It is well known that screening currents are induced in high-temperature superconducting tapes when they are located in a magnetic field that is changing with time. In recent years, the consequences of these screening currents have become better understood in the context of solenoids built from Rare-earth barium copper oxide (REBCO) double pancakes; specifically, the impact that can occur to the magnetic field distribution and the stress distribution. Pursuing this line of research further, a revisit of the 32-T prototype magnet in terms of order of energizing to the quench event has been investigated. A theoretical method and a finite element simulation have also been performed to study the stress/strain distribution, enthalpy margin, and ac losses affecting the quench of the superconducting solenoids. The analytical derivations of the hoop strain presented in this article are based solely on screening currents, excluding the general radial expansion caused by the transport currents.

Abstract The world’s highest-field dc magnets have, for more than 50 years, consisted of a combination of resistive and superconducting (SC) coils that we refer to as a ‘hybrid’. These magnets use SC technology for the outer coils, where the magnetic field is moderate, and resistive-magnet technology for the inner coils, where the field is highest. In such a configuration, higher fields have been attained than was possible with purely SC magnet technology, and lower lifecycle costs are attained than with a purely resistive magnet. The peak field available has been 45 T for over 20 years in Tallahassee, Florida, USA. There is presently a ‘revolution’ underway in hybrid magnet development. A second 45 T hybrid was completed in 2022 in Hefei, China that might be upgraded to 48 T in a few years. The high field lab in Grenoble, France is also testing a hybrid magnet intended to reach 43.5 T but which also might be upgraded to 46 T in a few years. In addition, the lab in Nijmegen, The Netherlands is presently assembling a hybrid magnet intended to operate at 46 T. Papers have been presented and published with conceptual designs of hybrid magnets with fields up to 60 T. Given the developments underway, this is an appropriate time to review the history of such systems, with a particular focus on the larger, more expensive part of the magnets: the SC outsert coils. The demands placed on the SC coils of these magnet systems are unique due to their coupling with resistive coils that are operated at very high stress and wear out regularly, resulting in large field transients and fault forces. The evolution of the technology used for the SC coils of these hybrid systems is presented, evolving from ventilated windings to cable-in-conduit to cryogen-free.

In the framework of the 40 T all-superconducting magnet project at the National High Magnetic Field laboratory, improvements have been made in a quench model and the quench code that has been constructed to simulate the behavior of a magnet consisting of an HTS inner coil set with an LTS background field. The quench model, initially developed for the NHMFL 32 T magnet project, incorporates additional details of the REBCO conductor on a localized level. These include nonuniform magnetic field distribution across each conductor turn and deformation of the turns due to screening currents which affects the true field angle applied to the conductor's a-b plane. Detailed measurements of the test coil's conductors have also been made to improve the simulations. These include the slope of the a-b plane using X-ray techniques and IC (B, θ, T) of each conductor length using torque magnetometry. The test coil generating a field up to 13 T is an insulated, two-in-hand REBCO coil with twelve modules. Deliberate quench tests were performed with it in self-field and with a 11.4 T background field. The results and discussion of the comparisons are presented.

The QCD (Quantum ChromoDynamics) axion emerged as one of the best-motivated dark matter candidates. In 2018, the Axion Dark Matter eXperiment (ADMX), one of the U.S. Department of Energy's "Gen 2" flagship dark-matter projects, demonstrated first sensitivity to the highly plausible "DFSZ" dark matter axion couplings over a small frequency range. We anticipate this development marks the first step in constructing yet more powerful experiments that can explore large swaths of the axion parameter space at high sensitivity and result in a discovery. But, realizing this requires advances in both our understanding of the theory and experiment design. Between 25 January and 27 January 2021, the "Axions Beyond Gen 2 Workshop" was held, where selected members of the community discussed our broad understanding of the QCD axion and charted a course for future experiments having sensitivity and mass reach well beyond the current "Gen 2" experiments. These proceedings are summaries of the topics presented and discussed.

The Screening Current Induced Field (SCIF) is useful to determine the gross field contribution of screening currents in a superconducting coil, specifically coils wound with large aspect ratio, single filament high temperature superconductors (HTS) such as Rare Earth Barium Copper Oxide (REBCO). Our numerical model accurately predicted the behavior of the SCIF shape and magnitude during energizing to full field but diverged on de-energization. We suggest that axial clamping and its associated consequences at high field are responsible for the difference in the computed SCIF upon de-energization. Simple methods of approximating this effect and comparisons between numerical and measurement results are presented.

High Temperature Superconducting (HTS) materials are now becoming incorporated into magnets that are being used for a variety of physics applications. Axion detection is a particularly attractive application for these conductors and there is significant promise that reliable systems can be built. However, there are still many challenges that are presently unresolved when it comes to building magnets of this scale from these materials. In particular, when a superconducting magnet quenches the energy stored in the magnetic field is converted into heat. If not controlled properly, the energy can be deposited in a non-uniform manner that results in excessive heating in some regions and damage to the magnet. For magnets using traditional Low Temperature Superconductors (LTS) methods of protecting the magnet during quench have been relatively well developed. For the HTS materials this development is presently underway, but no demonstrations protecting coils of the size needed for axion detection have yet been published.

The 32 T all-superconducting magnet of the National High Magnetic Field Laboratory (NHMFL) was successfully tested in December 2017 and it is expected to be soon available for users. This all-superconducting magnet, comprised of a high-temperature superconducting (HTS) insert and a low-temperature superconducting (LTS) outsert, is the first superconducting magnet reaching more than 30 T. One of the challenges facing this new magnet technology is the estimation of the screening currents, and the corresponding hysteresis losses in the two HTS coils. These coils are made of more than 20,000 turns of insulated REBCO conductor connected in series. The modelling of such system represents a significant challenge due to the huge computational load imposed by the size of the system. Up to now, only medium size magnets (made of units of thousands of turns/tapes) have been successfully modelled with methods based on the well-known H formulation of the Maxwell's equations. In the present work, a new model based on the T-A formulation and a homogeneous technique is proposed. This new approach greatly reduces the computational load and allows performing real-time simulations of large-scale HTS magnets on personal computers.

Screening currents have long been known to impact the stress state in tape-wound superconducting coils. In recent years the advent of REBCO tape has led to the development of tape-wound coils by a number of organizations. While several groups have been computing screening currents and ac losses in REBCO tape in a variety of applications for several years, little has been published about the stress due to the screening currents. This problem is challenging due to the need to analyze thousands of REBCO turns which are not bonded together. The T-A formulation of Maxwell's equations employing a homogenization technique enables efficient estimation of the current distribution while structural calculations employing contact elements allow conservative estimation of stresses. Computational results are compared with observed degradation in a test coil. Future coil designs that include the effects of screening current strains are proposed

The National High Magnetic Field Laboratory has launched an innovative project to develop a 40 T all superconducting user magnet. The first year funding was awarded by the National Science Foundation in September 2018. Consideration of a 40 T superconducting user magnet sets target specifications of a cold bore of 34 mm with a homogeneity of 500 ppm over a 1 cm diameter of spherical volume, a better than 0.01 T set-ability and stability, and with an ability to ramp up to full field 50,000 times over its 20 years design lifetime. It will be a fully superconducting magnet that can withstand quenches at its full 40T field and provide a very low noise environment for experimentalists. These capabilities will enable the 40 T SC magnet to support higher-sensitivity measurements than possible in present-day resistive and hybrid magnets; high-magnetic-field measurements that will be uniquely capable of addressing physics questions on a number of expanding frontiers in condensed matter physics. A 40 T SC magnet would enable more users to run long experiments at peak field with much less power consumption compared with resistive and hybrid magnets. However, realization of such a 40 T SC magnet requires magnet technology well beyond the present state-of-the-art. Initial analysis of different HTS magnet designs, based upon the three presently viable HTS conductors: REBCO, Bi-2212, and Bi-2223, has determined that each technology faces significant challenges. Hence, we decided that four HTS magnet technologies consisting of Insulated REBCO, No-Insulation REBCO, Bi-2212, and Bi-2223 would be developed in parallel and technology gaps based on major risks will be closed in the R&D phase. The candidate technologies will be narrowed down at the decision points. The objective and R&D activities of the 40 T all superconducting user magnet project are presented.

There are growing concerns about the stresses created by shielding currents in high field superconducting magnets fabricated from tape conductors leading to reduced performance and lifetime. This paper presents results of stress/strain calculations caused by shielding currents assuming the conductor deformations follow a linear constitutive relation. An anisotropic bulk approximation approach was used to calculate the electromagnetic field distributions in a REBCO high field coil with a stack of pancakes and a large number of turns first, and then the Lorentz force distribution and mechanical response characteristics were studied in the two-dimensional axisymmetric configuration. A new discrete contact mechanical model implemented by the finite element method, which is able to simulate the contact and separation behaviors between adjacent turns during the deformation, was proposed to analyze the distributions of hoop stress, hoop strain, radial stress and radial displacement in the coil. The influences of shielding current on those mechanical responses were obtained by comparing the simulation cases with and without taking shielding current into account. Besides, a continuum bulk mechanical model, which is parallel to the discrete contact mechanical model and treats the pancakes as continuum bulks, was modeled as well in order to understand the influences of different models on the simulation results. Furthermore, we studied the influences of a couple of practical factors (including the n-value, ramp rate, and operating mode of the REBCO coil winding) on the shielding current and hoop stress. A couple of novel and important conclusions were found. (1) Neglecting the shielding current behavior would significantly underestimate the maximum local hoop stress in a REBCO high field coil. (2) The continuum bulk mechanical model is not adequate for the stress analysis of dry-wound high field coils, by which unreasonably large tensile radial stresses could be obtained. (3) The highest local hoop stresses at the fully-charged moment and the fully-discharged moment are located in a certain pancake near the end of the coil winding and the end pancake, respectively. (4) Decreasing the n-value and the ramp rate of the REBCO coil could be two auxiliary ways to suppress the shielding current and maximum local hoop stress in the coil. (5) For the ramp-and-hold operating mode, the REBCO coil experiences the highest stress level at the moment when it right achieves the goal field. (6) The cycling operation of a REBCO high field coil can cause the tape experiencing alternative positive and negative hoop stresses and this may decrease the fatigue life of the tape and then the life of a magnet.

The National High Magnetic Field Laboratory (NHMFL) has completed testing and commissioning of a unique ultrahigh field magnet that provides 36 T in a 32 mm bore with field inhomogeneity and stability better than 1 ppm over a cylinder of 1 cm diameter and length and a duration up to a few hours. While the magnet meets all its primary performance goals, there were some unexpected observations during the testing and commissioning activities. Primarily, while the magnet reaches full current and field, we are unable to charge the magnet as quickly as was anticipated. Also, the magnet occasionally quenches for no apparent reason. Charging the magnet at slower rates reduces the probability of quenches.

The High Magnetic Field Laboratory in Nijmegen has been collaborating with the National High Magnetic Field Laboratory (NHMFL) in Florida on a 45 T hybrid magnet project. The primary scope of the collaboration was the design and manufacture of the hybrid magnet's superconducting cold mass. The 7.5 ton cold mass includes a single 13 T solenoid wound with high J(c) RRP Nb-3 Sn/Cu cable-in-conduit conductor. The coil will he forced flow supercritical helium and operated in parallel with a set of Bitter-disk resistive coils. Coil winding, reaction heat treatment, epoxy impregnation, and cold mass assembly has been completed at the NHMFL. The full cold mass has been delivered to Radboud University and will be assembled with the cryostat and interfaced with the system utilities.

It is a general belief that no insulation (NI) coil technology is a path to very high field superconducting coils. Recent experience has shown that there are aspects of NI coil design that, if not addressed, can possibly lead to coil failures. One potential problem area is the large transient currents that are associated with quench propagation in NI coils. In an attempt to understand and possibly find ways to minimize the potential for damage from quench transients, a parameter study was undertaken to examine the factors that influence the magnitude of transient currents during quench in NI coils. The characteristics of the transient currents are first examined. A study is then made of a set of test coils, looking at quench propagation and the transient current magnitude as a function of contact resistance, critical current, and importantly coil size. For each coil size, it is found that as the contact resistance increases, the magnitude of quench transient currents is reduced until a condition where effective quench propagation ceases, called the quench propagation limit (QPL). As the QPL is approached, the amplitude of the transient current is decreased and may provide a regime where quench induced stress can be effectively contained in coil designs. As coil size increases, the value of contact resistance associated with the limit of quench propagation increases as well. At large coil sizes that will be characteristic of high field REBCO magnets, the QPL extends to truly large values of contact resistance compared to values observed between bare conductors. The use of methods such as resistive films on conductors and co-wind steel will be required to increase contact resistance. In recognition of this development, the use of high contact resistance achieved in this manner is appropriately called resistive insulation coil technology.

The National High Magnetic Field Laboratory has commissioned a 36.1 T resistive/superconducting hybrid magnet with homogeneity and stability of 1 ppm over a 10 mm diameter spherical volume to be used for solid-state nuclear magnetic resonance (NMR). Most NMR magnets use single strands of superconducting wire carrying a few hundred amps and persistent joints and switches. This magnet uses a 20 kA superconducting cable in a steel conduit for the outer part of the magnet and copper-alloy sheet metal for the inner part of the magnet. While >15 hybrid magnets have been built worldwide, they typically have a field uniformity of ~250 ppm/cm DSV and stability might be no better than 50 ppm. To attain 1 ppm uniformity, current density grading was employed in the resistive coils to cancel the z2 term. In addition, coils were shifted after the first map to reduce the z1 term. Ferroshims and resistive shims were installed in the bore to attain <;1 ppm over 10 mm. The large inductance of the superconducting coil reduced the ripple sixfold compared with all-resistive magnets and essentially eliminated the 60 Hz ripple and its harmonics. An NMR lock reduced the low-frequency drift to attain ~0.1 ppm stability.

In 2005, the Committee on Opportunities in High Magnetic Fields issued a challenge to develop a 30 T high-resolution NMR magnet. In response, the National High Magnetic Field Laboratory (NHMFL) is investigating all three commercially available high-temperature superconductors including REBCO, Bi-2212 and most recently, a reinforced Bi-2223 conductor supplied by Sumitomo Electric, designated Type HT-NX. Recent investigations of Type HT-NX conductor at the NHMFL and by others suggest that operation at hoop stress above 400 MPa, and total strain above 0.7% may be feasible. We have fabricated a test coil from a single 240 m length of HT-NX. The coil was successfully operated to 19.5 T in a 14 T background field, with a total applied strain of 0.8% and coil current density of 243 A/mm(2). The coil was cycled 20 times from half the design current to full current without observed degradation.

The three goals of this paper are: 1) to evaluate the improvements in technology for increasing magnetic flux density (magnetic field) to 14 T in the next few years and eventually to 20 T; 2) to highlight neuroscience opportunities enabled by these advances; and, 3) to evaluate the physiological and biophysical effects associated with MRI at very high performance levels. Substantial recent advances in magnet technology including superconductor developments enable neuroscience goals that are not obtainable at contemporary magnetic fields. Ten areas of brain neuroscience include potential improvements in resolution for functional MRI(BOLD), diffusion weighted MRI, tractography, susceptibility weighted MR, neuronal architecture patterns related to human behavior, proton spectroscopy of small brain biochemicals, chemical exchange saturation transfer (CEST), dynamic contrast enhanced MRI, brain energy metabolism using 13C, 17O, and 31P; and brain electrolyte physiology using 23Na, 35Cl, and 39K. Physiological phenomena and safety aspects include: absorbed RF power, acoustic sound pressure levels, induced electric fields, Lorentz forces, magnetohydrodynamic forces, and biophysical phenomena in cells and tissues. Where feasible, effects are quantified for magnetic fields beyond 7 T with the conclusion that there are no foreseen barriers either in the technical or human safety aspects of brain MRI and MRS at fields up to 20 T. This conclusion is conditioned on results of recommended experiments to verify the predicted level of physiological effects beyond 9.4 T. This technology is predicted to enable quantification of biochemical components of the functioning brain not detectable heretofore.

This white paper summarizes the workshop "U.S. Cosmic Visions: New Ideas in Dark Matter" held at University of Maryland on March 23-25, 2017.

The system integration of a 36 T high field resistive-superconducting hybrid magnet system which will be used primarily for NMR studies is being finalized at the National High Magnetic Field Laboratory in Tallahassee, Florida. The hybrid magnet consists of a 23 T resistive insert coil set nested inside of a 13 T superconducting coil wound with high JC Nb3Sn/Cu cable-in-conduit conductor. The resistive and superconducting coils are connected electrically in series and operate at 20 kA. The combined system will enable 1-ppm level uniformity in a 1 cm DSV for condensed-matter NMR at a record level of 1.53 GHz. The resistive insert has four concentric Florida-Bitter coils and operates at 14 MW. The superconducting outsert operates with forced flow supercritical helium at 4.5 K and 3.5 bar. The superconducting protection system consists of two fully independent circuits, one employing voltage averaging comparisons between winding layers and the other a voltage comparison with a co-wound coil. Initial magnet testing of the resistive coils has been completed. The superconducting cold mass has been cooled to 4.5 K and the system is being prepared for combined hybrid magnet testing.

The National High Magnetic Field Laboratory has brought to field a Series-Connected Hybrid magnet for NMR spectroscopy. As a DC powered magnet it can be operated at fields up to 36.1T. The series connection between a superconducting outsert and a resistive insert dramatically minimizes the high frequency fluctuations of the magnetic field typically observed in purely resistive magnets. Current-density-grading among various resistive coils was used for improved field homogeneity. The 48mm magnet bore and 42mm outer diameter of the probes leaves limited space for conventional shims and consequently a combination of resistive and ferromagnetic shims are used. Field maps corrected for field instabilities were obtained and shimming achieved better than 1ppm homogeneity over a cylindrical volume of 1cm diameter and height. The magnetic field is regulated within 0.2ppm using an external 7Li lock sample doped with paramagnetic MnCl2. The improved field homogeneity and field regulation using a modified AVANCE NEO console enables NMR spectroscopy at 1H frequencies of 1.0, 1.2 and 1.5GHz. NMR at 1.5GHz reflects a 50% increase in field strength above the highest superconducting magnets currently available. Three NMR probes have been constructed each equipped with an external lock rf coil for field regulation. Initial NMR results obtained from the SCH magnet using these probes illustrate the very exciting potential of ultra-high magnetic fields.