High Magnetic Field Laboratory Research Articles

Screening Currents and Hysteresis Losses in the REBCO Insert of the 32 T All-Superconducting Magnet Using T-A Homogenous Model

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.

Conceptual Design for a Next Generation Resistive Large Bore Magnet at the NHMFL

The National High Magnetic Field Laboratory (NHMFL) has successfully operated its 20 Tesla 195 mm large bore magnet for over 20 years. Eventually, as there was a certain slowdown in demand for that magnet at the time, it had been decommissioned in 2016 and its two outer coils have been re-used for parts in a higher energy density configuration to facilitate the fast construction of the world record 41.5 Tesla 32 mm bore resistive magnet. Without any resistive large bore magnet providing fields in the 20 Telsa range available for the last two years, the demand or desire for such a facility has been steadily rising at our laboratory. Again, cost and schedule for the construction of such a magnet are very critical aspects under consideration. One elegant solution to keep these factors most manageable is to not design a new stand-alone magnet but to design one or a set of insert coils that is interchangeable with a smaller bore existing magnet at the NHMFL. Different alternative configurations for such a large bore resistive insert including two different existing magnets to serve as the outsert as well as different usable bore sizes have been considered on a preliminary level of detail for comparison only. Eventually, a more detailed conceptual design has been developed for a chosen magnet system. In this paper, the authors present a summary of the different alternative considerations as well as an introduction to the conceptual design of a next generation 195 mm Large Bore Magnet capable to produce well above 20 Tesla.

Design and Performance Estimation of a 20 T 46 mm No-Insulation All-REBCO User Magnet

This paper reports on a proposed design and estimation of performance of a 20 T no-insulation (NI) high temperature superconductor (HTS) standalone user magnet for the National High Magnetic Field Laboratory. It consists of a stack of 18 double pancake coils wound with REBCO tapes from two different vendors, SuNAM and SuperPower, chosen for their complementary in-field critical current, mechanical properties and cost properties. The inner and outer winding diameters and overall height of the magnet are 46 mm, 102.6 mm, and 163.4 mm, respectively. The magnet is designed to generate a 20 T central field at its nominal operating current of 316 A in liquid helium at 4.2 K. The inductance and stored energy are 1.89 H and 94.4 kJ, respectively. In addition to the detailed design parameters, this paper presents a stress analysis, NI charging analysis and post-quench analysis, followed by discussion of design parameters and estimated performance of the proposed 20 T NI HTS magnet.

Impact of Failing Insert Coils on the Mechanical Design of the HFML 45 T Hybrid Magnet System

The High Field Magnet Laboratory of the Radboud University is constructing a 45 T hybrid magnet system. Large fault forces may arise in the not so unlikely event of failure of parts of the resistive insert magnet. The measures taken to accept the fault forces dominate the design of the support structures, in particular those coupling the room temperature resistive insert magnet with the superconducting coil in the cryogenic environment. The largest fault forces arise when half of one or several insert coils become electrically short-circuited in a burn-out accident. In case the complete upper or lower half of all the resistive coils collectively disappear, the so-called ultimate axial fault force on the superconducting outsert coil may amount to several MN - calculated in a static approach. Traditionally these ultimate fault forces are reference for the mechanical design to guarantee that the structures can sustain these forces preventing catastrophic failure of the magnet's housing. We will argue that if one takes into account the time dependent behavior of the powering and protection systems after detection of a coil failure, the resulting fault forces will be much smaller than the ultimate fault forces predicted by static approach.

Abstract A26: Precise characterization and comparison of KRAS proteoforms by top-down mass spectrometry

Abstract The two KRAS isoforms, 4A and 4B, exhibit high sequence identity, differing mainly in their C-terminal “hypervariable regions”, which bear post-translational modifications (PTMs) governing their association with the plasma membrane and downstream signaling effectors. Moreover, the KRAS gene bears three “mutation hotspot” residues (Gly12, Gly13, and Gln61) represented in a high percentage of human cancers. The similarity between KRAS isoforms, in addition to HRAS and NRAS, renders linking KRAS mutations and PTMs significantly challenging by standard proteomic methods based on digestion of proteins into short peptides. However, intact modified protein forms, or proteoforms, of KRAS4A and KRAS4B can be identified and characterized by “top-down” (TD) proteomic methods, elucidating crosstalk between N-terminal mutations and C-terminal PTMs with precise molecular detail. Previously, we employed the TD approach as part of a novel workflow to precisely characterize KRAS4B proteoforms isolated from colorectal cancer cell lines and primary tumor samples by immunoprecipitation (IP) and subsequent analysis on a benchtop Orbitrap mass spectrometer (1). Here, we modified our IP-TD protocol to include KRAS4A, then applied our workflow to the characterization, quantification, and direct comparison of KRAS4A and KRAS4B proteoforms within the context of multiple cancer cell lines. We hypothesized that we would be able to identify distinct proteoform populations for each KRAS isoform, which could then be compared both within and between different contexts. We have thus far identified nine novel proteoforms of KRAS4A, which not only exhibits differential PTM to KRAS4B, but varies widely in both abundance and PTM profile across all cell lines evaluated. We have also applied IP-TD to the analysis of endogenous KRAS4A and KRAS4B proteoforms using a more powerful 21 Tesla FT-ICR mass spectrometer at the National High Magnetic Field Laboratory. The high mass resolving power, mass measurement accuracy, and spectral acquisition rate achieved at high field combined with use of efficient dissociation methods and an external fragment ion accumulator to improve signal-to-noise ratios of MS/MS spectra produced unparalleled results with respect to degree of KRAS proteoform characterization. We were able to identify up to ten new KRAS proteoforms per context, several of which appear to be mutation- or context-specific. These latest advances have broadened our understanding of KRAS biology and provided new insights into the potential roles of KRAS4A and KRAS4B proteoforms in colorectal and other cancers. Reference 1. Ntai I, Fornelli L, DeHart CJ, Hutton JE, Doubleday PF, LeDuc RD, van Nispen AJ, Fellers RT, Whiteley G, Boja ES, Rodriguez H, Kelleher NL. Precise characterization of KRAS4b proteoforms in human colorectal cells and tumors reveals mutation/modification crosstalk. Proc Natl Acad Sci U S A 2018 Apr 17;115(16):4140-5. Citation Format: Caroline J. DeHart, Lissa C. Anderson, Lauren M. Adams, Luca Fornelli, Christopher L. Hendrickson, Neil L. Kelleher. Precise characterization and comparison of KRAS proteoforms by top-down mass spectrometry [abstract]. In: Proceedings of the AACR Special Conference on Targeting RAS-Driven Cancers; 2018 Dec 9-12; San Diego, CA. Philadelphia (PA): AACR; Mol Cancer Res 2020;18(5_Suppl):Abstract nr A26.

Supercritical helium expansivity effects on magnet protection systems

The recent commissioning of the 36 T Series-Connected Hybrid Magnet at the National High Magnetic Field Laboratory has uncovered an undesirable consequence of using super critical helium for cooling of superconductors. This recent dry magnet cooling technology greatly reduces the LHe requirements of the magnet and may be utilized more frequently in the future. Because the supercritical helium does not complete a phase transition with the addition of heat, the fluid can quickly expand. The increased expansivity of supercritical helium over liquid helium for a given heat load is capable of creating larger pressure waves. The impact pressure for compressible helium gas flow is combined with the speed of sound for ideal gases to determine if a high-speed pressure wave is sufficient to explain the premature failure of the burst discs in the over-pressurization protection system. The addition of an impact pressure can explain, under the right conditions, a reduction of up to 40% in the burst disc pressure rating. The Grüneisen parameter is shown to relate to the expansivity and values for the parameter are given in the supercritical range. A proposal to reroute the pipe near the burst disc is made to mitigate the effects of the impact pressure.

Development of general expressions for the temperature and magnetic field dependence of the critical current density in coated conductors with variable properties

REBa2Cu3Oy coated conductors have recently become viable for high field superconducting magnets and this use may be the principal present driver of coated conductor development. Driving the transport critical current density (Jc) as high as possible has become one of the principal goals of CC manufacturers but this can only be done by developing highly engineered nanostructures that may not be easy to control in quantity production of long lengths. Protection of high field (B) magnets operating in the temperature (T) of 4–20 K range is challenging and one key data set needed for accurate quench modeling is a wide-ranging Jc(B, T) data set. At the National High Magnetic Field Laboratory (NHMFL), 12 km of REBCO tapes were purchased for the all-superconducting 32 T user magnet that successfully reached field recently. They were characterized at 4.2 K with field orientation B perpendicular to tape and at 18° off the tape-plane axis. Of the tapes selected for 32 T, three were chosen for additional Jc(B, T) characterization from 4.2 to 75 K in the B tape orientation in fields from 1 to 15 T. Although all tape lengths were bought to the same advanced pinning specification, in fact there was substantial variation of more than 2 in the low temperature, high field Jc. Here we probe the reasons for this variability in the context of measurements of the transport Jc(B, T) dependence of 3 representative samples from this distribution with Ginzburg–Landau models of vortex pinning using a power law for Jc(B) and an exponential temperature dependence for T < 45 K and 3 T < B < 15 T. A fourth tape from the 32 T magnet procurement with Jc outside this range was then selected to test the validity of our modelling. Using this extensive data set, the correlation between Jc(B, 4.2 K) and Jc(B, T) enabled us to predict Jc(B, T) for tapes procured for the 32 T magnet with an expected accuracy of 10% or less for T < 40 K and B up to 15 T. Transmission electron microscopy made clear that the BaZrO3 (BZO) size, volume fraction and density varied significantly across the range of conductors studied, suggesting that nano-structural control is difficult during coated conductor manufacture and that the resulting Jc variations may have to be accepted in procurement practice.

Design, Construction, and Operation of New Duplex Magnet at Pulsed Field Facility-NHMFL

The Pulsed Field Facility (PFF)- National High Magnetic Field Laboratory (NHMFL) in Los Alamos, New Mexico has developed and operated a several types of ultra-high field pulsed magnets, including 100 T Multi-shot, 60 T Long Pulse, and an array of small 65 T magnets for users. Separately powering (nested) coils allows to both reduce the driving voltages and have a further degree of control over the pulse duration and therefore the current carrying capacity in conductor to maximize the produced magnetic fields. Duplex design with two nested coils powered separately by two capacitors has been used at several pulsed field centers to increase the generated magnetic field. In the last year, PFF-NHMFL focused on developing such a magnet to generate maximum magnetic field up to 77 Tesla using existing 4 MJ capacitor bank. The magnet is expected to provide 75 tesla magnetic field during regular operation. A Metal Oxide Varistor (MOV) bank is used to protect the capacitor bank and its associated electrical components from the overvoltage in the case of a fault happening in the duplex magnet. This paper will outline the design, construction and operation of that magnet and the MOV bank.

The 40 T Superconducting Magnet Project at the National High Magnetic Field Laboratory

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.

Understanding quench in no-insulation (NI) REBCO magnets through experiments and simulations

Present research on no-insulation (NI) rare earth barium copper oxide (REBCO) magnets have demonstrated their ability to produce high fields due to their compact nature. NI magnets have often been demonstrated to be self-protecting. However, evidence of mechanical damage in recent high field magnets, suggests that there are some issues about quench that must be resolved for this otherwise promising technology. This article attempts to explain multi-physics phenomena occurring during the quench of an NI magnet that can be used to elucidate quench behavior through experiments and simulations. A lumped circuit model is used for the circuit analysis where each coil is modeled as a single inductor with variable quench resistance in series and characteristic contact resistance in parallel. Three case studies have been analyzed: (1) a 3 double pancake (DP) standalone magnet, (2) a 2 DP coil in 31 T background, and (3) a high temperature superconductor/low temperature superconductor (HTS/LTS) hybrid user magnet that consists of a 13 T HTS insert and a 6 T LTS background magnet. Lessons learned from these analyses include: (1) characteristic resistance of NI coil rises during quench with the temperature rise; (2) influence of Hall effect exists on the voltage rise during quench; (3) over-current during quench can over-stress the coil; and (4) quench propagation from one end of the magnet generates significant unbalanced forces. This approach is expected to be used in the preliminary design of an ultra high field (>40 T) user magnet currently under design at the National High Magnetic Field Laboratory.

Design of the Resistive Insert for the Nijmegen 45 T Hybrid Magnet

This paper presents the design of the resistive insert magnet for the 45 T hybrid magnet under construction at the High Field Magnet Laboratory (HFML) in Nijmegen. We describe the design process and tools used to obtain the coil and disk design, and we describe the mechanical design aspects of the housing for the magnet. We address the end turn issue and introduce an innovative new element to provide sufficient clamping of the coil at all times. Finally, we address possible fault scenarios and the magnetic misalignment forces associated with such faults and their impact on the housing.

Next-generation highly flexible round REBCO STAR wires with over 580 A mm−2 at 4.2 K, 20 T for future compact magnets

We report the latest developments of next-generation flexible round RE–Ba–Cu–O (REBCO, RE = rare earth) wire, driven by the needs of compact accelerator magnets requiring round isotropic wire with an engineering current density (Je) of 600 A mm−2 at 4.2 K, 20 T at a bend radius of 15 mm. We have developed a Symmetric Tape Round (STAR) REBCO wire using multiple layers of REBCO tapes specifically developed for this architecture, featuring a mechanically symmetric geometry with a 10–18 μm thick substrate wherein the superconductor film is positioned near the tape’s neutral plane for superior bend strain tolerance. Furthermore, each layer of REBCO tape is individually optimized for maximum bend strain tolerance. These ultra-thin substrate REBCO symmetric tapes enabled us to fabricate next-generation isotropic round wires of just 1.3 mm diameter and a critical current equivalent to commercial 12 mm wide REBCO tapes. The in-field performance of STAR wires of several configurations has been tested at National High Magnetic Field Laboratory to identify the most suitable architecture to meet the needs of high-field compact accelerators. At a bend radius of 15 mm, a six-layer STAR wire exhibits critical current of 778 A at 4.2 K in 20 T background field, which equals Je of 586.4 A mm−2 at a Lorentz force (FL) of 15.5 kN m−1 which is the highest reported Je value for REBCO wire in round geometry at this magnetic field. Similarly, a 12-layer STAR wire shows an Ic of 1156 A at 31.2 T, 4.2 K which corresponds to a Lorentz force of 36 kN m−1. Multiple tests of STAR wires at high magnetic field confirm a <0.1% variation in measured Ic. This level of reproducibility of the high performance of STAR wire in high magnetic fields at 4.2 K and small bend radius underscores the potential of STAR REBCO wire for use in compact accelerator magnet and related applications.

Fabrication of the Nb3Sn/Cu CICC Coil and Cold Mass for the Radboud University HFML 45 T Hybrid Magnet

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.

Design of a 30-T Superconducting Magnet for Quantum Oscillation Application

High field superconducting magnets, as important experimental facilities in condensed matter physics, provide high magnetic field for mapping the Fermi surface of metals, which is a basic approach to study many material properties like resistance, Hall resistance, and magnetic susceptibility. Up to now, the highest magnetic field ever achieved with superconducting magnets is 32 T, obtained at national high magnetic field laboratory (NHMFL) on Dec. 8, 2017, with a 17-T REBCO insert in a 15-T LTS background magnet. Institute of Electrical Engineering (IEE, CAS), has developed high field superconducting magnet for many years, and we successfully tested a 27.2-T superconducting magnet at IEE on Nov. 28, 2017. In the next step, we want to utilize our magnet techniques to fabricate a 30-T superconducting magnet for quantum oscillation application. The magnet will consist of a 15-T/160-mm LTS magnet and a 15-T/35-mm REBCO insert. Electromagnetic design to obtain a higher operation margin and detailed stress analysis will be introduced. Other design considerations will also be given in details and engineering issues will also be discussed.

Verification Testing of MQXFA Nb3Sn Wires Procured Under LARP

The High-Luminosity LHC Accelerator Upgrade Project (AUP) in the U.S. will construct quadrupole magnets to be delivered to CERN. An initial 3 tons, over 600 km total length of conductor was procured under the LHC Accelerator R&D Program (LARP) for this project. Programs for quality control(QC) at the supplier and quality verification (QV) at the laboratories were solidified into components of the overall quality plan for strand procurement under AUP. Measurements of the critical current (Ic) and residual resistance ratio (RRR), and related probes and techniques, are central to the quality plan. Described below is the verification testing that has taken place at the National High Magnetic Field Laboratory (NHMFL). Testing challenges are presented by the high sensitivity of these wires. In addition, new RRR test software was developed to accommodate challenges presented by meeting international standards with existing configurations of test strands.

Test Results of the 36 T, 1 ppm Series-Connected Hybrid Magnet System at the N&lt;sc&gt;HMFL&lt;/sc&gt;

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.

A Numerical Study of Quench in the NHMFL 32 T Magnet

The National High Magnetic Field Laboratory (NHMFL), Tallahassee, FL, USA, has developed, built, tested, and commissioned a 32 T all-superconducting user magnet system combining two series-connected high-field high-temperature superconductor (HTS) nested inner coils (insert) wound with SuperPower, Inc. REBCO tapes and a low temperature superconducting (LTS) outer magnet (outsert) composed of five coils (subdivided into 17 electrical sections). Protected-quench tests were performed at the NHMFL to analyze the reliability of the 32 T magnet during the superconducting-to-normal transition. The quench tests were performed at different values of transport current both in the HTS insert and in the LTS outsert. The University of Bologna, in collaboration with the NHMFL, has developed a quasi-3-D FE model suited for the analysis of quench in HTS magnets. The model was previously applied to the analysis of the experimental results of the quench tests carried out on the prototype coils, manufactured in the framework of the 32 T magnet R&D activities. In this paper, the numerical model is applied to analyze the quench initiation and propagation in the 32 T HTS insert. The dump of the transport current during quench is computed and compared with the experimental result. The most stressed regions within the insert windings are identified.

(Invited) Electronic Structure and Properties of Boron-Doped Endohedral Metalloheterofullerenes

Since the discovery of buckminsterfullerene in 1985,1 intensive research has been devoted to this family of closed-cage carbon nanostructures. Endohedral metallofullerene La@C60 was already detected the same year and La@C82 was isolated few years later.2 In 1999, the synthesis, isolation and characterization of the first clusterfullerene Sc3N@C80 using the Krätschmer-Huffman method was a milestone in the field of endohedral metallofullerenes (EMF).3 The characteristic structural and electronic properties of EMFs, as well as their reactivity, have been extensively analyzed by experimental and theoretical groups.4 In particular, different rules to predict the relative stability of fullerene cages, based on the ionic model, were proposed.5 A different way to modify the properties of fullerenes is by replacing a carbon atom in the caged network with a heteroatom. The resulting heterofullerene shows distinct electronic structure and, therefore, properties from the all-carbon cage precursor. Nitrogen-doped heterofullerenes have received much more attention and investigation than boron-doped heterocages.6 Few years ago, Dunk et al. reported facile gas-phase formation of C59B by atom exchange resulting from exposure of C60 to boron vapor by means of a pulsed laser vaporization cluster source, which was the first report of borafullerene formation directly from pristine C60.7 Recent application of this technique to endohedral monometallofullerenes and clusterfullerenes at the National High Magnetic Field Laboratory in Florida lead to the formation of boron-doped EMFs with exciting electronic properties. We here analyze the characteristic electronic structure of observed B-doped metalloheterofullerenes, rationalize their stabilities and discuss about their properties. References Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162-163.a) Heath, J. R.; O’Brien, S. C.; Zhang, Q.; Liu, Y.; Curl, R. F.; Kroto, H. W.;Tittel, F. K.; Smalley, R. E. J. Am. Chem. Soc. 1985, 107, 7779-7780; (b) Chai, Y.; Cuo, T.; Jin, C.; Haufler, R. E.; Felipe Chibante, L. P.; Fure, J.; Wang, L.; Alford, J. M.; Smalley, R. E. J. Phys. Chem. 1991, 95, 7564-7568.Stevenson, S.; Rice, G.; Glass, T.; Harich, K.; Cromer, F.; Jordan, M. R.; Craft, J.; Hadju, E.; Bible, R.; Olmstead, M. M.; Maitra, K.; Fisher, A. J.; Balch, A. L.; Dorn, H. C., Nature 1999, 401, 55.a) Rodríguez-Fortea, A.; Balch, A. L.; Poblet, J. M. Chem. Soc. Rev. 2011, 40, 3551-3563; b) Popov, A. A.; Yang, S.; Dunsch, L. Chem. Rev. 2013, 113, 5989-6113.a) Rodríguez-Fortea, A.; Alegret, N.; Balch, A. L.; Poblet, J. M., Nature Chem. 2010, 2, 955-961; b) Garcia-Borràs M., Osuna S., Swart M., Luis J.M., Solà M. Angew. Chem. Int.. Ed. 2013, 52, 9275-9278; c) Wang, Y.; Díaz-Tendero, S.; Martín, F. and Alcamí, M. J. Am. Chem. Soc., 2016, 138, 1551-1560.Lamparth, I.; Nuber, B.; Schick, G.; Skiebe, A.; Grosser, T.; Hirsch, A. Angew. Chem. Int. Ed. Engl. 1995, 34, 2257.Dunk, P. W.; Rodríguez-Fortea, A.; Kaiser, N. K.; Shinohara, H.; Poblet, J. M.; Kroto, H. W. Angew. Chem. Int. Ed. 2013, 52, 315-319.

Achieving 1 ppm field homogeneity above 24 T: Application of differential mapping for shimming Keck and the Series Connected Hybrid magnets at the NHMFL.

Powered resistive and resistive-superconductive hybrid magnets can reach fields higher than superconducting NMR magnets but lack the field homogeneity and temporal stability needed for high resolution NMR. Due to field fluctuations in powered magnets, commercially available mapping systems fail to produce maps of these magnets with sufficient reproducibility, thus hampering attempts to improve homogeneity of the field they generate. Starting with a commercial mapper, we built a mapping system which uses a two-channel (measurement + reference) mapper probe. We used this system to map and then to shim two magnets of Florida Bitter type at the National High Magnetic Field Laboratory in Tallahassee, FL. With a combination of passive (ferromagnetic) and active shims we achieved 2.3 ppm homogeneity in 1 cm diameter spherical volume (dsv) at 25.0 T in the Keck resistive magnet, and 0.9 ppm homogeneity in 1 cm dsv at 23.5, 28.2, and 35.2 T in the series-connected resistive-superconductive hybrid (SCH) magnet.

Magnetoresistance of cryogenic linear temperature sensor

The 40 T hybrid magnet was developed at High Magnetic Field Laboratory, Chinese Academic of Science, it is composed of 30 T resistive magnet insert and 10 T superconducting outsert which is cooled with forced flow of supercritical helium at 4.5 K supplied by cold box and valve box. The gaseous helium temperatures in valve box are measured by CLTS (Cryogenic linear temperature sensor). This paper presents the characteristics of CLTS, reports the magnetoresistance of the CLTS which is 5.8 m away from 40 T hybrid magnet centre when the 10 T superconducting outsert was in the field of excitation, and CLTS resistance offset for remanent magnetization. CLTS was not influenced by resistive magnet because of the small stray field of hybrid magnet 30 T resistive magnet.