Showing papers in "IEEE Transactions on Applied Superconductivity in 2017"

A High-Temperature Superconducting Maglev-Evacuated Tube Transport (HTS Maglev-ETT) Test System

Abstract: As a bellwether transportation mode, albeit with hot controversy, evacuated tube transport (ETT) can dramatically reduce air friction, and incorporating Maglev technology could also thoroughly eliminate wheel-rail friction. It is not difficult to imagine that incorporation of these two technologies could establish an innovative transportation system, with particular advantages in terms of high speed, safety, energy saving, and environmental protection. The integration of these two technologies has been a great challenge, however, due to the composite technology. To realize this revolutionary idea, we have successfully developed the first proof-of-principle prototype of a 45-m-long high-temperature superconducting Maglev evacuated tube transport (HTS Maglev-ETT) test system, called the “Super-Maglev,” based on the passive self-stable HTS Maglev conceived in our group in 2000. The system mainly consists of three parts: an HTS Maglev-vehicle-guideway coupling system with 1-t load capability at a levitation gap of 10 mm, a 45-m-long racetrack-type evacuated tube with a 2-m-diameter circular cross section pumped by a hybrid air extraction system, and a 3-m linear induction motor to provide sectional propulsion. The system can achieve a pressure as low as 2.9 kPa in the tube. Experiments show that air drag on the vehicle is greatly reduced at that low air pressure, and a maximum speed of 50 km/h was recorded on the 6-m-diameter test guideway. Theoretically, the reduction of the aerodynamic consumed power could reach as high as to 90% under 10 kPa. This “Super-Maglev” strongly demonstrates the feasibility and potential merits of the HTS Maglev-ETT transportation concept.

A Public Database of High-Temperature Superconductor Critical Current Data

Abstract: One of the major impediments to the industrial take-up of high-temperature superconductors is the paucity of comprehensive, reliable and relevant performance data on commercially available wires. To address this, the Robinson Research Institute is making available its internal database of wire performance data, acquired on our in-house 1 kA critical current measurement system. The database is freely accessible via the world wide web at http://www.victoria.ac.nz/robinson/hts-wire-database and allows download of both graphical images of the data as well as the underlying data files. The database will continue to be expanded into the future, and submissions of commercially available wires for independent characterization and inclusion are invited. We demonstrate the utility of these data in terms of a case study on the design of our 1.5 T 2G HTS MRI system and by outlining a comparison of correlations between low-temperature and 77 K performance across 2G HTS wires sourced from different manufacturers that demonstrates that this is useful only for wires of a particular type produced under similar conditions (for example, several batches produced using the same process). These examples highlight the need for complete characterization of different wires under the operating conditions of interest.

Investigating surface loss effects in superconducting transmon qubits

Abstract: Superconducting qubits are sensitive to a variety of loss mechanisms including dielectric loss from interfaces. By changing the physical footprint of the qubit, it is possible to modulate sensitivity to surface loss. Here, we show a systematic study of planar superconducting transmons of differing physical footprints to optimize the qubit design for maximum coherence. We find that qubits with small footprints are limited by surface loss and that qubits with large footprints are limited by other loss mechanisms, which are currently not understood.

Design Overview of DM Radio Pathfinder Experiment

Abstract: We introduce DM Radio, a dual search for axion and hidden photon dark matter using a tunable superconducting lumped-element resonator We discuss the prototype DM Radio Pathfinder experiment, which will probe hidden photons in the 500 peV (100 kHz)–50 neV (10 MHz) mass range We detail the design of the various components: the LC resonant detector, the resonant frequency tuning procedure, the differential SQUID readout circuit, the shielding, and the cryogenic mounting structure We present the current status of the pathfinder experiment and illustrate its potential science reach in the context of the larger experimental program

Optimization of Transition Edge Sensor Arrays for Cosmic Microwave Background Observations With the South Pole Telescope

Abstract: In this paper, we describe the optimization of transition-edge-sensor (TES) detector arrays for the third-generation camera for the South Pole Telescope. The camera, which contains ∼16 000 detectors, will make high-angular-resolution maps of the temperature and polarization of the cosmic microwave background. Our key results are scatter in the transition temperature of Ti/Au TESs is reduced by fabricating the TESs on a thin Ti(5 nm)/Au(5 nm) buffer layer and the thermal conductivity of the legs that support our detector islands is dominated by the SiOx dielectric in the microstrip transmission lines that run along the legs.

The 16 T Dipole Development Program for FCC

Abstract: A key challenge for a future circular collider (FCC) with centre-of-mass energy of 100 TeV and a circumference in the range of 100 km is the development of high-field superconducting accelerator magnets, capable of providing a 16 T dipolar field of accelerator quality in a 50 mm aperture. This paper summarizes the strategy and actions being undertaken in the framework of the FCC 16 T Magnet Technology Program and the Work Package 5 of the EuroCirCol.

Design and application of superconducting fault current limiter in a multiterminal HVDC system

Abstract: Voltage source converter based HVdc (VSC-HVdc) systems are prone to high short-circuit current during transmission line faults. The situation for multiterminal HVdc (MTDC) systems is worse. The characteristics of superconducting material are ideal to limit the fault current in HVdc systems. This paper presents a novel use of the resistive type of superconducting fault current limiter (SFCL) in the MTdc network with the function of limiting the high current. The working principles of fault current limiter and a three-terminal HVdc system are modeled in detail using PSCAD/EMTDC software. The hybrid operation of the SFCL in the three-terminal HVdc system is tested in this paper for the fault response of the MTdc system. The performances of SFCL under different fault conditions are analyzed. The simulation results show that the fault current is effectively restrained and the SFCL can act as an efficient protective device for VSC-based multiterminal HVdc systems.

Research and Development of the High Stable Magnetic Field ReBCO Coil System Fundamental Technology for MRI

Abstract: The superconducting magnet is effective to get a high stable and high magnetic field for magnetic resonance imaging (MRI). The current MRI superconducting magnet needed cooling in the liquid helium (4.2 K) to use NbTi superconducting wire. In the past few years, price increase and low availability of liquid helium has become a serious problem. Under such circumstances, the development of a high-temperature superconducting (HTS) coil dispensing with liquid helium cooling is greatly desired. The research and development project of the high stable magnetic field ReBCO coil system fundamental technology that started from the latter half of 2013 develops a ReBCO coil for 3 T MRI superconducting magnets. It gets a prospect of the practical use as the final aim. In this project, we will produce an HTS test coil of 300 mm bore experimentally and evaluate the magnetic field. This coil is cooled in less than 20 K by a GM refrigerator. We are going to make MRI used by the ReBCO coil field to evaluate the uniformity and stability of the magnetic field.

An Integrated Row-Based Cell Placement and Interconnect Synthesis Tool for Large SFQ Logic Circuits

Abstract: This paper presents a row-based design methodology covering cell placement, clock tree synthesis, and routing steps for large SFQ circuits. The proposed placement tool initiates by running a state-of-the-art CMOS placer, which places fixed-height but variable-width cells in rows on the chip. Cells in each row are then grouped together such that each group contains at most $k$ cells with the same logic level. Next, for clock routing, this paper proposes HL-tree, which adopts an H-tree with passive transmission line connections to distribute the clock to groups, and within each group, a linear path composed of splitters and Josephson transmission lines (JTLs) provides the clock to cells. Increasing $k$ reduces the chip area, but also may incur a performance loss. To evaluate the effectiveness of the proposed approach, place-and-route results of a 32-bit Kogge–Stone adder for different values of $k$ are reported. By using this new design methodology, the overall chip area can be reduced by 27% compared with the results of a conventional CMOS placement accompanied by an H-tree clock network.

Properties of Unshunted and Resistively Shunted Nb/AlOx-Al/Nb Josephson Junctions With Critical Current Densities From 0.1 to 1 mA/μm2

Abstract: We investigated current–voltage characteristics of unshunted and externally shunted Josephson junctions (JJs) with high critical current densities J c in order to extract their basic parameters and statistical characteristics for JJ modeling in superconducting integrated circuits as well as to assess their potential for future technology nodes. Nb/AlO x -Al/Nb junctions with diameters from 0.5 to 6 μm were fabricated using a fully planarized process with Mo or MoN x thin-film shunt resistors with sheet resistance R sq = 2 Ω/sq and R sq = 6 Ω/sq, respectively. We used our current standard MIT Lincoln Laboratory process node SFQ5ee to fabricate JJs with J c = 0.1 mA/μm2 and our new process node SFQ5hs (where “hs” stands for high speed) to make JJs with J c = 0.2 mA/μm 2 and higher current densities up to about 1 mA/μm2. Using LRC resonance features on the I–V characteristics of shunted JJs, we extract the inductance associated with Mo shunt resistors of 1.4 pH/sq. The main part of this inductance, about 1.1 pH/sq, is the inductance of the 40-nm Mo resistor film, while the geometrical inductance of superconducting Nb wiring contributes the rest. We attribute this large inductance to “kinetic” inductance arising from the complex conductivity of a thin normal-metal film in an electromagnetic field with angular frequency $\boldsymbol{\omega },$ ${{\mathbf \sigma }}(\boldsymbol{\omega }) = {\boldsymbol{\sigma }_0}/({1 + \boldsymbol{i\omega \tau }})$ , where ${{{\mathbf \sigma }}_0}$ is the static conductivity and $\boldsymbol{\tau }$ the electron scattering time. Using a resonance in a large-area unshunted high- J c junction excited by a resistively coupled small-area shunted JJ, we extract the Josephson plasma frequency and specific capacitance of high- J c junctions in 0.1–1 mA/μm2 J c range. We also present data on J c targeting and JJ critical current spreads. We discuss the potential of using 0.2-mA/μm2 JJs in very large scale integration single flux quantum circuits and 0.5-mA/μm2 JJs in high-density integrated circuits without shunt resistors.

Development of Superconducting Magnetic Bearing for 300 kW Flywheel Energy Storage System

Abstract: The world's largest-class flywheel energy storage system (FESS), with a 300 kW power, was established at Mt. Komekura in Yamanashi prefecture in 2015. The FESS, connected to a 1-MW megasolar plant, effectively stabilized the electrical output fluctuation of the photovoltaic (PV) power plant caused by the change in sunshine. The FESS uses a superconducting magnetic bearing (SMB) to levitate a heavy weight flywheel rotor without mechanical contact. The SMB consists of high-temperature superconducting coils (HTS coils) made of rare-Earth-Ba2Cu3Oy , superconducting tapes, and high-temperature superconducting plates (HTS plates) made of YBa2Cu3 Oy. The HTS plates in the rotor axis received the levitation forces in the magnetic field generated by the HTS coils in the stator. At first, the SMB was tested alone to check its cooling properties and to measure the levitation force. In the factory test, the SMB was confirmed to levitate the 4000-kg load, and its performance and basic reliability were verified. In the verification test, at Mt. Komekura, the FESS rotor reached a maximum of 2950 r/min, and the FESS was charged and discharged at 300 kW in the PV plant.

Superconducting Properties of 100-m Class Sr0.6K0.4Fe2As2 Tape and Pancake Coils

Abstract: Iron-pnictides are hotly studied since 2008 in the superconducting materials research area, due to their special properties and unclear mechanism. Big achievement has been made in the pnictide research during the past years. For practical uses, pnictide superconductor should be fabricated in a form that can be used for different devices. In this paper, 100-m class 7-core Sr 0.6 K 0.4 Fe 2 As 2 (Sr122) tapes have been made by using the powder-in-tube technique, which is reported for the first time. Clearly, an average J c of 1.3 × 10 4 A/cm 2 at 10 T was reached over the 115-m length, showing a high property and good uniformity of 100-m level Sr122 tapes. Using the 10-m long Sr122 tapes, two double-pancake coils were fabricated by a wind and reaction technique. No transport current could be measured for the coil made from the monofilamentary tape. However, transport I c was obtained in the coil made from the 7-filamentary tape. The factors that affect the superconducting property of the coil were discussed in this paper.

High-Speed Operation of Random-Access-Memory-Embedded Microprocessor With Minimal Instruction Set Architecture Based on Rapid Single-Flux-Quantum Logic

Abstract: We present design and experimental results of a rapid single-flux-quantum (RSFQ) bit-serial microprocessor with reduced-size embedded random access memories (RAMs) and with a minimal instruction set, called CORE e2h. The microprocessors called CORE e series have been developed for demonstrating small-scale program execution, such as loop calculation and sorting, in order to show the first prototype of a stored-program computer using the RSFQ technology. The CORE e2h is the most simplified variation of the CORE e series, which is equipped with only two registers, and can execute 13 instructions. The target clock frequency for bit-serial operation is 50 GHz, while the designed system clock cycle is 2 GHz. We carefully designed every component, implementing functionality using a small number of Josephson junctions with a small footprint. We fabricated several chips of the CORE e2h microprocessor integrated with two 128-bit shift-register-based RAMs on the same die. We experimentally obtained correct operations for all the instructions, and confirmed high-speed transfer between the instruction memory and controller unit and between the data memory and datapath at around 50 GHz.

HTS System and Component Targets for a Distributed Aircraft Propulsion System

Abstract: Distributed propulsion for aircraft is a radical concept in which the generation of thrust is decentralized from the main power sources. Various implementations of this concept are being considered as ways of reducing aircraft fuel burn and noise impact in the future, many relying on electrical transmission. However, the power required for aircraft propulsion is orders greater than the present electrical systems on aircraft. Efficiency and weight of the electrical components will be crucial for the feasibility of an aircraft with distributed propulsion and thus, HTS technology is of strong interest. This paper explains the interest in the technology and possible means of implementation in an aircraft, including considerations of the means of cryogenic cooling. Technology targets are presented on the basis of an aircraft that uses distributed propulsion to enable boundary layer ingestion and produces 50% of its thrust electrically.

AC-Biased Shift Registers as Fabrication Process Benchmark Circuits and Flux Trapping Diagnostic Tool

Abstract: We develop an ac-biased shift register introduced in our previous work (V. K. Semenov et al. , IEEE Trans. Appl. Supercond. , vol. 25, no. 3, 1301507, June 2015) into a benchmark circuit for evaluation of superconductor electronics fabrication technology. The developed testing technique allows for extracting margins of all individual cells in the shift register, which in turn makes it possible to estimate statistical distribution of Josephson junctions (JJs) in the circuit. We applied this approach to successfully test registers having 8-, 16-, 36-, and 202 thousand cells and, respectively, about 33-, 65-, 144-, and 809 thousand JJs. The circuits were fabricated at MIT Lincoln Laboratory, using a fully planarized process, 0.4-μm inductor linewidth and 1.33 × 106 cm−2 junction density. They are presently the largest operational superconducting SFQ circuits ever made. The developed technique distinguishes between “hard” defects (fabrication-related) and “soft” defects (measurement-related) and locates them in the circuit. The “soft” defects are specific to superconducting circuits and caused by magnetic flux trapping either inside the active cells or in the dedicated flux-trapping moats near the cells. The number and distribution of “soft” defects depend on the ambient magnetic field and vary with thermal cycling even if done in the same magnetic environment.

Technical and Economic Analysis of the R-Type SFCL for HVDC Grids Protection

Abstract: Mainly used in ac grids, resistive-type superconducting fault current limiters may be still more interesting for dc systems due to their capability to reduce high short circuit currents appearing in case of dc cable fault. This limiter reduces the breaking capability, speed, and energy requirements of the required dc circuit breaker allowing the implementation of electromechanical breakers for fault current interruption. These breakers have lower breaking capability, on-load losses, and investment costs in comparison with hybrid circuit breakers based on power electronics. This paper presents a technical and economic analysis of a superconducting fault current limiter used in a radial three-terminal high voltage dc grid to protect a cable link. Based on simulation studies using an electromagnetic transient program, an effective system protection will be demonstrated with special attention to the continuity of power flow through healthy parts of the grid.

Investigation of HTS Twisted Stacked-Tape Cable (TSTC) Conductor for High-Field, High-Current Fusion Magnets

Abstract: The critical currents of a twisted single-tape were measured at 4.2 K in fields up to 17 T. It was confirmed that the critical current of a twisted-tape was similar to that of a flat tape in c-axis fields. Based on the single-tape critical-currents, a 40-tape (4-mm width) twisted stacked-tape cable (TSTC) conductor was experimentally evaluated at fields up to 17 T at 4.2 K. It was found that the Lorentz load degradation of the TSTC conductor was negligible at the end of a cyclic test. The TSTC cabling method has been considered to be suitable for developing a high-field high-current REBCO conductor for magnet applications as well as power transmission cables. A method to fabricate the inner legs of a D-shape toroidal field coil using a TSTC conductor has been discussed. This method allows the conductor to properly resist the transverse Lorentz load and mitigate ac loss issues by adopting a parallel-HTS-tape flat cable configuration where necessary.

Design of a Canted-Cosine-Theta Superconducting Dipole Magnet for Future Colliders

Abstract: A four-layer canted-cosine-theta 16-T dipole has been designed as a possible candidate for future hadron colliders. The design maintains part of the future-circular-collider magnet requirements, i.e., a 50 mm clear bore and 16 T operating at 1.9 K. The magnet intercepts Lorentz forces with an internal structure of ribs and spars, minimizes conductor, and reduces the number of layers and magnet size by using wide cables. The role of iron and its impact on field and magnet size is discussed. A three-dimensional magnetic analysis was carried out for 1-in-1 and 2-in-1 designs including a structural analysis for the 1-in-1 case. Thoughts on future improvements during winding are also discussed.

Superconducting NbTin Thin Films With Highly Uniform Properties Over a ${\varnothing}$ 100 mm Wafer

Abstract: Uniformity in thickness and electronic properties of superconducting niobium titanium nitride (NbTiN) thin films is a critical issue for upscaling superconducting electronics, such as microwave kinetic inductance detectors for submillimeter wave astronomy. In this article we make an experimental comparison between the uniformity of NbTiN thin films produced by two DC magnetron sputtering systems with vastly different target sizes: the Nordiko 2000 equipped with a circular $\varnothing$ 100 mm target, and the Evatec LLS801 with a rectangular target of 127 mm $\times$ 444.5 mm. In addition to the films deposited staticly in both systems, we have also deposited films in the LLS801 while shuttling the substrate in front of the target, with the aim of further enhancing the uniformity. Among these three setups, the LLS801 system with substrate shuttling has yielded the highest uniformity in film thickness ( $\pm$ 2%), effective resistivity (decreasing by 5% from center to edge), and superconducting critical temperature ( $T_{\mathrm{c}}$ = 15.0 K–15.3 K) over a $\varnothing$ 100 mm wafer. However, the shuttling appears to increase the resistivity by almost a factor of 2 compared to static deposition. Surface SEM inspections suggest that the shuttling could have induced a different mode of microstructural film growth.

Design, Manufacture, and Test of an 80 kA-Class Nb3Sn Cable-In-Conduit Conductor With Rectangular Geometry and Distributed Pressure Relief Channels

Abstract: Within the frame of the R&D activities carried out in Europe for the toroidal field coils of the nuclear fusion device DEMO, a fundamental milestone was considered to be the demonstration of Nb3Sn cable-in-conduit conductors (CICC) performance in the demanding range of interest for DEMO. Among the different technological solutions envisaged, the present paper deals with a wind & react CICC solution, with rectangular geometry, thick steel jacket, and distributed pressure relief channel, designed to operate at 82 kA in a magnetic field of 13 T and with a current sharing temperature ${\rm{T}}_{{\rm{cs}}}\,{\rm{> \,6.5\,K}}$ . The main manufacturing steps of the prototype conductor are described in the present paper, carried out within industrial environment, partly using the facilities and procedures available for the manufacture of ITER conductors. A sample was designed for the EDIPO facility at the Swiss Plasma Center, Switzerland, in the configuration usually adopted for the test of ITER poloidal field conductors, where the two straight conductor legs are part of the same cable length, with a continuous transition through a bottom hairpin-type joint, thus avoiding any resistive connection. The conductor has been characterized in terms of dc performance at relevant operating conditions and the absence of any performance degradation with electro-magnetic load cycles has been verified, thus, qualifying the proposed technological solution. AC losses and thermo-hydraulic tests have also been carried out, providing relevant information for further coil design.

Mechanical Performance of Short Models for MQXF, the Nb3Sn Low-β Quadrupole for the Hi-Lumi LHC

Abstract: In the framework of the Hi-Lumi LHC Project, CERN and U.S. LARP are jointly developing MQXF, a 150-mm aperture high-field Nb3Sn quadrupole for the upgrade of the inner triplet of the low-beta interaction regions. The magnet is supported by a shell-based structure, providing the preload by means of bladder-key technology and differential thermal contraction of the various components. Two short models have been produced using the same cross section currently considered for the final magnet. The structures were preliminarily tested replacing the superconducting coils with blocks of aluminum. This procedure allows for model validation and calibration, and also to set performance goals for the real magnet. Strain gauges were used to monitor the behavior of the structure during assembly, cool down and also excitation in the case of the magnets. The various structures differ for the shell partitioning strategies adopted and for the presence of thick or thin laminations. This paper presents the results obtained and discusses the mechanical performance of all the short models produced up to now.

Comparison of Superconducting Fault Current Limiter and Dynamic Voltage Restorer for LVRT Improvement of High Penetration Microgrid

Abstract: For a high penetration microgrid, improving its low-voltage ride-through (LVRT) capability under some minor or temporary faults can contribute to reinforcing power support and reducing network instability. In this paper, the comparison of a superconducting fault current limiter (SFCL) and a dynamic voltage restorer (DVR) for LVRT capability enhancement of a 10-kV microgrid is conducted. Concerning the microgrid which includes distributed photovoltaic generation, energy storage and loads, the effects of the SFCL and the DVR are compared in detail, and related theoretical analysis, simulation study, and economical evaluation are carried out. From the demonstrated results, the suggested two devices can both assist the microgrid to achieve the LVRT operation, but the economics of the DVR are weaker than the SFCL. Moreover, compared to the DVR, the SFCL enables the energy storage unit to offer better control effects in power stabilization. From technical and economic perspectives, the SFCL is a more preferable choice than the DVR.

Design and Application of the SFCL in the Modular Multilevel Converter Based DC System

Abstract: There is an urgent need for the fault current limiter to guarantee the stability of the dc system. For this reason, the improved inductive superconducting fault current limiter (SFCL) is designed and its working principle is addressed. By analyzing the transient characteristics of the dc fault in modular multilevel converter based dc system, the current-limiting effects of the proposed SFCL are studied in detail. According to the current-limiting requirements and the coordination among the converter, the fault current limiter and the dc circuit breaker, the desired impedance of the SFCL is discussed in this paper. Finally, the effectiveness and correctness of the analyses have been verified by the simulation results based on the PSCAD/EMTDC simulation

Conductor specification and validation for high-luminosity LHC quadrupole magnets

Abstract: Author(s): Cooley, LD; Ghosh, AK; Dietderich, DR; Pong, I | Abstract: The high-luminosity upgrade of the large hadron collider (HL-LHC) at CERN will replace the main ring inner triplet quadrupoles, identified by the acronym MQXF, adjacent to the main ring intersection regions. For the past decade, the U.S. LHC Accelerator RaD Program, LARP, has been evaluating conductors for the MQXFA prototypes, which are the outer magnets of the triplet. Recently, the requirements for MQXF magnets and cables have been published in [P. Ferracin et al., IEEE Trans. Appl. Supercond., vol. 26, no. 4, Jun. 2016, Art. no. 4000207], along with the final specification for Ti-alloyed Nb3Sn conductor determined jointly by CERN and LARP. This paper describes the rationale beneath the 0.85-mm-diameter strand's chief parameters, which are 108 or more subelements, a copper fraction not less than 52.4%, strand critical current at 4.22 K not less than 631 A at 12 T and 331 A at 15 T, and residual resistance ratio of not less than 150. This paper also compares the performance for ∼100 km production lots of the five most recent LARP conductors to the first 163 km of strand made according to theHL-LHCspecification.Two factors emerge as significant for optimizing performance and minimizing risk: a modest increase of the subelement diameter from 50 to 55 μm, and a Nb:Sn molar ratio of 3.6 instead of 3.4. The statistics acquired so far give confidence that the present conductor can balance competing demands in production for the HL-LHC project.

Test of an 8.66-T REBCO Insert Coil with Overbanding Radial Build for a 1.3-GHz LTS/HTS NMR Magnet.

Abstract: A 1.3-GHz/54-mm LTS/HTS NMR magnet, assembled with a three-coil (Coils 1-3) 800-MHz HTS insert in a 500-MHz LTS NMR magnet, is under construction. The innermost HTS insert Coil 1 has a stack of 26 no-insulation (NI) double pancake (DP) coils wound of 6-mm-wide and 75-μm-thick REBCO tapes. In order to keep the hoop strains on REBCO tape <;0.6% at an operating current I op of 250 A and in a field of 30.5 T, we overbanded each pancake in Coil 1 with a 6-mm-wide, 76-μm-thick 304 stainless steel strip: 7-mm-thick radial build for the central 18 pancakes, while 6-mm-thick for the outer 2 × 17 pancakes. In this paper, Coil 1 was successfully tested at 77 K and 4.2 K. In the 77-K test, the measured critical current was 35.7 A, determined by an E-field criterion of 0.1 μV/cm. The center field magnet constant decreased from 34.2 to 29.3 mT/A, when I op increased from 5 to 40 A. The field distribution at different I op along the z-axis was measured. The residual field distributions discharged from 10 and 20 A were recorded. In the 4.2-K test, Coil 1 successfully generated a central field of 8.78 T at 255 A. The magnet constant is 34.4 mT/A, which is same as our designed value. The field homogeneity at the coil center within a ±15-mm region is around 1700 ppm. This large error field must be reduced before field shimming is applied.

Numerical Study on Magnetization Characteristics of Superconducting Conductor on Round Core Cables

Abstract: This paper presents numerical study on magnetization characteristics of conductor on round core (CORC) cable. Numerical models introduced in this paper are based on finite-element method (FEM) with simplified structure, optimized assumptions and settings. Magnetization performance of CORC cables is represented by both full coupled model and isolated model. These numerical models are verified by experiments on CORC cables made of commercial HTS tapes. On basis of this, two topics are discussed in this paper: one is the magnetic shielding effect of superconducting layers in CORC cables; the other one is the end effect of magnetization loss in short CORC cables. Conclusions obtained in this paper will be helpful to understand the working mechanism of CORC cable and also useful in design of large-scale magnets based on CORC cables.

Experimental Production of a Real-Scale REBCO Magnet Aimed at Its Application to Maglev

Abstract: We fabricated a REBCO magnet that demonstrates a magnetomotive force of 700 kA above 30 K. This magnet is comparable to the low temperature superconducting (LTS) on-board magnet of the maglev with respect to coil size (racetrack-shaped, 1070 mm wide, 500 mm high) and magnetomotive force. The REBCO magnet does not have a radiation shield and is cooled by a single-stage Gifford-McMahon (GM) cryocooler. A 6-mm-wide coated conductor (SuperPower Inc. SCS6050-AP) is used for the coil winding. The number of turns and the wire length are 2800 and approximately 7600 m, respectively. In this coil fabrication, we utilized thermoplastic resin instead of epoxy impregnation. The REBCO magnet was excited at 35 K, and a magnetomotive force of 700 kA was successfully achieved. In addition, a magnetomotive force of 750 kA was also achieved at 32 K.

Quench Analysis of a Multiwidth No-Insulation 7-T 78-mm REBCO Magnet

Abstract: Due to the self-protecting feature, no-insulation (NI) high-temperature superconductor magnets have been regarded as a reliable option to generate high fields, yet their intrinsic charging delay still remains a major drawback. To apply the NI technique for actual high-field user magnets, however, postquench transient behavior of such magnets need to be fully understood, particularly the electromagnetic interaction among magnetically coupled subcoils. Recent publications have shown successful simulations of the transient behavior of a single NI coil or a multicoil magnet using distributed network models. Even though these approaches are very accurate, they often require substantial computation time, especially when multiple iterations are required during design or analysis of an NI magnet having a large number of coils. This paper presents a simple circuit approach that may be effective for quench simulation of multicoil NI magnets. Each NI subcoil in a magnet is lumped into a single inductor with a resistor in series and a resistor in parallel. This approach has allowed us to simulate the whole magnet system within reasonable time without compromising the understanding of mutual interactions of all of the subcoils after quench, namely change in parameters such as voltage, current, and temperature with respect to time and coil to coil normal zone propagation in an electromagnetic manner. We verified the proposed approach by analyzing the quench process in a previously built 7 T, 78 mm all-REBCO NI magnet (by MIT), and by doing the first ever comparison between the simulated results with the data measured from actual experiment.

Development of a Novel Soldered-Stacked-Square (3S) HTS Wire Using 2G Narrow Tapes With 1 mm Width

Abstract: Based on the pre-existing assembled, stacked, or twisted structures of high temperature superconducting (HTS) wires, a novel soldered-stacked-square (3S) HTS wire is suggested and developed through narrowing, stacking, and soldering processes in this study. For the fabrication processes of the 3S wire, a narrow 2G tape with 1 mm width is applied, and the stacking and soldering processes are completed simultaneously. To confirm the performance of the 3S HTS wire, several 3S samples are fabricated. For the 3S HTS samples, we evaluate typical electrical and mechanical properties, and investigated for the feasibility of the 3S HTS wire in the HTS applications. The test results show that measured critical currents of two 3S wire samples are 170 and 255 A, and their curves are typical ones of HTS tapes. The measured self-field ac loss is also reasonable, which is analyzed from both theoretical models of the elliptical and thin strip equations. Moreover, the minimum bending diameter for the 3S samples is approximately 60 mm, and the allowable axial tensile force is 350 N. These mechanical properties of 3S wire are satisfied for the HTS applications significantly. Finally, the smallest round structure of the HTS wire is conceptually suggested for the 3S HTS wire.

Frequency Dependent Behavior of a Dynamo-Type HTS Flux Pump

Abstract: The high-T c superconductor (HTS) dynamo is a type of superconducting flux pump that can inject a dc current into a superconducting magnet coil, without requiring physical connection to an external power supply. Here, we report experimental results from an HTS dynamo employing 46-mm wide coated conductor stator wire. We characterize the output of the device as a function of frequency, and show the effect of slitting the stator wire into two or three parallel equivalent strips. At operating frequencies above ~100 Hz, we observe that the internal resistance and open-circuit voltage ceases to increase linearly with frequency, and that the short-circuit current is greatly reduced. Experimental waveform data of the output open-circuit voltage enables us to identify three distinct and different signature waveforms, which mark the low-frequency, midfrequency, and high-frequency operating regimes. These regimes appear to arise due to the saturation of circulating currents within the stator wire, followed by a rise in temperature of the wire. We conclude that dissipation due to circulating currents plays an important role in depressing the output of this wide stator device at high frequencies. This effect is in marked contrast to results previously reported for a similar HTS dynamo employing narrower stator wires.