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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 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.

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.

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 JC RRP Nb 3 Sn/Cu cable-in-conduit conductor. The coil will be 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.

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 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

In 2005 the Committee on Opportunities in High Magnetic Fields (COHMAG) 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 (HTS) 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 14T in the next few years and eventually to 20T; 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 7T 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 20T. This conclusion is conditioned on results of recommended experiments to verify the predicted level of physiological effects beyond 9.4T. This technology is predicted to enable quantification of biochemical components of the functioning brain not detectable heretofore.

Helmholtz Zentrum Berlin HZB operates two large scale facilities the research reactor BER 2 and the syn chrotron source for soft X rays BESSY 2. This year HZB s neu tron instrument suite around BER 2 has been strengthened by a unique high magnetic field facility for neutron scattering. Its main components are the High Field Magnet HFM , which is the most powerful dc magnet for neutron scattering worldwide, and the Extreme Environment Di ffractometer EXED , which is a dedicated neutron instrument for time of flight technique. The hybrid magnet system is projected according to the special geo metric constraints of analyzing samples by neutron scattering in a high field magnet. Following our past experience, only steady state fields are adequate to achieve the goals of the project. In particular, inelastic scattering studies would virtually be excluded when using pulsed magnets. The new series connected hybrid magnet with a horizontal field orientation was designed and constructed in collaboration with the National High Magnetic Field Laboratory NHMFL , Tallahassee, FL, USA. With a set consisting of a su perconducting cable in conduit coil and different resistive coils of conical shape, maximum fields between 26 31 T are possible with cooling power between 4 and 8 MW for the resistive part. A series of commissioning activities of the magnet components and the technical infrastructure systems 20 kA power supply, water cooling, and 4 K Helium refrigerator was completed at HZB. The maximum field achieved with a 4 MW resistive coil was 26 T

An initiative to design and build magnetic resonance imaging (MRI) and spectroscopy (MRS) instruments at 14 T and beyond to 20 T has been underway since 2012. This initiative has been supported by 22 interested participants from the USA and Europe, of which 15 are authors of this review. Advances in high temperature superconductor materials, advances in cryocooling engineering, prospects for non-persistent mode stable magnets, and experiences gained from large-bore, high-field magnet engineering for the nuclear fusion endeavors support the feasibility of a human brain MRI and MRS system with 1 ppm homogeneity over at least a 16-cm diameter volume and a bore size of 68 cm. Twelve neuroscience opportunities are presented as well as an analysis of the biophysical and physiological effects to be investigated before exposing human subjects to the high fields of 14 T and beyond.

The National High Magnetic Field Laboratory has developed and operated large, high-field dc and pulsed magnets for research in condensed matter physics. We are now developing three resistive/superconducting hybrid magnets with fields ranging from 25 to 45 T, and are developing concepts for hybrid magnets up to 60 T as well human-head MRI magnets up to 20 T and repetitively pulsed magnets to reach 60 T every 30 sec.

The final assembly of the Series-Connected Hybrid magnet system for the Helmholtz-Zentrum Berlin for Materials and Energy (HZB) has occurred with the integration of the superconducting cold mass, cryostat, resistive Florida-Bitter coils, and the cryogenic, chilled water, power, and control subsystems. The hybrid magnet consists of a 13-T superconducting $\hbox{Nb}_{3}\hbox{Sn/CICC}$ coil and a set of 12-T resistive, water cooled coils at 4.4 MW. Much of the cryostat and cold mass functional requirements were dictated by the electromagnetic interactions between the superconducting and resistive coils. This includes the radial decentering and axial aligning forces from normal operations and a 1.1 MN fault load. The system assembly was an international achievement with the cold mass being completed at the NHMFL in the USA, cryostat to cold mass interfaces made at Criotec Impianti in Italy, and final assembly at the HZB in Germany.

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 36.1 T. 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 48 mm magnet bore and 42 mm 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 1 ppm homogeneity over a cylindrical volume of 1 cm diameter and height. The magnetic field is regulated within 0.2 ppm 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 1.5 GHz. NMR at 1.5 GHz reflects a 50% increase in field strength above the highest superconducting magnets available presently. 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.

We performed a feasibility study on a high-strength Bi2-x Pb x Sr2Ca2Cu3O10-x (Bi-2223) tape conductor for high-field solenoid applications. The investigated conductor, DI-BSCCO Type HT-XX, is a pre-production version of Type HT-NX, which has recently become available from Sumitomo Electric Industries (SEI). It is based on their DI-BSCCO Type H tape, but laminated with a high-strength Ni-alloy. We used stress-strain characterizations, single- and double-bend tests, easy- and hard-way bent coil-turns at various radii, straight and helical samples in up to 31.2 T background field, and small 20-turn coils in up to 17 T background field to systematically determine the electro-mechanical limits in magnet-relevant conditions. In longitudinal tensile tests at 77 K, we found critical stress- and strain-levels of 516 MPa and 0.57%, respectively. In three decidedly different experiments we detected an amplification of the allowable strain with a combination of pure bending and Lorentz loading to ≥ 0.92% (calculated elastically at the outer tape edge). This significant strain level, and the fact that it is multi-filamentary conductor and available in the reacted and insulated state, makes DI-BSCCO HT-NX highly suitable for very high-field solenoids, for which high current densities and therefore high loads are required to retain manageable magnet dimensions.

Recently, significant improvement in the strain tolerance of Bi-2223 conductor has been achieved by lamination with high strength nickel alloy. The conductor, supplied by Sumitomo Electric and designated Type HT-NX, is now commercially available in lengths sufficient for manufacture of high-homogeneity solenoids. A program to fully exploit the improved conductor properties is now underway at the National High Magnetic Field Laboratory (NHMFL). Five coils are being made, the last of which is to demonstrate an NMR measurement approaching 1 GHz and 1 ppm over 10-mm volume. In so doing, we expect to demonstrate critical current fraction, and strain similar to that expected in 30-T NMR magnets. The coils will be tested inside an existing 16 Tesla large-bore background magnet at the NHMFL. The design of the NMR demonstration coil is presented first, with expected values for field, homogeneity, and strain given. A technology development program is then outlined, which includes fabrication of four test coils to test various design features, develop fabrication tooling, and train personnel.

Recent activities that were conducted on the Helmholtz Zentrum Berlin Series-Connected Hybrid outsert coil included the reaction heat treatment and vacuum-pressure impregnation. A thermocouple was wound into the windings for diagnostic purposes during processing of the coil. To maintain a reasonable temperature differential across the coil, the ramp rate was limited to approximately 2.7°C/hr. Epoxy impregnation was conducted with an epoxy developed in-house. The epoxy is transferred in vacuum but cured with one atmosphere gage pressure applied. Subsequently, the winding hardware was removed including the stainless steel mandrel through a machining operation. After assembly of voltage breaks and voltage taps, Paschen tests to 4 kV were conducted successfully that qualified the ground insulation. In addition, surge tests to qualified the internal insulation with 3 kV applied across the leads were conducted. Impedance measurements were conducted to confirm the internal insulation integrity.

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.

To extend its user's facilities, the High Field Magnet Laboratory (HFML-EMFL) at the Radboud University is in the process of building a 45-T hybrid magnet. The magnet system will consist of a 22 MW 32.7 T resistive insert and a 600-mm-bore 12.3 T superconducting outsert magnet, and the design was significantly adjusted after a thorough design revision in 2011. The HFML hybrid magnet will be operated with separate current sources for the superconducting and resistive coils (20 kA at 10 V and 40 kA at up to 550 V, respectively). The outsert coil is a solenoid layer wound with all-Nb 3 Sn/Cu cable-in-conduit conductor (CICC), cooled by a forced flow of supercritical helium and operated at 20 kA. Similar to the series-connected hybrids for the HZB (Berlin, Germany) and the NHMFL (Tallahassee, FL, USA), the HFML outsert coil contains three grades of conductor. All CICC grades are based on high-current density Nb3Sn strand produced by Oxford Superconducting Technology. The CICC production and qualification program has been completed successfully. The coil will be wound at the NHMFL and, after heat treatment and impregnation, sent to Nijmegen for integration into the cryostat. In this paper, the design choices and current status of the program are presented.

The National High Magnetic Field Laboratory (NHMFL) is designing a series-connected hybrid magnet, which has a 40 mm diameter vertical warm bore with a cylindrical profile. The magnet will generate a 36 T field with 13 MW power for a high homogeneity version (1 ppm homogeneity) or >;40 T for a high field version. This hybrid shares the design of superconducting coil with another SCH designed for Berlin, Germany. The cryostat design is quite different however. In this paper the design of the NHMFL cryostat is presented. The main features are described at first followed by the discussion of the FEA models and results.

The National High Magnetic Field Laboratory (NHMFL) has designed and is constructing a Series-Connected Hybrid (SCH) magnet system in Tallahassee, FL. Before the construction of the magnet system can begin many obstacles have to be solved through hardware design and winding practices. The hardware has to have the strength to handle the stresses of winding as well as retaining maximum functionality. The NHMFL will overcome these issues by running several analysis calculations and by producing three model coils for practicing functionality. Two model coils have been built and necessary changes to design and winding procedures have occurred through the practices. These changes will be presented and have been implemented into our winding procedures.

The National High Magnetic Field Laboratory (NHMFL) has several resistive solenoid magnet projects underway presently, including upgrade of four existing magnets, design of the conical bore insert for the Helmholtz Center Berlin (HZB), design of insert coils of the Series-Connected-Hybrid (SCH) for NHMFL and design of resistive split magnet. The upgrades and the design of the conical bore insert are discussed in detail while other programs are presented briefly.

A 45 T Hybrid Magnet System is being developed at the Nijmegen High Field Magnet Laboratory as part of the Nijmegen Center for Advanced Spectroscopy. The 45 T Hybrid Magnet System will be used in combination with far-infra-red light produced by a Free Electron Laser under construction directly adjacent to the High Field Magnet Laboratory. The superconducting outsert magnet will consist of three CICC coils wound on a single coil form, using Nb$_{3}$Sn strands. A test program for strand and cable qualification is underway. The CICC will carry 13 kA and the coils will produce 12 T on axis field in a 600 mm warm bore. The nominal operating temperature will be 4.5 K maintained with forced-flow supercritical helium. The insert magnet will produce 33 T at 40 kA in a 32 mm bore consuming 20 MW, and will consist of four coils. The insert magnet will be galvanically and mechanically isolated from the outsert magnet. Complete system availability for users is expected in 2014. In this paper we will report on the conceptual design of the 45 T Hybrid Magnet System.

The cable-in-conduit conductors (CICC) designed for the National High Magnetic Field Laboratory (NHMFL) and Helmholtz Zentrum Berlin (HZB) Series-Connected Hybrid magnets have been qualified. The superconducting coils for the hybrid magnets consist of three CICC configurations graded for different designed fields. Representative samples with the same Nb3Sn type, cable pattern, and CICC configuration were fabricated and the current sharing temperature (TCS) and critical current were measured with the intention to quantify the level of degradation from cyclic, transverse electromagnetic loads. In addition the intrinsic strain for the CICC was determined by measuring the TCS and IC as a function of applied longitudinal strain. Two conductor grades, for the middle and low fields regions of the coil (MF and LF), are tested at the NHMFL in a split magnet that is equipped with a system that can apply up to 250 kN force in a direction longitudinal to the samples. The samples are hydraulically connected to a cryogenic system which delivers temperature controlled supercritical helium. The reduction in TCS from electromagnetic load cycling is shown to be insignificant and measurements as a function of strain shown that the MF and LF CICC have intrinsic strains of approximately 0.65% and 0.70% respectively.

The National High Magnetic Field Laboratory (NHMFL) is designing two series-connected hybrid magnets, one for the Helmholtz Center Berlin (HZB) and the other for the NHMFL. The one for HZB has a horizontal, conical warm bore with a 30 degree opening angle for neutron scattering experiments. The one for the NHMFL has a 40 mm diameter vertical warm bore with a cylindrical profile. The design of the HZB cryostat will be completed this year. In this paper the design of the HZB cryostat is presented. The results of a structural analysis performed for normal operation and for fault scenario are discussed. The main features of the NHMFL cryostat are described shortly in the introduction section.

The NHMFL presently has three magnet projects using Cable-in-Conduit Conductor (CICC) underway: first for the magnet lab in Tallahassee, FL, second for the Helmholtz Centre Berlin for Materials and Energy (HZB), Germany, and the third for the Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory, TN. The original design of an active shield coil using NbTi superconducting CICC for the NHMFL SCH is replaced by a much lower-cost iron shield composed of 100 mm thick iron-walls positioned symmetrically around the magnet. The SCH fringe field and the effect of iron magnetization on central field homogeneity are discussed under the alternative shield design concept. Nonlinear magneto-static analysis is performed for various shield configurations to ensure the levels of fringe field and the magnetic forces on surrounding instrumentation are acceptable to magnet users. The minimum iron volume and magnetic forces among iron walls are obtained for the design of shield supporting structure. The field uniformity over 1 cm diameter spherical volume (DSV) for both axial and radial directions is examined to meet the design requirements. In addition, effects of current leads, bus bars and surrounding magnets such as the 45-T hybrid to field uniformity are evaluated and the best option for iron shielding is presented.

The National High Magnetic Field Laboratory (NHMFL) in Tallahassee, Florida has designed and is now constructing two Series Connected Hybrid (SCH) magnets, each connecting a superconducting outsert coil and a resistive Florida Bitter insert coil electrically in series. The SCH to be installed at the NHMFL will produce 36 T and provide 1 ppm maximum field inhomogeneity over a 1 cm diameter spherical volume. The SCH to be installed at the Helmholtz Center Berlin (HZB) in combination with a neutron source will produce 25 T to 30 T depending on the resistive insert. The two magnets have a common design for their cable-in-conduit conductor (CICC) and superconducting outsert coils. The CICC outsert coil winding packs have an inner diameter of 0.6 m and contribute 13.1 T to the central field using three grades of CICC conductors. Each conductor grade carries 20 kA and employs the same type of Nb3Sn superconducting wire, but each grade contains different quantities of superconducting wires, different cabling patterns and different aspect ratios. The cryostats and resistive insert coils for the two magnets are different. This paper discusses the progress in CIC conductor and coil fabrication over the last year including specification, qualification and production activities for wire, cable, conductor and coil processing.

Two series-connected hybrid (SCH) magnets are under development at the National High Magnetic Field Laboratory. The first SCH is for the Helmholtz Centre Berlin (HZB) in Germany. The HZB SCH will be a horizontal bore, 30 T magnet and will be used for neutron scattering experiments. The second SCH is for the National High Magnetic Field Laboratory (NHMFL) in Tallahassee, FL. The NHMFL SCH will be a vertical bore, 36 T magnet. Both SCH Magnets combine a set of resistive Florida-Bitter coils with a superconducting outsert coil constructed of cable-in-conduit conductor (CICC). The two SCH magnets are designed for various operating scenarios including those with multiple ramp cycles at various rates. Both of the superconducting magnets are forced flow cooled with supercritical helium at 4.5 K. A standard refrigerator with a capacity of about 150-200 W at 4.5 K will be used to supply the cooling power and the forced mass flow rate. The cryogenic system of the SCH magnet consists of a helium refrigerator, a valve box with subcooler, a magnet cryostat and cryolines. In this paper, the design of the cryogenic system is described.

The National High Magnetic Field Laboratory (NHMFL) has been funded to design and construct two series-connected hybrids (SCH), one each for the NHMFL in Tallahassee, FL and the Helmholtz Centre Berlin for Materials and Energy (HZB), Germany. The original designs for both hybrids contained three concentric coils comprising a Nb3Sn superconducting cable-in-conduit (CIC) main-field outsert coil, a resistive main-field insert coil and a NbTi superconducting CIC shield coil. The fringe field requirements for both facilities were met by designing the NbTi shield coil to match the 9.6 times 106 A-m2 dipole moment of the main field coils at an average radius of 1.16 m. Subsequent, alternate magnetic shielding concepts for both hybrids have been investigated with the objective of reducing the anticipated $700 k NbTi shield coil cost. The HZB fringe field requirements were targeted to a local area of concern at the adjacent Spin Echo facility, centered 15.65 m from the hybrid magnet center. An alternate, lower-cost shield concept comprised a set of four smaller shield coils positioned orthogonally around the HZB spin echo facility with a resultant total 85% to 90% reduction in required dipole moment compared to the original NbTi shield coil. The fringe field requirements at the NHMFL were not changed, but a lower-cost iron shielding option (not described here) located around the NHMFL hybrid was found to be a viable alternative. As a result, the NbTi CIC coil can be eliminated from the common superconducting magnet design for both the HZB and the NHMFL SCH's in favor of viable lower cost shielding options.

Performance verification of the Nb3Sn cable-in-conduit conductor (CICC) for the series-connected hybrid magnets at the National High Magnetic Field Laboratory (NHMFL) and Helmholtz Centre Berlin is performed through short sample testing. The superconducting outsert coil consists of three CICC configurations, graded for the applied magnetic field. The CICC for the high field section of the coil is tested in the SULTAN facility at EPFL-CRPP. Measurements of the current sharing temperature at field current combinations comparable to what is expected in the magnet are made. Electromagnetic cycling is performed to investigate the Nb3Sn strand sensitivity to transverse loads. In addition, measurements of AC loss and pressure drop along the conductor are made and compared to thermal-hydraulic computations.

The National High Magnetic Field Laboratory is developing resistive-superconducting hybrid magnets both for internal use and for installation at other facilities. The Tallahassee magnet will have a vertical bore and provide 36 T in a 40-mm bore with 1-ppm homogeneity over a 10-mm diameter spherical volume. The Berlin version will provide a horizontal field of 25 T in a converging-diverging bore configuration suitable for neutron-scattering experiments. A design study is underway for a third magnet for Oak Ridge that will be similar to the Berlin version but provide >30 T. The three magnets will use very similar ~ 13 T Nb3Sn CICC coils for the superconducting outserts. The resistive insert magnets will be different configurations operating at different power levels. In designing the magnet systems we have developed a new numerical model to predict the critical current of Nb3Sn CICC's, tested several conductors in-house and abroad, designed cryostats and refrigeration systems, and developed new resistive magnet technology. An overview of the innovations and present status is presented.

The NHMFL presently has three series-connected-hybrid (SCH) projects, two of which are entering a construction phase. The design of the SCH outsert magnets is based on a CICC configuration using cable of multi-filamentary Nb3Sn and Cu strands inside a stainless steel jacket cooled by forced flow supercritical helium at 4.5 K. The outsert coil design is presented along with the design criteria for the evaluation of conduit and conductor insulation components. The results of detailed finite element analysis performed for normal, quench and fault load conditions are discussed to ensure the levels of stress and strain in conduit and insulations satisfy the design criteria. The conduit evaluation of fatigue life and fatigue crack growth rate (FCGR) is performed and the results are discussed for a possible cyclic lifetime and a minimum detectable flaw size for the SCH outsert.

A parametric study has been conducted to quantify the effect in performance of cable-in-conduit conductors (CICC's) to changes in cable and conduit design. Measurements of current sharing temperature and critical current as a function of electromagnetic cycling and longitudinal strain were systematically performed on CICC's with common Nb3Sn internal tin strand. The designs varied in void fraction (0.30 or 0.36), long or short cable twist pitch, cable core patterns (6 around 1 or triplet), and conduit material property (stainless steel 316 or Haynes 242). Measurements were performed at the NHMFL in a test facility for conductor characterization with capability to 12 T, 20 kA, and 250 kN axial tensile load now modified to deliver temperature controlled supercritical helium to the CICC samples.

The National High Magnetic Field Laboratory is developing several powered magnets employing novel configurations for use in photon and neutron scattering experiments. First is a split resistive magnet being built for Far-Infrared Scattering at the NHMFL in Tallahassee. This magnet has spurred the development of the novel Split Florida-Helix (SFH) technology. High-field test coils of the SFH concept have been designed and built. Test results are presented. Second, Series-Connected Hybrid magnets with horizontal, conical bores are being designed for neutron scattering experiments at the Hahn-Meitner Institute in Berlin and the Spallation Neutron Source in Oak Ridge, TN. A new resistive magnet technology, the Conical Florida-Bitter (CFB), is being developed suitable for use as the resistive insert of these magnets. A high-field CFB test coil has been designed and is under construction. The conceptual design of the eventual hybrid system is presented along with the detailed design of the high-field test coil.

The NHMFL is investigating magnetic shimming techniques that can be used in high-field resistive and hybrid magnet systems. An advantage of locating powered shims at the inner bore of the high-field magnet is that the ampere-turns required are orders of magnitude lower than required if the shims are located at the outer diameter of the overall magnet system. However, challenges that must be met are those of minimizing the radial space of the shim assembly, providing sufficient cooling to the shims, and implementing a feasible fabrication process for the complicated shim geometry. This paper describes a method for using powered resistive shims at the inner bore of the magnet system. Locating the shims in this region allows the shims to access pre-existing cooling water at the resistive magnet inner diameter. A novel shimming circuitry is described that has been developed in combination with a fabrication process that has been successfully demonstrated. The concepts and engineering design of this resistive shimming technique are presented.

Resistive and resistive-superconducting hybrid magnets can generate dc magnetic fields much higher than conventional superconducting NMR magnets but the field spatial homogeneity and temporal stability are usually not sufficient for high-resolution NMR experiments. Hardware and technique development addressing these issues are presented for high-resolution NMR at magnetic fields up to 40 T. Passive ferromagnetic shimming and magic-angle spinning are used effectively to reduce the broadening from inhomogeneous magnetic field. A phase correction technique based on simultaneous heteronuclear detection is developed to compensate magnetic field fluctuations to achieve high spectral resolution.

The MagLab series-connected hybrid and the HFML Nijmegen hybrid outer superconducting coils each require a pair of binary current leads, comprised of a nitrogen vapor-cooled resistive section and a high-temperature superconducting (HTS) section, configured to minimize the heat load to the coils. The HTS sections of both pairs of leads are made with stacked and soldered Bi-2223 tapes supplied by Sumitomo. The MagLab fabricated and tested the tape stacks for both programs. The procedures and apparatus for fabricating the stacks and making the critical current measurements are described. Critical current and n-value measurement results are reported.

A new set of resistive magnets is being installed using technology developed for our 45-T hybrid magnet. Test results will be presented for the two new 20-MW magnets: the first providing 31 T in a 50-mm bore, the second 35 T in a 32-mm bore. Fabrication status is provided for a third high-homogeneity magnet expected to provide 28 T with inhomogeneity of less than 50 ppm over a 10-mm diameter spherical volume in a 32-mm bore. A new 36-T, 40mm bore high-homogeneity series-connected hybrid is also in the Engineering Design Phase and concepts are being developed for a 55-T hybrid magnet

An innovative hybrid magnet configuration is being developed at the NHMFL, consisting of a Florida-Bitter resistive magnet nested within a cable-in-conduit conductor (CICC) superconducting magnet to provide high fields for less power than traditional hybrid magnets. The resistive and superconducting magnets, connected in series, will be capable of producing 23.1 T and 13.8 T respectively for a total central field of 36.9 T. The CICC uses a cable of multifilamentary Nb3Sn/Cu strands inside a superalloy jacket that confines flowing supercritical helium in direct contact with the cable strands. The design of the magnet system is presented along with the design criteria used to evaluate the superconducting magnet and its integral components. The results of a structural analysis performed using finite elements for normal operational and fault loads are discussed for the most critical component, the conduit

The National High Magnetic Field Laboratory in Tallahassee, Florida, USA has designed, built and tested a high field, split pair, Repetitively Pulsed Magnet (RPM) suitable for neutron scattering experiments. RPM magnets have the advantage of lower average power and lower construction costs than DC magnets. The NHMFL RPM program intends to build magnets with higher field and larger scattering space than those available elsewhere. Details of design and testing of the first prototype are presented

The NHMFL is developing conceptual designs of powered magnets with novel configurations. Conical hybrid magnets are being considered using a novel Conical Florida-Bitter technique. A resistive split magnet is being developed and will be installed at the powered magnet facility in Tallahassee. This magnet will be used for scattering experiments with far-infrared light as well as rotation experiments

The world's highest-field dc magnets have, for roughly the past thirty years, consisted of resistive-superconducting hybrid magnets. These magnets use superconducting 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 are attained than is possible with purely superconducting magnet technology, and lower lifetime (capital and operating) costs are attained than with a purely resistive magnet. The resistive coils of these magnets represent the pinnacle of high-field resistive-magnet technology and have been the focus of much of the resistive magnet technology development over the past thirty years. The evolution of high-field resistive magnet technology is presented, focusing on the development of hybrid inserts.

The National High Magnetic Field Laboratory in Tallahassee, Florida designs, builds and operates the world's highest field dc resistive magnets, providing fields up to 33 T in purely resistive systems and up to 45 T in resistive-superconducting hybrids. The next generation of magnets is presently being designed and used technology developed for our hybrid to upgrade the field in our various resistive magnets. Coil designs are presented for the following 20 MW dc systems: 1) a new 50 mm bore magnet expected to provide 32 T, 2) a new 32 mm bore magnet expected to provide 35 T, and 3) a new high-homogeneity magnet expected to provide 30 T with inhomogeneity of 50 ppm or less over a 10 mm diameter spherical volume.

The National High Magnetic Field Laboratory in Tallahassee, Florida and the National Superconducting Cyclotron Laboratory in East Lansing, Michigan have been collaborating on the design and construction of a large-gap, compact superconducting sweeper magnet for nuclear physics experiments. The magnet consists of two NbTi D-shaped coils and 20 tons of magnet iron in a "C" configuration. The coils were successfully bucket tested in their support structures during the summer of 2002. The design of the cryostat was completed in late 2002 and construction is nearly completed. System testing is expected to occur late in 2003.

A superconducting dipole, designed for use as a sweeper magnet for nuclear physics experiments, is being constructed by the National High Magnetic Field Laboratory for operation at the National Superconducting Cyclotron Laboratory (NSCL). The magnet consists of two D-shaped NbTi coils and will operate at at a peak mid-plane field of 3.95 T in a gap of 140 mm. Peak field on the conductor will be 6.25 T. The winding pack current density is 143 A/mm2. A multi-particle beam will enter the magnet from the upstream side. The neutrons continue straight through to a neutron detector. The charged particles will be swept 43 degrees on a one meter radius into a mass spectrometer. Status of manufacturing and results of single-coil tests are presented.