Series-Connected Hybrid Magnet Research Articles

Conical Insert Design for the HZB Series-Connected Hybrid

The National High Magnetic Field Laboratory (NHMFL) is designing series-connected hybrid magnets for the Helmholtz Center Berlin (HZB). The hybrid has a horizontal, conical warm bore with a 30 degree opening angle for neutron scattering experiments. The resistive insert includes two coils. The outer coil is a regular bitter coil, while the inner one has a varied inner radius to meet the requirement of large opening angle. Such technology as in the inner coil was developed in the NHMFL, and called Florida conical bitter technology. The conical configuration makes the cooling hole length varied with radius causing unique hydraulic design challenge. This paper presents the detail design features of the conical insert along with many analysis results.

Quality Assurance Tests of ${\rm Nb}_{3}{\rm Sn}$ Wires for the Series-Connected Hybrid Magnets

About 450 km of high JC Nb3Sn wires have been procured for the construction of two series-connected hybrid magnets by the National High Magnetic Field Laboratory (NHMFL). In addition to the quality assurance tests by the wire manufacturer, a receiving quality assurance inspection with a predetermined sampling frequency is carried out at the NHMFL. These tests include wire diameter, twist pitch, Cu/non-Cu ratio, IC, residual resistance ratio, hysteresis loss, and IC strain dependence. In this paper, the test procedures are described and the test results are presented. Based on these test results, all the wires have been accepted.

Series-Connected Hybrid Outsert Coil Winding Hardware Design

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.

Cryostat Design for the NHMFL Series-Connected Hybrid

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.

Eddy-Current Analysis of Cold Mass and Thermal Shield for Series-Connected Hybrid Magnet

The National High Magnetic Field Laboratory currently has three series-connected hybrid (SCH) magnet projects, where resistive coils are connected in series with superconducting coils using cable-in-conduit-conductor (CICC) underway: first for the magnet laboratory in Tallahassee, FL; second for the Helmholtz Zentrum Berlin for Materials and Energy (HZB), Germany; and the third for the Spallation Neutron Source at the Oak Ridge National Laboratory, TN. The one for HZB has a horizontal conical bore with a 30^ opening angle for neutron scattering experiments. During power supply trip, superconducting magnet quench, resistive insert short, and insert fault, transient electromagnetic effects as a result of fast decay of the coil current introduce a significant amount of eddy current and Lorentz force on the conductive components of the cryostat such as the metallic cold-mass magnet frame and the 50-K thermal radiation shield. Although the eddy-current heating is not a concern during quench and fault operations, the eddy-current-induced Lorentz forces need to be taken into account in the structural design of the SCH cryostat. In this paper, a detailed eddy-current analysis for the HZB magnet during abnormal operations has been performed for its cryostat based on the dry magnet design concept. The nonuniform eddy-current distribution from the finite-element analysis implies that local hot spots may develop under abnormal operations. The eddy-current-induced Lorentz forces are quantified to ensure the strength and stability of the cryostat structure and, most importantly, safety of the SCH magnet during abnormal conditions.

Qualification Measurements of the Mid-Field and Low-Field CICC for the Series-Connected Hybrid Magnet With Effects of Electromagnetic Load Cycling and Longitudinal Strain

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.

Cryostat Design for the HZB and NHMFL Series-Connected Hybrids

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.

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.

Recent Progress of the Series-Connected Hybrid Magnet Projects

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.

Electro-mechanical modeling of Nb 3Sn CICC performance degradation due to strand bending and inter-filament current transfer

Performance degradation of Nb 3Sn cable-in-conduit conductors (CICCs) is a critical issue in large-scale magnet design such as the International Thermonuclear Experimental Reactor (ITER) and the series-connected hybrid (SCH) magnets currently under development at the National High Magnetic Field Laboratory (NHMFL). Not only the critical current is significantly lower than expectations but also the voltage–current characteristic is observed to have a much broader transition from a single strand to a CICC cable. The variation of conductor voltage–current characteristic as a result of cable electromagnetic, mechanical and thermal interactions is challenging to model. In this paper, we use a new numerical model, called the Florida electro-mechanical cable model (FEMCAM) benchmarked against 40 different conductor tests, to study the influence of bending strain and current non-uniformity on the critical current and n-value of Nb 3Sn strands and CICC cables. The new model combines thermal bending effects during cool-down, electromagnetic bending effects during magnet operation and current transfer in strands with filament fracture. The n-value of a strand under bending is derived from integration of filament critical current over strand cross-section for full and no current transfer. The cable n-value is obtained from the power law relation of cable electric field and critical current curve. By comparing numerical results with measurements of advanced Nb 3Sn strands and various CICC cables, we demonstrate that FEMCAM is self-consistent in modeling inter-filament current transfer. The new model predicts that I c degradation of bent strands initially follows closely full current transfer but starts deviating and falls between full and no current transfer with an increasing bending strain. The results agree with recent TARSIS measurements for less than 1% bending strain mostly interested in practice. The strand n-value degradation from FEMCAM with no filament current transfer agrees better with measurements than that from full current transfer. Finally, FEMCAM simulated cable n-values are compared with various CICC measurements. The results imply that FEMCAM is a useful tool for the design of Nb 3Sn-based CICCs and both thermal bending and electromagnetic bending play important roles in CICC performance.

Design of Cryogenic System for SCH Magnets

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.

Characterization of High $J_{c}$${\rm Nb}_{3}{\rm Sn}$ Strands for the Series-Connected Hybrid Magnet

The series-connected hybrid magnet under construction at the National High Magnetic Field Laboratory uses cable-in-conduit-conductor with high Jc Nb3Sn strands. One of the candidate Nb3Sn strands is made by the restacked-rod process with a nominal Jc of ~ 2400A/mm2. We characterized the Jc as a function of axial and transverse strain, magnetic field and temperature. The Jc-strain measurements have been carried out by a straight pull device and two different Walters spring probes. In addition, we have investigated the effect of the Nb3Sn reaction heat treatment temperature on the irreversible strain. We found that the irreversible strain increases with the heat treatment temperature. Our results indicate that, in addition to its high J c, this type of strand has moderate strain sensitivity, therefore is suitable for the application of the series-connected hybrid magnet.

Mechanical Analysis of the Superconducting Outsert for the Series Connected Hybrid Magnets

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.

Development and Testing of Electrical Joints for the 36-T Series-Connected Hybrid Magnet

Here, we report the development of the CICC joint design for the 36-T Series-Connected Hybrid Magnet. A novel solder-less single-box praying-hands joint has been designed to meet the mission of the SCH. A prototype sample joint, Florida Solder-less Joint A (FSJ-A), was manufactured and tested. The low DC resistance confirmed the feasibility of the concept design. In addition, a simple model describing the current transient behavior of the pray-hand joint is presented. A comparison with the experimental data is also included.

Current Sharing and AC Loss Measurements of a Cable-in-Conduit Conductor With ${\rm Nb}_{3}{\rm Sn}$ Strands for the High Field Section of the Series-Connected Hybrid Outsert Coil

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.

Florida electro-mechanical cable model ofNb3Sn CICCs for high-field magnet design

Performance degradation of Nb3Sn cable-in-conduit-conductors (CICCs) is a critical issue in large-scale magnet designsuch as in the International Thermonuclear Experimental Reactor (ITER) andthe series-connected hybrid (SCH) magnets currently under development at theNational High Magnetic Field Laboratory (NHMFL). The critical current Ic ofNb3Sn conductors is strongly affected by thermal pre-strain in strand filaments in a CICC fromdifferential thermal contraction between strands and conduit during cooling down afterheat treatment. Mitchell and Nijhuis recently introduced strand bending underlocally accumulated Lorentz force for the interpretation of observed transverseload degradation, defined as the Ic reduction due to strand bending and contactstress at strand crossing with respect to the expected Ic from strand data atthe thermal compressive strain. In this paper, a new numerical model of CICCperformance has been developed based upon earlier work by Mitchell and Nijhuis.The new model, called the Florida electro-mechanical cable model (FEMCAM),combines the thermal bending effects during cooling down and the electromagneticbending effects during magnet operation, as well as effects due to strand filamentfracture. We present the FEMCAM formulation and benchmark the results againstabout 40 conductor tests of first-cycle performance and 20 tests that include cyclicloading. We also consider the effects of different jacketing materials on CICCperformance. We conclude that FEMCAM can be a helpful tool for the design ofNb3Sn-basedCICCs and that both thermal bending and transverse bending play important roles in the performanceof Nb3Sn CICCs.

Transient Stability Analysis of the Superconducting Outsert in the NHMFL Series Connected Hybrid Magnet System

Transient stability analysis of the superconducting outsert of a 36 T series-connected hybrid (SCH) magnet being developed at the NHMFL is performed. The latest design version of the SCH outsert primary coil will consists of three radial sections, designated as a low-field, a midfield and a high-field ones, each wound with a different superconducting cable-in-conduit conductor (CICC) using a cable of multi-filamentary strands inside a stainless steel jacket that confines slowly flowing supercritical helium (under 3 atm pressure at 4.5 K). There will be also an outer shield coil wound with a CICC that uses NbTi/Cu strands. The outsert cooling system concept delivers helium to all of the winding layers (each layer has an inlet and outlet), aiming to sustain a wide range of duty cycles required by diverse science experiments, conducted with the magnet. The transient stability is discussed in terms of temperature margin, limiting current, and energy margin as well and analyzed for several operation scenarios/duty cycles and situations resulting in different patterns of deposition and evacuation of heat due to AC losses in the outsert sections.

The Powered Scattering-Magnet Program at the NHMFL

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.

Cooling Conditions Analysis of the Superconducting Outsert in the Series Connected Hybrid Magnet System

The cooling system design for the superconducting outsert of a 36 T Series-Connected Hybrid (SCH) magnet being developed at the NHMFL is discussed and analyzed. The outsert will be wound with a superconducting cable-in-conduit conductor (CICC) that uses a cable of multi-filamentary Nb3Sn/Cu strands inside a high-strength alloy jacket that confines slowly flowing supercritical helium (under 3-3.5 atm pressure at 4.5 K) in direct contact with the cable strands. The cooling design option with the highest flow to all the winding layers is selected (each layer has an inlet and outlet), aiming to sustain a wide range of duty cycles required by diverse science experiments, conducted with this magnet. Typically, many experiments will need multiple cycles (charging/discharging) with short, albeit variable, waiting time before each one. The charging cycles would have ramp rates of about 30 A/s to relatively high ramp rates, up to 500-600 A/s. The different ramp rate characteristics will result in different patterns of deposition and evacuation of heat due to AC losses in the outsert windings. All of these cases are carefully analyzed and discussed.

Structural Design for the NHMFL Series Connected Hybrid Magnet System

The series connected hybrid (SCH) Magnet developed at the NHMFL will provide high field (36 T), high field quality (1 ppm stability and homogeneity) and a moderately large bore (40 mm inside the resistive shim coils) for users interested in medium resolution NMR studies. The SCH comprises a superconducting outsert built with Nb3Sn cable-in-conduit conductor and a resistive insert using Florida-Bitter technology. The insert and outsert magnets are connected electrically in series, which mitigates many of the engineering challenges for such systems, but mechanical design of the structural components linking these remains critical to the performance of the superconducting outsert. We present detailed design and analysis of these components with an assessment of their ability to meet the functional requirements presented by potentially normal, off normal and fault conditions during operation.