, even though the field distribution shows a uniformity of 0.37% within this study.

Optimization of surface bonding methods for fiber Bragg grating sensors at cryogenic temperatures

Optical fibre Bragg grating (FBG) sensors have been used for cryogenic temperature and strain measurements in a variety of applications, e.g., hotspot detection in high-temperature superconductor magnets. In such application, the sensitivity of the FBG and its reflection spectrum strongly depend on the methods of application, chiefly the adhesion and the elasticity of the bond, and the birefringence effect. To investigate those features, Draw Tower Gratings (DTG®) with Ormocer coating and FemtoSecond Gratings (FSG) with polyimide coating are mounted in the V-shaped grooves and on the surface of copper substrates using Stycast and Apiezon N. Their performance is investigated between 293 K and 77 K. The FBG sensors mounted in the V-grooves can achieve higher temperature sensitivities than those on the surface of the copper due to enhanced thermal strain transfer. It is found that although there is no obvious difference between the temperature sensitivity of the Ormocer and polyimide coated FBGs at cryogenic temperatures, the polyimide coated FBGs show superior reflection spectra, whose peak intensity remains strong at 77 K. Cryogenic vacuum grease Apiezon N is shown to bond the fibre well to the copper at cryogenic temperatures, which also reduces the birefringence effect induced from the non-uniform strain across the FBG. This paper provides practical knowledge of bonding FBG sensors in cryogenic environments for temperature and/or strain measurements.

Stable operation characteristics and perspectives of the large-current HTS STARS conductor

The High-Temperature Superconducting (HTS) magnet option has been explored for fusion reactors as well as for next-generation fusion experimental devices. The Stacked Tapes Assembled in Rigid Structure (STARS) conductor uses HTS tapes with simple stacking without twisting and transposition. A practically applicable STARS conductor is presently being developed with an operating current of 18 kA at 20 K temperature and ~15 T magnetic field. The conductor is required to have a high current density of 80 A/mm2. For the second stage of the conductor development, internal electrical insulation is applied between the copper stabilizer casing and the outer stainless-steel jacket, and a 6-m conductor sample was fabricated in a solenoid coil shape with a 600-mm diameter and three turns. The coiled sample was tested in 8 T, 20 K using a facility equipped with a maximum 13-T, 700-mm bore solenoid coil. Excitation up to the rated current of 18 kA was successfully attained with stable operation. The characteristics of the conductor observed during the excitation test are discussed.