High-temperature Superconducting Insert Research Articles

Analytical Modeling of Magnetization Losses in Twisted Stacked HTS Conductors

The European post-ITER fusion demonstrator reactor (DEMO) design activities have been ongoing for several years in the framework of the EUROfusion Consortium. The designs proposed for the DEMOnstration Fusion Power Plants (DEMO) magnets include options based on high-temperature superconducting (HTS) technology, namely as an Insert in the Central Solenoid (CS). This work describes the development of tools for the calculation of magnetisation losses in a twisted-stacked HTS cable, which represents one of the most promising configurations of HTS fusion Cable-in-Conduit conductors (CICC). This geometry cannot be tackled with the straightforward application of 1D or 2D models, given their inability to describe the twisting of the tape stacks. However, the methodology proposed in this work allows one to treat the twisting of the conductors by discretizing them in a set of straight pieces. A set of analytical equations is developed to compute the instantaneous power losses in the slab approximation. This fast, easy-to-use, open-access tool, can be employed for the computation of AC losses in the central solenoid modules of a tokamak. In particular, we have applied the model to study the HTS insert of the DEMO central solenoid.

A homogenised anisotropic J-model for accelerating computations of screening current profile in large-scale HTS magnets

Owing to the induced excessive strain, plastic deformation, and distortion of the magnetic field in high-temperature superconducting (HTS) tapes, there is a rapidly growing research interest in the screening current profile in high-field superconducting magnets. The J-model was developed to calculate the current density distributions in HTS stacks and cables, which improves the calculation efficiency because no air domain is considered in the calculation regions. However, for a large-scale high-field magnet wound with hundreds and thousands of layers of HTS tapes, there is still a big challenge for J-model simulation involving prohibitive time consumption and memory requirements. This study proposes an improved J-model approach based on a homogenised anisotropic (HA) scheme for large-scale HTS magnet systems to decrease the number of elements and enhance calculation efficiency. The reliability of the improved HA J-model was verified by solving 2000-tape HTS stacks, and a significant improvement in computational performance was achieved. Furthermore, the HA J-model was applied to a large-scale scenario of a 32 T hybrid all-superconducting magnet at the National High Magnetic Field Laboratory to numerically capture the electromagnetic responses. The magnetic flux and current densities in the HTS insert coils were obtained completely, and the screen current induced field, which is the most concerning, showed a good agreement with the literature. Meanwhile, the time consumption by the HA J-model is approximately half that of the traditional J-model and even less than 1–2 orders of magnitude of the others, such as the traditional T-A model and H model, with acceptable accuracy. The improved HA J-model may provide an appropriate prior design with rapid and real-time calculations for large-scale magnet systems.

Multi-physics quench simulation of no-insulation HTS insert coils with high background magnetic field

Compared with traditional insulated High Temperature Superconducting (HTS) coil, no-insulation (NI) HTS coil has higher thermal stability and self-protection capability, so that is considered as an important way to generate high steady-state magnetic field. This paper focuses on the experimental phenomena that mechanical damage always occurs in NI coils during quench with high background field. A multi-physics quench simulation model is established for NI insert coils, which contains a 2-d axisymmetric partial element equivalent circuit (PEEC) model for electromagnetic calculation and two axisymmetric finite element models for thermal and mechanical analysis. Based on these models, we analyzed the electromagnetic, thermal and mechanical phenomena of insert coils during quench propagation and recovery. The simulation results show that mechanical damage may occur in the outer superconducting zone near the normal zone in no-insulation coils during quench, and thermal strain caused by temperature rise during quench may exacerbate mechanical damage. Meanwhile, quench at different single-pancake coils has different effects on the mechanical performance of the entire insert coils. Based on the electromagnetic simulation results, we also analyzed the time delay of the traditional terminal voltage quench detection method and its causes. This study analyzed the multi-physics quench phenomenon of no-insulation HTS insert coils. And the results imply that hoop strain will increase to the critical strain due to high magnetic field and current redistribution caused by radial current shunt during quench, resulting in mechanical damage. Additionally, radial current shunt and induced voltage will retard quench propagation velocity, resulting in the time delay in terminal voltage quench detection.

Design of CCT6: A Large Aperture, Nb$_3$Sn Dipole Magnet for HTS Insert Testing

We present the design of a four-layer, Canted Cosine Theta (CCT) Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_3$</tex-math></inline-formula> Sn dipole magnet as part of the general R&D program for high field superconducting magnets supported by the US Magnet Development Program (US-MDP). Future testing with High-Temperature Superconducting (HTS) inserts in a hybrid configuration motivates the design’s large clear aperture of 120 mm and target operating dipole field of 12 T at 4.2 K. First, we present a graded magnetic design utilizing existing Nb <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$_3$</tex-math></inline-formula> Sn cables which results in 30% operating current margin at the target field. We then share results from a 2D mechanical analysis which makes use of an idealized external structure to provide initial guidance for design studies. This mechanical analysis is extended to demonstrate compatibility of the CCT6 design with a mechanical support structure, based on key and bladder technology, currently considered for use within multiple US-MDP high-field magnet programs.

Conceptual Design Studies of an HTS Insert for the DTT Central Solenoid

The “Divertor Tokamak Test” (DTT) facility is an experimental fusion reactor being built in Frascati (IT) in the framework of the European Fusion Roadmap. The DTT Central Solenoid comprises six independently energized modules, wound from Nb3Sn Cable-In-Conduit Conductors (CICC). The future addition of a High Temperature Superconductor (HTS) insert would enhance the CS performance, in terms of magnetic flux density and magnetic field produced in the machine and would represent a fundamental technology demonstration toward high-field fusion. Along with the LTS cables, the Central Solenoid complex aims to reach 18 Wb at the pre-magnetization phase, being 16.54 Wb with the LTS CS only. Each LTS module is internally divided into three sub-modules: High Field (HF), Medium Field (MF) and Low Field (LF), each with a dedicated CICC cable optimized for the intense electromagnetic loads during the plasma scenarios. The LTS cables adopt a 316LN steel jacket and a fiberglass-reinforced resin insulation layer. The conceptual design presented here concerns an HTS insert, to be possibly placed inside the CS bore in the machine, employing a Round-In-Square (RIS) configuration of the conductor with either aluminum alloy or steel jackets. The study carried out for the design of the HTS insert and its pre-compression structure, the main electromagnetic and structural analyses and the design solution are presented in this work.

Metal-as-insulation HTS coils

:In this article, we summarize what we have learned about metal-as-insulation (MI) winding behavior and technical challenges. High-temperature superconductor (HTS) materials are highly thermally stable. This feature compared to classical low temperature superconductor (LTS) enables the use of MI technology to improve the protection against quenches. We firstly present a review of what has been done worldwide since 2011, which is the first time that a co-winding of a metallic tape without insulation was used in HTS pancake coils. In this review we also focus on the turn-to-turn contact resistivity parameter and the ranges obtained by groups worldwide. Then, we give details of our NOUGAT insert, a LNCMI-CEA-Néel Institute MI HTS insert built in 2018. We describe the specific conception of the magnet including the compact magnetic shielding. The high number of tests performed in high resistive background fields between 2018 and 2021 gives us a unique data set on this technology for ultra-high field (UHF) magnets; even in case of a background field fast drop at 19 T. This magnet is the first REBCO solenoid of this size using this technology, and tested intensively at such high magnetic fields (up to 32.5 T) so far. Finally, we discuss about the self-protection feature of MI coils and we propose a passive protection way for high turn-to-turn resistivity values.

Electromagnetic design and stress analysis of an 8 T no-insulation HTS insert coil for 20 T magnet

An 8 T no-insulation (NI) high temperature superconducting (HTS) insert has been designed using a 12 T low temperature superconducting (LTS) magnet to build a 20 T magnet. This paper focuses on the electromagnetic design and stress analysis of the 8 T HTS insert coil. The preserve elitist genetic algorithm was adopted to design a coil with the least cost in conjunction with the finite element program. The pre-stress during the winding, the thermal stress during the cooling down, and the electromagnetic stress considering screening current effect during the charging of the HTS insert coil were analyzed and discussed in this paper.

A High-Efficiency Protecting Scheme for HTS Inserts in Case of Background Magnet Quenches

For extremely high field magnets, the LTS/HTS structure is an efficient and economical solution. In an LTS/HTS magnet system, the LTS (Low-temperature superconducting) background field magnet is deeply mutual inductance coupled with the HTS (High-temperature superconducting) insert magnet. A sudden background LTS magnet quench may potentially destroy the double pancakes (DPs) of the insert HTS magnet because of the induced current and the resulting additional electromagnetic stress. To improve the safety and stability of the LTS/HTS magnet system, this article proposed a new passive protecting scheme of the LTS/HTS system. In this scheme, a magnetic dam is arranged between the insert magnet and the background field magnet. When the background field magnet quenches, the magnetic dam can help to slow down the magnetic flux variation in the insert magnet. According to the simulation results, a magnetic dam can significantly reduce the instantaneous stress rise of the insert magnet by 65.3protective effect of the magnetic dam was also verified by the experiments with a Bi-2223 insulating coil magnet. The structural parameters affecting the isolating effect of the magnetic dam were also discussed. Based on these work, researchers can further improve the safety and stability of the LTS/HTS magnet system.

Achievement of 26.5 T at 1.8 K and 24.0 T at 4.4 K in a Free Bore of 68-mm Diameter: Successful Commissioning of the HOMER II LTS/HTS High-Field Facility Upgrade

As an upgrade for higher fields, a REBCO high-temperature superconductor (HTS) insert coil with a remarkable cold bore of 68-mm diameter has been constructed with the goal to be routinely operated to attain total fields of 25 T and beyond in HOMER II. HOMER II is the most advanced superconducting high-field facility of the Institute for Technical Physics at the Karlsruhe Institute of Technology. With its basic magnet configuration, consisting of NbTi and (NbX) <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn low-temperature superconductor solenoid sections, HOMER II can generate a magnetic field of 20 T in a large bore of 185-mm diameter at a Helium bath temperature of 1.8 K. The REBCO HTS insert coil was manufactured using the rarely implemented concept of layer winding technology and consists of five nested solenoids. In this article, we report on the design of the insert, its construction, and commissioning as well as related aspects. In the initial operation period, magnetic fields of up to 26.5 T were obtained at 1.8 K and 24.0 T at 4.4 K, respectively, without training steps and quenching or other problems.

Minimum Volume Optimal Design Method of Hybrid MRI Magnet

The purpose of this paper is to present a minimum volume optimal design method of hybrid magnetic resonance imaging (MRI) magnet using field superposition. The hybrid MRI magnet made by high-temperature superconductor (HTS) and low-temperature superconductor (LTS) is the optimal approach of producing higher magnetic field, which also reduces volume than LTS. In this study, the field superposition theory combined with linear programming and nonlinear optimization is introduced in advance to get minimum volume. Considering the different advantages of LTS magnets and HTS magnets, three hybrid MRI magnet configurations are introduced: (1) HTS insert and LTS outsert: (2) Minimum HTS magnet under LTS magnet: (3) Minimum LTS magnet under HTS magnet. In addition, the homogeneity over the central sphere volume and the scope of 5 Gauss fringe field are considered in actively shielding MRI design. Finally, the three different configurations are compared and a 7 T hybrid actively shielding MRI magnet is presented. The designed magnet homogeneity and 5 Gauss fringe field are validated by finite element simulation method, which illustrates the efficiency and the flexibility of the proposed optimal method.

Prototype REBCO Z1 and Z2 shim coils for ultra high-field high-temperature superconducting NMR magnets

We present promising results of novel high-temperature superconducting (HTS) shim coil prototypes that circumvent the size and strength limitation of our earlier innovative HTS shim concept based on 46-mm wide REBCO tape. The HTS shim coil is placed inside the HTS magnet, mainly for ultra-high-field (> 1 GHz or 23.5 T) NMR magnets, and thus unaffected from the windings’ diamagnetic wall effects. One full-scale version will be applied to clean up Z1 and Z2 harmonic errors in the MIT 1.3-GHz high-resolution NMR magnet composed of an 835-MHz HTS insert, while another version for an MIT 1-GHz microcoil NMR magnet whose small-scale model we are currently building. The prototype sets were wound with a 2-pile, 1.03-mm wide, 0.30-mm thick REBCO conductor. Operated at 77 K, the Z1 shim set generated a 1st harmonic field strength of 179 kHz/cm at 70 A, while the Z2 shim set, composed of two pairs, Z21 and Z22, generated the 2nd harmonic field of 141 kHz/cm2 at 50 A. Together with discussion on technical challenges for this REBCO shim coil concept, we demonstrate its feasibility for the next generation of ultra-high-field (UHF) HTS NMR magnets.

AC losses and induced fields in HTS coil wound using two-ply coated conductors

AC losses and screening-current-induced fields (SCFs) in a high temperature superconducting (HTS) inserts for high field magnet are evaluated theoretically. The HTS inserts are composed of stacked pancake coils, which are wound using two-ply conductors, where the superconducting-layer sides of two rare-earth-based coated conductors are attached to each other without electrical insulation. The theoretical formulas of AC loss and SCF in the two-ply conductor are derived for the simultaneous applications of a transport current and an external magnetic field parallel to its broad face. By taking into account the magnetic-field profile in the HTS insert and the magnetic-field dependency of critical current density in the coated conductor, the AC losses and SCFs are estimated using the theoretical formulas for monotonical increase in a central field up to 25.5 T in 60 minutes in combination with low temperature superconducting outsert coils. It is guessed that the AC loss estimated for the HTS insert wound using the two-ply conductor could be cooled by using prepared cryocoolers. The magnitude of SCF in the HTS insert wound using the two-ply conductor becomes smaller than that for the single-tape conductor estimated previously.

Insert magnet and shim coils design for a 27 T nuclear magnetic resonance spectrometer with hybrid high and low temperature superconductors

An extremely-high field nuclear magnetic resonance (NMR) spectrometer has the potential to significantly augment the resolution and sensitivity of the atomic resonance spectrum, which will significantly boost the development of a range of scientific areas. The success of an extremely-high field NMR spectrometer depends to a great extent on the development of a superconducting magnet system. A high-temperature superconducting (HTS) insert magnet and low-temperature superconducting (LTS) shim coils design scheme for a 27 T NMR spectrometer is proposed in this work. A LTS background magnet was used to generate a 15 T magnetic field in the central area and the designed HTS insert magnet contributed the remaining 12 T magnetic field. The HTS insert magnet adopted notch double-pancake (DP) optimization to improve the rough magnetic field distribution in the central area to a highly-homogeneous level, and shim compensation schemes were proposed for the hybrid magnet to eliminate the inhomogeneous magnetic field components in the practical fabrication. The HTS insert magnet design had a current margin of 45.64% in terms of the perpendicular magnetic field intensity and the maximum hoop stress was reduced to 260 MPa with binding. There were in total eight shim coils, including two Nb3Sn zonal coils located between the insert magnet and the background magnet and six NbTi tesseral coils placed outside the background magnet. The magnet system will be fabricated in the near future.

Effect of Crossover Turns in Double-Pancake Winding on Magnetic Field Homogeneity of HTS NMR Magnet

Field homogeneity of a superconducting magnet is a critical factor for nuclear magnetic resonance (NMR) spectrometers. For REBCO high-temperature superconducting (HTS) coil fabrication, layer-wound is considered a better option than double-pancake (DP) winding, in terms of field homogeneity. Nonetheless, DP winding is preferred to layer-wound owing to the ease of manufacturing and the difficulty in making long REBCO tapes. However, DP winding has an inevitable crossover turn between the top and bottom layers. For magnets with an HTS insert used in NMR spectrometers, the crossover turn could be critical for the field homogeneity. In this paper, the magnetic field generated by the crossover turn is derived based on the Biot-Savart law, and the effect of crossover turns in stacked DPs on the field homogeneity of an NMR-class HTS magnet is analyzed.

Recent Development of the 25 T All-Superconducting Magnet at IEE

Institute of Electrical Engineering, Chinese Academy of Sciences has been focusing on high-field all-superconducting magnet project for many years. No-insulation (NI) winding technology, despite of its charging delay time, makes the magnet of much higher current density, and more important, self-protected. Based on the NI dry-winding technology, a 25 T/32 mm bore superconducting magnet was designed as a combination of 15 T/160 mm bore low temperature superconductor magnet and a 10 T/32 mm bore high temperature superconductor insert. A prototype coil, composed of 15 double pancakes (DPs), was tested at 4.2 K. Test results show that the REBCO insert can generate a 9 T central field in a 15 T background field, and 11.2 T, self-field. While fully evaluation of this insert shows that bad splice joints and premature quenches occurred during the test. We have repaired the bad splice joint, replaced some broken DPs, and rebuilt some double-wound DPs to lower the current density at the top/bottom of the insert to improve the operation margin. After that, we will retest the magnet system. The latest design, construction, and test results of the 25 T all-superconducting magnet will be fully introduced in this paper.

The Effect of Winding Conditions on the Stress Distribution in a 10.7 T REBCO Insert for the 25.7 T Superconducting Magnet

A 25.7 T/32 mm all-superconducting nuclear magnetic resonance magnet, consisting of a 10.7 T/32 mm rare earth-barium-copper-oxide (REBCO) high temperature superconducting (HTS) insert and a 15 T/160 mm low temperature superconducting magnet, has been manufactured at the Institute of Electrical Engineering, Chinese Academy of Sciences. There is a large electromagnetic force induced in the HTS insert after the superconducting magnet is energized, which may cause the mechanical failure of the REBCO tape. To protect the HTS insert during its energization, it is necessary to apply suitable winding tension on the REBCO tape and overband. In this paper, the effect of winding conditions on the stress distribution in the HTS coil was investigated, in which the winding tension applied on the REBCO tape and overband on the HTS insert were included. A checking test was conducted after the superconducting magnet quenched at 25.7 T, which shows that the HTS coil did not burn out and suffer catastrophic damage. The optimized pretension load and overband conditions utilized to wind the HTS coil could protect it from the excessive electromagnetic force when the central field exceeded 25 T.

Electromagnetic Design of HTS Insert for Ultrahigh Field NMR Magnet

In this paper, an electromagnetic design of a 27 T nuclear magnetic resonance (NMR) magnet composed of a low-temperature superconducting outsert magnet and a high-temperature superconducting (HTS) insert magnet is carried out. An optimization code is improved according to the fabrication form of the HTS magnet. Concerns about the radial magnetic field, the hoop stress, the current margin, length of tape used, and outer diameter of the HTS insert are investigated with respect to the magnet length, inner radius, and current density. The screening current effect is taken into consideration for the sensitivity analysis. The results predict that the larger the inner radius of the HTS insert, the better the real field homogeneity can be obtained. Screening current effect can deteriorate the field homogeneity zonally by about 1 order of magnitude.

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.

Influence of the LTS Outsert Shape on AC Losses in a REBCO HTS Insert

A superconducting magnet system composed of a REBCO high-temperature superconducting (HTS) insert and low-temperature superconducting (LTS) outsert has been designed and fabricated. When the HTS insert is charged up/down or exposed to an alternating magnetic field, it will suffer large ac losses which impart a negative effect on the thermal stability of the superconducting magnet system. The influence of the LTS outsert shape on ac losses was investigated. The calculations are carried out in three steps. First, the Fabry's coil-shape factor was adopted to design the LTS outsert. The inner radius, the outer radius, and the height of the LTS outsert were varied independently to obtain the configurations with the same center magnetic field, albeit of different shapes. The magnetic field dependence of the REBCO tape's Ic-value was introduced accordingly. Secondly, the ac losses generated in the HTS insert were studied based on a slab model, which takes the temperature variation into consideration. Thirdly, the results for ac losses with different LTS outserts were compared and discussed through three different cases.

Design and optimization of a HTS insert for solenoid magnets

With the availability of High-Temperature Superconducting (HTS) prototype cables, based on high-performance REBCO Coated Conductor (CC) tapes, new designs can now be made for large bore high-field inserts in superconducting solenoids, thus extending the magnet operating point to higher magnetic fields. In this work, as an alternative approach to the standard trial-and-error design process, an optimization procedure for a HTS grading section design is proposed, including parametric electro-magnetic and structural analyses, using the ANSYS software coupled with a numerically-efficient optimization algorithm. This HTS grading section is designed to be inserted into a 12T large bore Low-Temperature Superconducting (LTS) solenoid (diameter about 1m) to increase the field up to a maximum value of at least 17T. The optimization variables taken into consideration are the number of turns and layers and the circle-in-square jacket inner diameter in order to minimize the total needed conductor length to achieve a peak field of at least 17T, while guaranteeing the structural integrity and manufacturing constraints. By means of the optimization, an optimal 360m total conductor length was found, achieving 17.2T with an operating current of 22.4kA and a coil comprised of 18×12 turns, shortened of about 20% with respect to the best initial candidate architectural design. The optimal HTS insert has a bore compatible with manufacturing constraints (inner bore radius larger than 30cm). A scaled HTS insert for validation purposes, with a reduced conductor length, to be tested in an advanced experimental facility currently under construction, is also mentioned.