HTS Insert Research Articles

Mechanical Support of the NHMFL 32 T Superconducting Magnet

The HTS insert for the 32 T superconducting magnet is comprised of two dry wound, double pancake REBCO coils of different lengths, nested within an LTS outsert. The high fields present within the HTS coils create large axial compression forces and significant radial expansion. The mechanical solutions implemented to account for the effects of axial and radial coil movement are described. These solutions are applied throughout many of the mechanical subsystems in and between the HTS coils, including electrical terminations, electrical bussing, external reinforcing of the windings, and suspension of the HTS coils within the outsert. Experiences from the assembly in preparation for the initial operation of the magnet are also shared.

Test of an 8.66-T REBCO Insert Coil with Overbanding Radial Build for a 1.3-GHz LTS/HTS NMR Magnet.

A 1.3-GHz/54-mm LTS/HTS NMR magnet, assembled with a 3-coil (Coils 1-3) 800-MHz HTS insert in a 500-MHz LTS NMR magnet, is under construction. The innermost HTS insert Coil 1 has a stack of 26 no-insulation (NI) double pancake (DP) coils wound of 6-mm wide and 75-μm thick REBCO tapes. In order to keep the hoop strains on REBCO tape < 0.6% at an operating current Iop of 250 A and in a field of 30.5 T, we overbanded each pancake in Coil 1 with a 6-mm wide, 76-μm thick 304 stainless steel strip: 7-mm thick radial build for the central 18 pancakes, while 6-mm thick for the outer 2×17 pancakes. In this paper, Coil 1 was successfully tested at 77K and 4.2 K. In the 77-K test, the measured critical current was 35.7 A, determined by an E-field criterion of 0.1 μV/cm. The center field magnet constant decreased from 34.2 mT/A to 29.3 mT/A, when Iop increased from 5 A to 40 A. The field distribution at different Iop along the z-axis was measured. The residual field distributions discharged from 10 A and 20 A were recorded. In the 4.2-K test, Coil 1 successfully generated a central field of 8.78 T at 255 A. The magnet constant is 34.4 mT/A, which is same as our designed value. The field homogeneity at the coil center within a ±15-mm region is around 1700 ppm. This large error field must be reduced before field shimming is applied.

Design Study of a 10-T REBCO Insert Solenoid

As a first step to develop HTS insert technology, we plan to build a 10-T REBCO solenoid that will be tested at the French National High Magnetic Field Laboratory of Grenoble in France (LNCMI) at 4.2 K, in the aperture of a 20-T resistive magnet. Two winding approaches are being considered for the HTS insert, i.e., the no-insulation (NI) and metallic insulation (MI) winding technics. Both methods allow turn-to-turn bypassing current but differ in their turn-to-turn resistance value by three orders of magnitude. In this paper, we provide a comparative study between the two winding options using numerical simulations applied to a preliminary design of the insert, which features a cold bore diameter of 50 mm and is made of 15 double-pancake coils wound from a 6-mm REBCO tape.

HTS Dipole Magnet for a Particle Accelerator Using a Twisted Stacked Cable

High- ${T}_{{c}}$ superconducting (HTS) dipole magnets are expected to be used in the upgrade of existing particle accelerators. In the framework of EuCARD-2 project, a new dipole-generating field between 17 and 20 T is under consideration with an HTS insert providing 5 T. This has to provide a good field quality at two-thirds of 40-mm beam aperture. The HTS stacked cable is the simplest way to acquire the large current and a strong mechanical cable. A twist for a half-turn in the magnet ends partially transposes the stacked tapes and improves the current distribution. However, the twisted stacked cable exposes the ReBCO tape to magnetic fields perpendicular to their wide face, where the engineering current density ${J}_{{e}}$ is lower. In addition, the different strength and direction of forces cause shear stress between the tapes. The HTS twisted stacked insert is designed with a 4 $\times$ 4-mm block-coil stacked with 4-mm-wide tape. The 46-turns of stacked cable, which is almost aligned to the background field, achieve a 5 T with the required field quality. The 3-D magnet ends are designed with the racetrack and flared-ends coils. It consists of six double pancakes. One of the mechanical conditions in the coil-ends, whose block-coil is divided into four tapes, has been analyzed with the Fresca2 LTS outsert. The HTS insert is able to withstand forces on its own.

A Parametric Study on Overband Radial Build for a REBCO 800-MHz Insert of a 1.3-GHz LTS/HTS NMR Magnet.

A high-resolution 1.3-GHz/54-mm low-temperature superconducting/high-temperature superconducting (HTS) nuclear magnetic resonance magnet (1.3 G) is currently being built at Francis Bitter Magnet Laboratory, Massachusetts Institute of Technology. One of its key components is an 800-MHz HTS insert (H800) comprising three nested coils. Each coil is a stack of double-pancake coils wound with 6-mm-wide 75-μm-thick REBCO tape. For this H800 generating its self-field of 18.6 T and being exposed to a total field as high as 30.5 T, overbanding each pancake coil is necessary to keep the conductor strain at < 0.6%. Although electromagnetic and mechanical details of the H800 had been considered during its design stage, a parametric study on the overband radial build considering winding tension effect should further confirm the results of our previous analysis. Thus, in this paper, based on Maxwell's equations and the equilibrium equations for mechanical deformation, we examine stress levels that the H800 experiences as H800 undergoes winding-energizing sequences during operation at 1.3 GHz. We also discuss the effects of overband radial build and winding tension on conductor stress in each coil. Finally, based on this analysis, we may further optimize the stainless-steel overbanding and winding tension on each H800 coil.

A High-Resolution 1.3-GHz/54-mm LTS/HTS NMR Magnet.

A high-resolution 1.3-GHz/54-mm low-temperature superconducting/high-temperature superconducting (HTS) nuclear magnetic resonance magnet (1.3 G) is currently in the final stage at the Massachusetts Institute of Technology Francis Bitter Magnet Laboratory. Its key component is a three-coil (Coils 1-3) 800-MHz HTS insert comprising 96 no-insulation (NI) double-pancake coils, each wound with a 6-mm-wide GdBCO tape. In this paper, after describing the overall 1.3-G system, we present innovative design features incorporated in 1.3 G: 1) an NI winding technique applied to Coils 1-3 and its adverse effect in the form of charging time delay; 2) persistent-mode HTS shims; 3) a "shaking" magnet; and 4) preliminary results of Coil 1 operated at 4.2 K.

AC Losses of an HTS Insert in a 25-T Cryogen-Free Superconducting Magnet

A 25-T cryogen-free superconducting magnet (25T-CSM) is being developed at the High Field Laboratory for Superconducting Materials, Institute for Materials Research, Tohoku University. In the case of a cryogen-free superconducting magnet, the coil temperature rises during a sweep of an operating current due to ac losses. Hence, the ac loss estimation is very important for the cooling design. The critical current density and the magnetization of Gd123 tapes at 4.2 K were measured in order to estimate the ac loss of a Gd123 insert of the 25T-CSM. The ac loss is actually the hysteresis loss, which are calculated from the J c properties taking the magnetic field distribution in the Gd123 coil into account. Regarding the effect of tape stacking (or winding) in a pancake coil, the slab approximation can be used for hysteresis loss calculation. In the case of slab model, the full penetration field at the center of the tape becomes higher than the maximum applied magnetic field in the most part of the coil. As a result, the hysteresis loss increases with increasing a magnetic field when the magnet energizes. The hysteresis losses assuming the slab model, however, show an opposite field dependence to those calculated from the strip model without the stacking effect. Hence, the ac loss of 5 W is estimated when the magnet energizes to 25.5 T within 60 min.

Reducing Modeling Domain to Speed-Up Quench Simulations of HTS Coils

Three-dimensional quench analyses of large magnet systems, such as low- and high-temperature superconductor hybrids for particle accelerators of future, are computationally very heavy when cable level details are taken into account. Mesh based modelling methods, like finite-element method, will result in a large amount of elements and consequently degrees of freedom in these simulations. In this paper we simulate the quench using mesh structures including cable and insulation layers in some parts of the coil system. Elsewhere we consider the coil as a homogenous mixture of cable and insulation. Futhermore, in some simulations we completely neglect parts of the magnet system. Then we study how these modelling decisions affect the simulated hot spot temperature rise and development of resistive voltage. Using this kind of geometry adaptation in an appropriate way we can considerably shorten simulations without losing reliability of the results. In this paper we compare how modeling decisions related to representing the magnet in a modelling software affect the important quench parameters and computation time. Examples are computed with an in-house programmed quench software for a 6T HTS insert designed within the EuCARD project.

The Manufacture and Test of a GdBCO Insert Coil for an 11 T Superconducting Magnet

In this paper, an 11 T hybrid (LTS/HTS) magnet was designed and developed. The LTS background magnet generates 9 T and the HTS insert coil generates 2 T. The inner diameter of the HTS coil is 40 mm. The outer diameter of the LTS magnet is 176 mm. The height of the inner HTS magnet and the outer LTS magnet are 100 and 200 mm, respectively. The LTS magnet and the HTS coil were fabricated using NbTi wire and GdBCO tape, respectively. The LTS background magnet was fabricated without mandrel, thus reducing shear stress, and using a pre-preg material between each layer. Pre-preg is a thin, glass-woven fabric impregnated with a resin. The electromagnetic field analysis presented here was performed with respect to the structure and the operating current of both the LTS magnet and HTS coil. The hybrid (LTS/HTS) magnet was operated at liquid helium temperature 4.2 K. After several quench, the maximum magnetic field of the center was 10.51 T.

Observations from the Analyses of Magnetic Field and AC Loss Distributions in the NHMFL 32 T All-Superconducting Magnet HTS Insert

A 17 T high-temperature superconducting two-coil magnet (insert) to be operated in a 15 T low-temperature superconducting multisection magnet (outsert) is the most demanding part of the National High Magnetic Field Laboratory all-superconducting 32 T magnet system. The HTS coils are of the pancake type and to be wound with REBCO coated conductors/tapes manufactured by SuperPower, Inc. The distribution of AC losses in the HTS windings during the magnet charging/discharging process are computed and analyzed with due regard for the AC loss density dependence on the magnetic field and the field angle. The calculations are based on the measured magnetization of a representative sample against magnetic field and field angle. The results enable determination of heat load on the magnet and its cryogenic system. Since the magnet is of the pool-cooled type, a related helium vapor bubble problem can develop owing to the high field and field gradients, and the diamagnetic susceptibility of helium.

A New High-Temperature Superconducting (HTS) 700-MHz Insert Magnet for a 1.3-GHz LTS/HTS NMR Magnet.

Upon the totally unexpected theft of the original 600 MHz HTS insert that occurred in December 2011, we were forced to examine our entire 1.3 GHz NMR Magnet program anew and determined that a combination of a 600 MHz LTS magnet and a 700 MHz HTS insert (H700) would yield a 1.3 GHz LTS/HTS magnet that meets the technical specifications consistent with economic constraints. Although this new 700 MHz HTS insert still comprises, as H600, a YBCO inner coil and a Bi2223 outer coil, it incorporates innovative design features. In addition to presenting the major design parameters of the new H700, we discuss here its key electromagnetic and mechanical issues.

No-Insulation (NI) HTS Inserts for > 1 GHz LTS/HTS NMR Magnets.

A no-insulation (NI) technique has been applied to wind and test a NI HTS (YBCO) double-pancake coil at 4.2 K. Having little detrimental effect on field-current relationship, the absence of turn-to-turn insulation enabled the test coil to survive a quench at a coil current density of 1.58 kA/mm2. The NI HTS coil is compact and self-protecting, two features suitable for large high-field magnets. To investigate beneficial impacts of the NI technique on >1 GHz LTS/HTS NMR magnets, we have designed six new NI HTS inserts for our ongoing 1.3 GHz LTS/HTS NMR magnet, which require less costly LTS background magnets than the original insulated HTS insert. A net result will be a significant reduction in the overall cost of an LTS/HTS NMR magnet, at 1.3 GHz and above.

On the 600 MHz HTS Insert for a 1.3 GHz NMR Magnet.

We update here on the status of one of the main components of a 1.3 GHz NMR magnet currently being developed at the MIT Francis Bitter Magnet Laboratory (FBML): the 600 MHz HTS insert coil. Started in 2000 as a 3-phase program and currently in its final phase (Phase 3A), the HTS insert will first be installed in the bore of a 500 MHz all LTS magnet, generating a field of 25.8 T (1.1 GHz). In Phase 3B, we will place the HTS insert in the bore of a 700 MHz LTS magnet, achieving our ultimate goal of completing a high-resolution 1.3 GHz magnet. The HTS insert, built as a stack of double pancakes (DP), consists of two concentric stack of coils, the inner coil wound with YBCO tape and the outer coil with Bi2223/Ag tape. The series-connected coils, immersed in a bath of liquid helium at atmospheric pressure, will be operated in driven mode. The paper describes detailed aspects of: 1) design and fabrication of both coils, 2) testing of individual DPs at 77 K to characterize the current carrying capabilities and magnetic performance of each DP coil, 3) techniques and elements utilized in the DP-to-DP splice procedure and, 4) splice dissipation.

A 1.3-GHz LTS/HTS NMR Magnet-A Progress Report.

In this paper we present details of a 600 MHz HTS insert (H600) double pancake (DP) windings. It will first be operated in the bore of a 500 MHz LTS magnet, achieving a frequency of 1.1 GHz. Upon completion of H600, we will embark on the final phase (Phase 3B) of a 3-Phase program began in 2000: completion of a high resolution 1.3 GHz LTS/HTS magnet. In Phase 3B, the H600 will be coupled to a 700 MHz LTS magnet to achieve the ultimate frequency of 1.3 GHz. The HTS insert is composed of two concentric stacks of double pancakes, one wound with high strength BSCCO-2223 tape, the other with YBCO coated conductor. Details include conductor and coil parameters, winding procedure, DPs mechanical support and integration to the background 500 MHz LTS magnet. Test results of individual DPs in LN2 are also presented.

An Experimental Study on the Joint Methods Between Double Pancake Coils Using YBCO Coated Conductors

In these days, some researches on the HTS insert coil/LTS background coil (hybrid coil) for nuclear magnetic resonance (NMR) system have been conducted. Magnetic field homogeneity is very important in NMR system. Because the direction of the joint part between a stack of double pancake coils (DPCs) can cause magnetic field homogeneity problem, the joint methods between the DPCs are important. YBCO coated conductor tapes are manufactured in various widths.They can be used as an auxiliary superconducting tape at the joint part between the DPCs for enhancing magnetic field homogeneity. The joint characteristics between the DPCs can vary with the conditions of the auxiliary tape such as numbers, widths, and directions. In this work, we conducted joint experiments under various conditions of the auxiliary tape and analysed the joint characteristics. The joint methods in this work are expected to be used to fabricate an insert coil in NMR system.

Progress on the Development of a 5 T HTS Insert Magnet for GHz Class NMR Applications

The Korea Basic Science Institute (KBSI) has initiated the development of a 5 T HTS insert magnet system. The objective of this program is the design and fabrication of HTS insert magnet for GHz class LTS/HTS NMR applications. Compact and efficient HTS magnet system is designed to be inserted into 15 T conduction-cooled superconducting magnet with 100 mm room-temperature bore. In this paper, the status of the HTS insert magnet development in KBSI is presented. The design issues of the 5 T HTS insert magnet including 2G HTS conductor properties with additional reinforcement and insulation, coil configuration with supporting structures, magnetic winding and bending stress-strain, detect and activate protection, and spatial and temporal field performance are discussed. In addition, the design of cryostat for 5 T HTS insert magnet is presented.

Modular Test Facility for HTS Insert Coils

The final beam cooling stages of a Muon Collider may require DC solenoid magnets with magnetic fields in the range of 40-50 T. In this paper we will present a modular test facility developed to investigate very high fields using double pancake coils made of commercially available 2G HTS materials. The conductor performance is presented, together with magnetic calculations and an evaluation of Lorentz forces distribution in the coils when inserted in an outer solenoid. Finally, results from tests in nitrogen and helium of a double pancake coil are presented up to an external magnetic field of 14 T.

An Analytical Technique to Elucidate Field Impurities From Manufacturing Uncertainties of an Double Pancake Type HTS Insert for High Field LTS/HTS NMR Magnets.

This paper addresses adverse effects of dimensional uncertainties of an HTS insert assembled with double-pancake coils on spatial field homogeneity. Each DP coil was wound with Bi2223 tapes having dimensional tolerances larger than one order of magnitude of those accepted for LTS wires used in conventional NMR magnets. The paper presents: 1) dimensional variations measured in two LTS/HTS NMR magnets, 350 MHz (LH350) and 700 MHz (LH700), both built and operated at the Francis Bitter Magnet Laboratory; and 2) an analytical technique and its application to elucidate the field impurities measured with the two LTS/HTS magnets. Field impurities computed with the analytical model and those measured with the two LTS/HTS magnets agree quite well, demonstrating that this analytical technique is applicable to design a DP-assembled HTS insert with an improved field homogeneity for a high-field LTS/HTS NMR magnet.

Nonlinear Behavior of a Shim Coil in an LTS/HTS NMR Magnet With an HTS Insert Comprising Double-Pancake HTS-Tape Coils.

This paper reports results, experimental and analytical, of the nonlinear behavior of a shim coil in the presence of an HTS coil assembled with double-pancake (DP) HTS-tape coils. The experimental results are from: 1) operation of a 700 MHz LTS/HTS NMR magnet (LH700) consisting of a 600 MHz LTS NMR magnet (L600) equipped with superconducting shim coils and a 100 MHz DP-assembled HTS insert (H100) and; 2) an experiment with a room-temperature (RT) Z1 shim coil coupled to a 50 MHz DP-as-sembled HTS insert (H50). A field mapping theory with a harmonic analysis is applied to interpret both results. Based on experimental results and analyses, we conclude that the screening-current-induced field (SCF) generated by a DP-assembled HTS insert is primarily responsible for the nonlinear behavior, including strength degradation, of a shim coil.

Stress Analysis of a High Temperature Superconductor Coil Wound With Bi-2223/Ag Tapes for High Field HTS/LTS NMR Magnet Application

The electromagnetic stress distribution inside a HTS insert is one of the key issues for construction of a high field high/low temperature superconductor (HTS/LTS) magnet. The rmiddotJmiddotB formulae is widely used for the stress calculation of the coil, due to its simplicity. However, the assumption of this calculation makes it impossible to consider the effects of the bobbin, an epoxy impregnated structure, on stress of the coil. In this work, stress analysis methods are reported. The governing equation defined from the force equilibrium state was adopted and used to analyze what the stress of the HTS coil wound with Bi2223/Ag conductor. The validness of the stress analysis method was confirmed by comparison with experimental results, especially in the coil with epoxy impregnation. The numerical and experimental procedures are described and both results are discussed.