Series-Connected Hybrid Research Articles

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.

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.

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.

Modeling of the Performance of $\hbox{Nb}_{3}\hbox{Sn}$ CICCs for High Field Magnet Design

Accurate prediction of the performance of cable-in-conduit-conductors (CICCs) is challenging but critical for the design of high field magnets such as the series connected hybrid (SCH) magnet currently under development at the NHMFL. With various CICC tests performed for EFDA dipole magnet and ITER, we present the Florida electro-mechanical cable model (FEMCAM) and demonstrate that it is a self-consistent model incorporating important effects caused by differential thermal contraction and filament fracture to better predict CICC performance.

Development of the Superconducting Outserts for the Series-Connected-Hybrid Program at the National High Magnetic Field Laboratory

The National High Magnetic Field Laboratory (NHMFL) has embarked on an innovative program to develop a number of Series-Connected-Hybrid magnetic systems. In this novel concept, a set of resistive coils (insert) and a set of superconducting cable-in-conduit conductor (CICC) coils (outsert) are electrically driven in series, rather than independently as in previous hybrid systems. Presently NHMFL is working on 3 different projects. The first, funded by the National Science Foundation (NSF) with an $11.7 million grant, is related to the construction of a cylindrical-bore Series-Connected Hybrid (SCH), for high field nuclear magnetic resonance (NMR), condensed matter physics, biology and chemistry, to be located at the Magnet Lab's Tallahassee location. The second, funded by the Hahn-Meitner Institute (HMI) in Berlin, relates to the engineering design for a horizontal bore Series-Connected Hybrid to be used in a neutron diffraction experimental system. Finally, the third, also funded by NSF, is a conceptual and engineering study for an SCH that would be used for neutron scattering experiments at Oak Ridge National Laboratory's Spallation Neutron Source (SNS), similar to the HMI version. Because of the main points in common between the 3 different systems, our final goal is to develop a superconducting magnetic system suitable for the 3 applications. We believe this is possible for the magnet although, due to the different field/bore orientations, some differences in the mechanical design of the cryostat and resistive inserts will exist. In this paper we will give an overview of the R&D strategy devised to develop these superconducting magnetic systems, together with a report on the present status of such program.

Critical Current Longitudinal and Transverse Strain Sensitivities of High $J_{C}$${\rm Nb}_{3}{\rm Sn}$ Conductors

Characterizing critical current of strands as function of a strain is very important for large high field superconducting magnet applications such as the superconducting outsert coil of the series-connected hybrid at the NHMFL and the ITER magnets. Apparatuses for measuring versus longitudinal strain and transverse stress have been developed and used at the NHMFL. We have characterized the strain sensitivities of a few candidate strands for the series-connected-hybrid. In addition, irreversibility strains are measured for the recently developed ITER high strands. The different strain sensitivities for different strands are discussed.

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.

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.

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.

New high field magnet for neutron scattering at Hahn-Meitner Institute

The Berlin Neutron Scattering Center BENSC at the Hahn-Meitner-Institute (HMI) is a user facility for the study of structure and dynamics of condensed matter with neutrons and synchrotron radiation with special emphasis on experiments under extreme conditions. Neutron scattering is uniquely suited to study magnetic properties on a microscopic length scale, because neutrons have comparable wavelengths and, due to their magnetic moment, they interact with the atomic magnetic moments. Magnetic interactions and magnetic phenomena depend on thermodynamic parameters like magnetic field, temperature and pressure. At HMI special efforts are being made to offer outstanding sample environments such as very low temperatures or high magnetic fields or combination of both. For the future a dedicated instrument for neutron scattering at extreme fields is under construction, the Extreme Environment Diffractometer ExED. For this instrument the existing superconducting magnets as well as a future hybrid system can be used. The highest fields, above 30 T will be produced by the planned series-connected hybrid magnet system, designed and constructed in collaboration with the National High Magnetic Field Laboratory, Tallahassee, FL.

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 series‐connected hybrid triple converter system

AbstractThis paper proposes a hybrid multiconverter system aiming at a large‐capacity high‐voltage high‐efficiency converter system free from harmonics in its input/output. The system consists of n (n: integer) GTO converters and a single neutral‐point‐clamped IGBT converter connected in series by n + 1 output transformers. The GTO converters operate in a square‐wave switching mode while the IGBT converter operates in a PWM switching mode. The former produces a base part of the resultant output voltage. The latter not only produces an additional part of the resultant output voltage, but offsets the voltage harmonics generated by the GTO converters. A basic voltage control strategy for the hybrid converter system is proposed, and its validity is verified by experimental results. © 2005 Wiley Periodicals, Inc. Electr Eng Jpn, 153(3): 64–70, 2005; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/eej.20210

Application of a Series-Connected Hybrid Multi-Converter System to STATCOM

This paper gives an analysis and design of a static synchronous compensator (STATCOM) using a series-connected hybrid multi-converter system. The hybrid multi-converter system consists of two GTO converters and an IGBT converter connected in series by transformers. The proposed STATCOM has advantages that the harmonic components of the output voltage/current would be less than a conventional configuration STATCOM composed of a 12-, 24-, or 48-pulse converter; the total switching losses would be less than a PWM STATCOM; and the response would be as rapid as a PWM STATCOM. One problem of the hybrid configuration STATCOM is that it tends to operate in an over-modulation range if a considerable compensating reference change occurs. To solve this problem, this paper also proposes a variable DC voltage control strategy, where the DC voltage is adaptively adjusted in accordance with the required reactive power. The validity of the proposed configuration STATCOM and control strategies are verified by simulation and experimental results.

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.

Three-phase Series-connected Hybrid Converter System

In this paper, a 3-phase series-connected hybrid converter system is introduced. The converter system is constructed by a GTO converter and an IGBT converter connected in series by two output transformers aiming at a high voltage high efficiency converter. The GTO converter operates in a square-wave switching mode at the same switching rate as a reference output voltage in order to minimize the device switching losses. The IGBT converter is built in an NPC (neutral-point-clamped) arrangement and operates in a PWM switching mode at a high switching rate. The former produces a base part of the total output voltage. The latter not only produces an additional part of the total output, but also offsets the harmonic components generated by the GTO converter. The control strategy for the system is proposed, and the experiment on an inverter, rectifier and STATCOM operation is demonstrated using the proposed hybrid converter. The experimental results verified the validity of the proposed hybrid configuration and its control strategy.

Low resolution NMR magnets in the 23 to 35 T range at the NHMFL

The National High Magnetic Field Laboratory in Tallahassee, FL, USA has for several years provided low resolution NMR magnets to the condensed matter physics community. There are presently two magnets available, one that provides 24.5 T in a 32 mm bore with inhomogeneity of about 100 ppm over a 10 mm Diameter Spherical Volume (DSV) and another that provides 25 T in 52 mm bore with inhomogeneity of 12 ppm over a 1 cm DSV. The NHMFL is presently constructing the third magnet in this series that should provide field above 29 T with inhomogeneity less than 50 ppm over a 1 cm DSV. The NHMFL is also planning to build a series-connected hybrid to enable low resolution NMR in the 35 T range. History of the existing magnets and design of the 29 T magnet are presented along with analysis and conceptual design of magnets with uniformity in the 1 ppm range and fields in the 35 T range.