45-T Hybrid Research Articles

Upgrade of the NHMFL Cryogenic System and the Index Heating Test Results on the 45-T Hybrid

The 45-T Hybrid at the National High Magnetic Field Laboratory (NHMFL) has been the only facility in the world capable of providing a dc field of above 40 T. The 45-T hybrid magnet, consisting of a superconducting outsert and resistive insert, was commissioned in 2000 reaching a field of 45 tesla. The cryogenic system for the NHMFL has been in service since 1994 and has provided cryogenic service for the 45-T hybrid superconducting outsert since testing in 1998. It was recently upgraded, replacing a majority of the original system. The 45-T Hybrid was cooled down successfully with the new cryogenic system in February 2014. The new cryogenic system has been proved more efficient and reliable in the past years, providing increased availability to the users. After the upgrade of the cryogenic system, a standard index heating test was performed on the outsert of the hybrid. This test is mainly for evaluating the performance of the outsert, particularly the degradation of the innermost Nb3Sn coil of the outsert that suffered an unprotected quench in 2000 and degradation since then. The results of index heating tests in the past are compared and presented.

Progress in the Development of the HFML 45 T Hybrid Magnet

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.

Test of the MF-CICC Conductor Designed for the 12-T Outsert Coil of the HFML 45-T Hybrid Magnet

A 45-T hybrid magnet is being built at the High Field Magnet Laboratory of the Radboud University in Nijmegen, The Netherlands. The hybrid magnet consists of a 12-T cable-in- conduit-conductor (CICC) Nb 3Sn superconducting outsert and a 33-T resistive insert magnet. To verify the CICC design, a thorough testing has been completed in the SULTAN facility at Swiss Plasma Center, EPFL in Villigen (Switzerland) for the medium- grade conductor of the outsert. In two test campaigns, the dc cable performance (current-sharing temperature, critical current), the ac loss, and the conductor's performance stability during cyclic loading and after one warmup and cooldown cycle have been investigated. Two different cable layouts were tested—one with a very short twist pitch (STP) and the second one with a long twist pitch (LTP) cabling pattern. As both conductors were made of the same Nb 3Sn strand and underwent the same heat treatment and sample preparation procedure, the effect of the twist pitch on the ac loss and on the dc performance with respect to cyclic loading could be reliably evaluated. The test results show that both cable layouts are actually very robust. The cable could withstand 2000 load cycles and the warmup and cooldown cycle without any significant degradation of the dc performance, and even the overloading at BI product (field multiplied by current) approximately two times larger than those foreseen during magnet operation did not lead to a big performance change. Small differences between the STP and LTP options have been observed, indicating that the STP conductor withstands high electromagnetic loads better than the LTP one.

Study of Fault Conditions During Heat Treatment of Nb3Sn Strand

Heat treatment of large-scale superconducting coils is limited by the power of the furnace and the weight of the superconducting coils. For the superconducting coils to be used in a 45-T hybrid magnet, the practical temperature curve will lag behind the setting temperature curve, particularly for the lowest temperature stage (210 °C). There is also a risk that the heat treatment might be interrupted due to a general power failure or equipment failure. The highest temperature stage (640 °C) is the key stage to form Nb3Sn crystals; thus, interruption of the heat treatment during the 640 °C stage is expected to cause a great impact on the performance of a Nb3Sn superconducting magnet. This paper compares the performance of Nb3Sn strand samples given the recommended heat treatment with samples subjected to simulated temperature lag or simulated power failure. All of the samples had Residual Resistance Ratios, critical currents, and calculated heat losses that were satisfactory for use in the superconducting part of the 45-T hybrid magnet.

Water Cooled Resistive Magnets at CHMFL

The steady high magnetic field facility was founded by the National Development and Reform Commission of China (NDRC) in 2008 and is undertaken by the High Magnetic Field Laboratory of the Chinese Academy of Sciences. The project includes the construction of a series of water-cooled resistive magnets, superconducting magnets, and a hybrid magnet. Up-to-date, five water-cooled resistive magnets have been successfully installed. In addition, the six coils of the resistive magnet insert for the 45-T hybrid magnet have also been assembled and will be tested early next year. Two new world records of water-cooled resistive magnets have been achieved, i.e., 38.5 T/O32 and 35 T/O50 mm. Design, optimization, and assembly of the magnets, as well as their characterization are described in detail in this presentation.

Quench Analysis of the Hybrid-Magnet Superconducting Outsert

Quench propagation analysis of the superconducting outsert for the 45-T hybrid magnet being built at the High Magnetic Field Laboratory, Chinese Academy of Sciences was performed. The superconducting outsert was wound with a Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn cable-in-conduit conductor cabled in a 316LN jacket cooled with supercritical helium at 4.5 K. The superconducting outsert can produce 11.0-T central magnetic field at an operating current of 13410 A. An active protection circuit with a dump resistor of 0.272 Ω was adopted to transfer the stored magnetic energy during a dump operation. In this paper, the magnetic field and strain of the superconducting outsert were analyzed. During a quench, the hot spot temperature of the cable and the helium pressure inside a jacket for different quench delay times were introduced.

Iron Magnetic Shielding of the Series Connected Hybrid Magnet

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

The National High Magnetic Field Laboratory, established in 1990 with support from the National Science Foundation, the State of Florida, and the US Department of Energy, is a facility open to external users around the world. The experimental capabilities are distributed in three campuses. In Tallahassee, Florida, continuous magnetic fields are produced by means of superconducting and resistive magnets reaching fields of up to 33T (resistive), and 45T (hybrid). EMR, ICR, and a 900MHz wide bore NMR magnet are also available. The facility in Gainesville, Florida, is devoted to generating extremely low temperatures in the presence of external magnetic fields (15T, down to 0.4mK), and large MRI imaging capabilities. In Los Alamos, New Mexico, a 9 kV-capable capacitor bank and a number of different liquid Nitrogen-cooled resistive magnets produce repetitive pulses up to 75 T and now a single-shot pulsed up to 300T.

The Next Generations of Powered Solenoids at the NHMFL

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

A New Series-Connected Hybrid Magnet System for the National High Magnetic Field Laboratory

The National High Magnetic Field Laboratory (NHMFL) proposes to build a new hybrid magnet system with three specific goals: 1) address the needs of users requiring high homogeneity, high temporal stability, and long residence times at moderately high fields; 2) enhance user service through use of a single 10 MW power supply (allowing simultaneous operation of multiple systems); and 3) make the system attractive in terms of combined capital and operating costs. The basic configuration are series-connected hybrid, wherein resistive insert and superconducting outsert are connected in series and powered by a single 10-MW unit (20 kA and 500 V) of the NHMFL dc power system, a configuration with advantages for handling faults and off-normal operating conditions and with improved temporal stability. Specific design goals are 35 T, 10-ppm uniformity over a 10-mm DSV, and access for 40-mm diameter probes. By powering the superconducting outsert with 20-kA high-temperature-superconductor (HTS) current leads, cryogenic loads are kept small, substantially smaller even than the present 45-T Hybrid. The net result is a magnet design: 1) with capability for both higher field and higher field quality than present resistive-only systems, 2) that permits simultaneous service of two or more users, 3) that is sufficiently compact to fit within the standard resistive-magnet cell, 4) whose ease of operation is comparable to resistive-only systems, and 5) whose lifetime cost (including construction and operation) can be significantly lower than comparable resistive-only systems. We discuss details of the existing conceptual design, the methodology for its creation, the perceived development needs, and the projected program to realize this system.

The NHMFL 45-t hybrid magnet system: past, present, and future

A hybrid magnet system, combining superconducting and resistive magnet technologies for the production of 45-T steady field in a 32-mm room-temperature bore, has been operated successfully as a user facility for the past two years at the National High Magnetic Field Laboratory in Tallahassee, Florida. The superconducting outsert is a 710-mm bore magnet with demonstrated capability for 100-MJ stored energy, over 14-T field on axis, and nearly 16-T maximum field at the windings. This system continues to provide to researchers the highest steady field available anywhere in the world, even after degradation of the superconducting outsert, which resulted from an "unprotected" quench in July 2000. This paper reviews the important specifications and design features for the superconducting outsert, its nearly 3-year history of operation, as well as plans for its repair and future upgrades.

Analysis of observations during operation of the NHMFL 45-T hybrid magnet system

The NHMFL hybrid magnet system, which was designed to produce steady field in excess of 45 T in a 32-mm, room-temperature bore, was first tested in December 1999. Since then, the system has served users of the NHMFL at the full design currents in both the superconducting outsert (10 kA) and the resistive insert (67 kA), reaching a combined field of 45.2 T. This magnet system combines both superconducting and resistive magnet technologies, which, whether taken together or separately, define new states of the art. Operating alone, the superconducting outsert has been charged repeatedly to 10 kA, corresponding to a maximum field of nearly 16 T at its 710-mm winding i.d. More recently, operation of the outsert has been limited to 8 kA as a consequence of degradation suffered during an “unprotected” quench, but insert upgrades and higher-current operation (up to 74 kA) have allowed the system to provide 45 T to users still. Because the system was designed from the outset as a facility rather than an experiment, there is a minimum of instrumentation––in fact there is none internal to the steel vessel housing the superconducting magnet. Therefore, projections of internal conditions in the superconducting magnet are deduced from detailed analysis of observations from coil voltage taps and various external temperature sensors and pressure transducers. We present these with comments regarding their value in future magnet design as well as an introduction for more complete analysis by complex computer codes.

Investigation of index heating as source of quench in the NHMFL 45-T hybrid superconducting outsert

The superconducting outsert of the NHMFL 45 T is cooled by static He II contained in cable-in-conduit conductors, which must be capable of extracting any internal heat (primarily from hysteresis losses) during a ramp to full field. In the event that the ramping losses exceed the cooling capacity of the He II, the long cooling channels become partially or completely filled by He I and even the relatively weak heating of index losses can lead to slow thermal runaway and a delayed quench. A numerical analysis is presented using a one-dimensional model of heat transfer in a column of superfluid helium. The computational results are compared with experimental data, showing good agreement with the observed quench behavior.

Structural optimization of the superconducting outsert of a 45-T hybrid magnet

The National High Magnetic Field Laboratory (NHMFL), Tallahassee, Florida, in collaboration with the Francis Bitter National Magnet Laboratory, MIT, is constructing a 45 T hybrid magnet system using cable-in-conduit conductor (CICC) technology for the superconducting coils. In this technology, superconducting wires are cabled and placed in a steel conduit that acts as the principal load-bearing member of the solenoid. In this paper, a structural optimization of the CICC is performed with the aim of reducing the coil mass required to generate the same field strength. The constraints for the optimization are the upper limits values (allowable limits) for the three state variables: the average and peak values of equivalent von Mises stress, and hoop strain. Different geometric and material options were analyzed for optimum performance. Global-local finite element technique was used to conduct the stress analysis. First, a global analysis, using effective material properties, was performed; followed by a local structural analysis. This procedure was iteratively applied until one of the state variables just reached its allowable limit, thus yielding the optimized design. Significant reductions were achieved in mass and stored energy of the magnet as a result of the optimization. © 1997 Elsevier Science Limited

AC losses in superconducting Nb/sub 3/Sn and NbTi CIC conductors

AC losses measurements and calculation on large scale Cable-in-Conduit (CIC) conductors have been carried out at NHMFL In particular samples of the Nb/sub 3/Sn and NbTi conductors developed for the NHMFL 45-T Hybrid Magnet superconducting outsert have been tested. A calorimetric technique involving superfluid helium (He II) has been used. The samples have been tested with applied field variation rates varying in the range between 0.01 T/s to 30 T/s. From results it appears that for most of the samples, at high field variation rates, the coupling currents are so intense to locally saturate the superconducting strands. As no theories describing the saturation regime in CIC conductors exist, we tried to extend the theories developed for monolithic conductors to the more complicated CICC geometry. It emerges by defining an effective coupling time constant and an effective radius for the conductor that it is possible to fit the data with good accuracy. Effective coupling time constants between 3 ms and 200 ms, depending on the strands surface condition and the cable void fraction, have been measured for the Nb/sub 3/Sn conductors, while a time constant of 16 ms has been determined for the NbTi sample. The experimental results and the theoretical analysis are reported in the paper.

The potential for SAGBO cracking in a Nb3Sn wind-and-react solenoid using a CICC with Incoloy 908 jacket and glass fabric insulation

The 45-T Hybrid Magnet System being constructed for the National High Magnetic Field Laboratory (NHMFL) in collaboration with the Francis Bitter National Laboratory is composed of a resistive insert and superconducting outsert. Two of the outsert solenoids use Nb3Sn and will be wound with cable-in-conduit conductors (CICCs) using the wind-and-react technique. A candidate for the jacket material is Incoloy 908, but an obstacle to this choice is its sensitivity to Stress Accelerated Grain Boundary Oxidation (SAGBO). The conditions for SAGBO-residual stress, elevated temperature, and exposure to oxygen (even in extremely low levels)-are difficult to avoid in coils fabricated by the wind-and-react process. The glass fabric insulation, applied during winding, is at high temperature, both a source of oxygen (primarily in the form of absorbed water vapor) and, being highly compacted by the winding, an impediment to its removal. In attempt to solve this problem NHMFL has developed a pre-baking procedure in vacuum at 730 C, to be applied to the glass fabric before to insulate the coil. In order to test the efficiency of pre-baking and to check the existence of possible other features that also lead to SAGBO, a dummy coil, wound with a CIC conductor jacketed with Incoloy 908, has been manufactured. The coil has been heat treated in very high purity helium atmosphere. The results are reported.

An Overview and Status of the NHMFL 45-T Hybrid Project.

An Overview and Status of the NHMFL 45-T Hybrid Project.

Cryogenic System for the 45-T Hybrid Superconducting Magnet.

Cryogenic System for the 45-T Hybrid Superconducting Magnet.

AC loss measurement of the 45-T hybrid/CIC conductor

The results of calorimetric measurements of the AC loss in a cable-in-conduit (CIC) conductor under development for the a 45T hybrid superconducting outsert magnet are reported. The technique is unique in that the absorbed energy is determined by measurement of the temperature rise in the He II contained in the conductor. The sample is located in a separate cryostat and placed in the bore of a 7 T superconducting dipole magnet. A change in transverse field results in heating which produces an easily measurable temperature increase of the helium surrounding the conductor. Samples of Nb/sub 3/Sn CIC conductor developed for the high field portion of the 45 T hybrid magnet system have been measured. The time constant measured for this sample is compared to analysis based on a combination of hysteresis and eddy current losses.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Characterization of multifilamentary Nb/sub 3/Sn superconducting wires for use in the 45-T hybrid magnet

The National High Magnetic Field Laboratory (NHMFL), in collaboration with the Francis Bitter National Magnet Laboratory, is constructing a 45-T hybrid magnet system. Here we report the results of studies to characterize candidate Nb/sub 3/Sn superconductor for use in the superconducting outsert coils. We have determined the effects of strain and magnetic field on the critical current, measured ac losses, and measured residual resistivity ratios (RRR). Critical currents in excess of 1000 A mm/sup -2/ non-Cu have been measured in sample wires at 12 T and 4.2 K combined with a hysteresis loss of less than 500 mJ cm/sup -3/ We also present predicted 1.8 K performance, based on empirical models, and present a methodology for calculating sample holder prestrain.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>