Showing papers in "IEEE Transactions on Applied Superconductivity in 2007"

Highly Sensitive and Easy-to-Use SQUID Sensors

Abstract: We have developed a novel family of low-noise superconducting quantum interference devices (SQUIDs) to cover a wide range of applications. These sensors are robust and easy to use without compromising noise performance. They are optimized for operation with our high-speed direct-coupled flux-locked loop (FLL) electronics XXF-1. For the readout of cryogenic detectors, series arrays of 16 SQUIDs with 3 nH input inductance were designed which can be cooled down and operated in the Earth's field without magnetic shield. A compact gradiometric design allows the chips to be mounted directly on a Cu block at the cold stage of a mK cryostat without degradation in noise. A current noise level of 9 or 5 is achieved at 4.2 K or 300 mK, respectively. Ultra-high-speed operation was demonstrated by using a cold FLL electronics prototype in close proximity to the SQUID array. An extremely high FLL bandwidth of 350 MHz was achieved which outperforms any flux-locked SQUID before by more than an order of magnitude. For applications requiring a larger input inductance of up to 2 , integrated two-stage sensors were developed consisting of a single front-end SQUID with double-transformer coupling read out by a 16-SQUID array. These sensors are very convenient to use as their voltage-flux characteristic is essentially single-SQUID-like. Devices optimized for 4.2 K operation have a coupled energy resolution around 50 times Planck's constant . Heavily shunted devices for low-temperature operation typically achieve 80 or 8 at 4.2 K or 300 mK, respectively. An optional current limiter (Q-spoiler) at the input reduces the input current in pulsed applications like nuclear magnetic resonance or magnetorelaxometry. Integrated rf filters and resistor-capacitor shunts across the input coil result in smooth, well-behaved voltage-flux characteristics even at low temperatures 4.2 K. For magnetic field sensing applications, integrated miniature multiloop magnetometers were designed with maximized field resolution. For a 3 mm 3 mm chip size, a noise level of 3.6 is obtained at 4.2 K.

Modeling the Electrical and Thermal Response of Superconducting Nanowire Single-Photon Detectors

Abstract: We modeled the response of superconducting nanowire single-photon detectors during a photodetection event, taking into consideration only the thermal and electrical properties of a superconducting NbN nanowire on a sapphire substrate. Our calculations suggest that heating which occurs after the formation of a photo-induced resistive barrier is responsible for the generation of a measurable voltage pulse. We compared this numerical result with experimental data of a voltage pulse from a slow device, i.e. large kinetic inductance, and obtained a good fit. Using this electro-thermal model, we estimated the temperature rise and the resistance buildup in the nanowire, and the return current at which the nanowire becomes superconducting again. We also show that the reset time of these photodetectors can be decreased by the addition of a series resistance and provide supporting experimental data. Finally we present preliminary results on a detector latching behavior that can also be explained using the electro-thermal model.

Multi-Element Superconducting Nanowire Single-Photon Detector

Abstract: A multi-element superconducting nanowire single photon detector (MESNSPD) is presented that consists of multiple independently-biased superconducting nanowire single photon detector (SNSPD) elements that form a continuous active area. A two-element SNSPD has been fabricated and tested, showing no measurable crosstalk between the elements, sub-50-ps relative timing jitter, and four times the maximum counting rate of a single SNSPD with the same active area. The MESNSPD can have a larger active area and higher speed than a single-element SNSPD and the input optics can be designed so that the detector provides spatial, spectral or photon number resolution.

ROEBEL Assembled Coated Conductors (RACC): Preparation, Properties and Progress

Abstract: RBCO (R = Y or rare earth element) coated conductors (CC) are the most promising HTS materials for future high field coils operated at moderately high temperature (40-50 K). Coils are planned for the second generation of fusion reactors (DEMO, "DEMOnstrator") and beyond. A ROEBEL bar concept for a high current (kA-class) low AC loss cable is the most suitable assembling technique for conductors in magnet windings due to the flat rectangular cross section. The presented RACC-cable technique (RACC=ROEBEL assembled coated conductors) works with pre-shaping of tapes into strands with the ROEBEL specific meander geometry. The usually very good bending properties of the CC support the assembling procedure of the RACC-cable. We report on a 16 strand RACC-cable with 19 cm transposition length made from CC material from the commercial supplier SuperPower which reached 1020 A transport critical current at 77 K (Jeng = 11.3 kAcm-2). The basic properties of the virgin YBCO tapes and the shaped strands like orientation and field dependent transport currents, current homogeneity and bending effects, were investigated and correlated with the measured properties of the RACC-cable. Calculation of the self field effects by means of a model adapted to the specific RACC-cable geometry and in particular taking into account the current distribution in the cable, explained the 30% current reduction in the cable quantitatively.

Recent Progress in Second-Generation HTS Conductor Scale-Up at SuperPower

Abstract: YBa2Cu3Ox (YBCO) second-generation (2G) HTS conductors have been produced in lengths over 300 m in pilot scale facilities established at SuperPower. High throughput pilot-scale manufacturing has been demonstrated with tape speeds at or above 30 m/h of 12 mm wide tape (corresponding to 90 m/h of 4 mm wide conductor) in all steps. A 322 m long conductor with a minimum critical current value of 219 A/cm has been produced, which corresponds to a critical current x length value of 70,520 A-m. A 270 m long, 4 mm wide conductor with an end-to-end critical value of 100 A has also been demonstrated. In a campaign to manufacture 2G conductor for the Albany cable project, SuperPower has produced 12,470 m of conductor that meets or exceeds the specification for piece length (42.4 m) and critical current (100 A/cm). In fact, more than 55% of the conductor produced is at least 100 m in piece length and more than 27% greater than 200 m in piece length. In addition to scaling up 2G conductors to high-throughput pilot-scale manufacturing, we have demonstrated high critical currents in short samples produced by Metal Organic Chemical Vapor Deposition (MOCVD). Critical current values of 557 A/cm have been achieved in 2.1 micrometer thick films in 12 mm wide, 10 cm long tapes. These samples exhibit a critical current value of 116 A/cm at 1 T and 76 K, in the orientation of field parallel to the c-axis. We have also constructed a 4-pancake coil that generated a magnetic field of 1.1 T at 77 K and 2.4 T at 64 K. A Fault Current Limiter (FCL) assembly has been successfully constructed and tested at high power levels. A prospective current of 90 kA was successfully limited to 32 kA within 1 ms without any HTS element failure.

Design, Fabrication, and Test of a 5-kWh/100-kW Flywheel Energy Storage Utilizing a High-Temperature Superconducting Bearing

Abstract: The Boeing team has designed, fabricated, and is currently testing a 5-kWh/100-kW flywheel energy-storage system (FESS) utilizing a high-temperature superconducting (HTS) bearing suspension/damping system. Primary design features include: a robust rotor design utilizing a composite rim combined with a metallic hub to create a 164-kg rotor assembly without critical resonances within the normal operating range, a closed-loop passive HTS bearing suspension/damping system, and a brushless 100-kW motor/generator. The Boeing FESS provides true isolation for the critical load as the 3-phase 480-V AC input power is converted to 600-V DC and re-inverted to clean, 3-phase 480-V AC output power by the inverter power electronics. When fully charged, the system is capable of responding within 4 ms for the uninterruptible protection of critical digital loads. Low losses and stable operation were demonstrated at rotational speeds exceeding 15,000 rpm.

Development and Demonstration of a HTS Power Cable to Operate in the Long Island Power Authority Transmission Grid

Abstract: The US Department of Energy is currently funding the design, development and demonstration of the first long length, transmission level voltage, cold dielectric, high temperature superconductor power cable. The cable is designed for permanent installation in the long island power authority (LIPA) grid and will be able to carry 574 MVA at a voltage of 138 KV. The project is led by American Superconductor and the team is comprised of Nexans, Air Liquide and LIPA. This paper will describe the goals of the project, an overview of the cable design, refrigeration system and site design. It will also include the statistics of the HTS wire manufactured for this project, the results of the qualification testing on a prototype 30 meter cable and the preparation work of the refrigerator system. The cable will be completed and put into service in the LIPA grid in 2007.

Tunability of Superconducting Metamaterials

Abstract: Metamaterials are artificial structures with unique electromagnetic properties, such as relative dielectric permittivity and magnetic permeability with values less than 1, or even negative. Because these properties are so sensitive to loss, we have developed metamaterials comprised of superconducting waveguides, wires, and split-ring resonators. An important requirement for applications of these metamaterials is the ability to tune the frequency at which the unique electromagnetic response occurs. In this paper we present three methods (unique to superconductors) to accomplish this tuning: temperature, dc magnetic field, and rf magnetic field. Data are shown for dc and rf magnetic field tuning of a single Nb split-ring resonator (SRR). It was found that the dc field tuning was hysteretic in the resonant frequency data, while the quality factor, was less hysteretic. The rf power tuning showed no hysteresis, but did show suppression of the at high power. Magneto-optical images reveal inhomogeneous magnetic vortex entry in the dc field tuning, and laser scanning photoresponse images for a YBa2Cu3O7-6SRR reveals the current distribution in the rings.

Middle-Infrared to Visible-Light Ultrafast Superconducting Single-Photon Detectors

Abstract: We present an overview of the state-of-the-art of NbN superconducting single-photon detectors (SSPDs). Our devices exhibit quantum efficiency (QE) of up to 30% in near-infrared wavelength and 0.4% at 5 mum, with a dark-count rate that can be as low as 10-4 s-1. The SSPD structures integrated with lambda/4 microcavities achieve a QE of 60% at telecommunication, 1550-nm wavelength. We have also developed a new generation of SSPDs that possess the QE of large-active-area devices, but, simultaneously, are characterized by low kinetic inductance that allows achieving short response times and the GHz-counting rate with picosecond timing jitter. The improvements presented in the SSPD development, such as fiber-coupled SSPDs, make our detectors most attractive for high-speed quantum communications and quantum computing.

Triaxial HTS Cable for the AEP Bixby Project

Abstract: Ultera has installed a single 200-meter long high temperature superconducting (HTS) 3-phase triaxial design cable at the American electric power (AEP) Bixby substation in Columbus, Ohio. The cable connects a 138/13.2 kV transformer to the distribution switchgear serving seven outgoing circuits. It was designed to carry 3000 Arms. Testing of 3- to 5-meter length prototype cables, including a 5-meter prototype with full scale terminations tested at ORNL was conducted prior to the manufacture and installation of the AEP triaxial cable. These prototypes were used to demonstrate the crucial operating conditions including steady state operation at the 3000 Arms design current, high voltage operation, high voltage withstand and 110 kV impulse, and overcurrent fault capability. A summary of the results from the thermal analysis and testing conducted by Ultera and ORNL will be presented. Some analysis of the cable thermal-hydraulic response based on the testing that were used to determine some of the cable cryogenic system requirements are also presented.

Development of Saturated Iron Core HTS Fault Current Limiters

Abstract: We have been carrying out a saturable iron core reactive type superconducting fault current limiter (SFCL) development program since 2002. The major two disadvantages that people used to be foretold for a saturable iron core reactive type SFCL are the massive use of iron (resulting in large size, heavy weight, and high cost) and the high induced voltage hazard to the dc superconducting coil (this may damage the current supply of the dc bias) as a fault takes place. We have found the ways to deal with these two problems, making such kind of equipment reliable and cost effective. In this paper, we will report the technical data and testing results of a 3 phase lab testing model. Some key design parameters of the 35 kV/100 MVA prototype will also be presented.

Digital Channelizing Radio Frequency Receiver

Abstract: HYPRES is developing a class of digital receivers featuring direct digitization at radio frequency (RF). Such a receiver consists of a wideband analog-to-digital converter (ADC) modulator and multiple digital channelizer units to extract different frequency bands-of-interest within the broad digitized spectrum. The single-bit oversampled data, from either a lowpass delta or bandpass delta-sigma modulator, are applied to one or more channelizers, each comprising digital in-phase and quadrature mixers and a pair of digital decimation filters. We perform channelization in two steps, the first at full ADC sampling clock frequency with rapid single flux quantum (RSFQ) digital circuits and the second at reduced (decimated) clock frequency with commercial field programmable gate array (FPGA) chips at room temperature. We have demonstrated lowpass and bandpass digital receivers by integrating an ADC modulator and a channelizer unit on the same chip at clock frequencies up to 20 GHz. These 1-cm2 single-chip digital-RF receivers contain over 10,000 Josephson junctions. The channelizing receiver approach can be extended to include multiple ADC modulators and multiple channelizer units on a multi-chip module.

NbN Superconducting Single-Photon Detectors Prepared on Single-Crystal MgO Substrates

Abstract: We report the fabrication of NbN superconducting single photon detectors (SSPDs) and the development of a receiver system operating in a standard transport helium dewar. The NbN- SSPDs consisted of a 3.5 to 6.7-nm-thick NbN ultrathin film meander line and a 150-nm-thick NbN thin-film coplanar waveguide line. The NbN ultrathin films were deposited on single-crystal MgO substrates by reactive dc-magnetron sputtering to grow the NbN ultrathin films epitaxially, and the films were formed so that they had 80 to 200-nm-wide meander lines, which covered a 20 times 20 or 50 times 50 mum2 area. All devices demonstrated good superconductivity, e.g., a device with a 3.9-nm-thick and 80-nm-wide line had a Tc of 10.8 K, an Ic of 19 muA, and a Jc of 5.9 times 106 A/cm2, which meant there was no serious deterioration in superconductivity compared with unprocessed films. We describe a reliable connection between an optical fiber and the detector, a setup which was used for first tests with pulsed light.

High Throughput Processing of Long-Length IBAD MgO and Epi-Buffer Templates at SuperPower

Abstract: SuperPower has successfully transferred from low throughput IBAD (Ion beam assisted deposition) YSZ technology to high throughput IBAD MgO technology. Pilot-scale IBAD system and pilot-scale epi-buffer deposition system, each with capabilities of producing single-piece lengths over 1000 m have been established. Helix tape handling approach instead of wide tape approach is chosen due to its immense advantages over wide tape approach. High-performance IBAD MgO buffer stack and their processes have been developed together with high throughput where each layer in the buffer stack is processed at 40 m/h tape speed or higher during our Phase I scale up. Both pilot systems are in routine production mode since March 2006. More than 14 km IBAD MgO buffer tapes of 12 mm width were produced for Albany cable project. 700-800 m IBAD MgO tape per run is routinely produced in pilot IBAD system with single piece tape length ~570 m. 400-550 m epi-buffer tape per run is routinely produced in pilot buffer system. High throughput IBAD MgO buffer gives better texture of YBa2Cu3O7 (YBCO) film compared with IBAD YSZ, and superior superconducting properties. A high critical current of 557 A/cm in short tapes and a world record critical current times length value of > 70, 000 A - m were obtained with the high throughput IBAD MgO buffers.

A Compact HTS 5 kWh/250 kW Flywheel Energy Storage System

Abstract: Flywheel energy storage systems (FESS) are expected to contribute to uninterruptible power supplies (UPS) and power quality tasks significantly. We present design and the component results of a compact 5 kWh/250 kW HTS flywheel whereby the rotor will be totally magnetically stabilized. The design is optimized for highly integrated functionality of rotor body, generator/motor and bearings. The heart of the FESS is an ironless high power PM generator/motor delivering 250 kW. An advanced flywheel body manufactured from graphite fiber is stabilized by two magnetic bearings. A prototype 200 mm HTS bearing has been constructed and tested up to a load of 1 ton axially and 0.47 ton radially capable to carry the rotor weight of about 0.5 t. With a coefficient of friction (COF) of about 10-6 the rotational drag was low. A passive PM bearing giving 80 N/mm radial stiffness/per pole was studied. HTS-PM damping response dependent on the temperature shows Lehr factors of 5-10% damping between 50 and 80 K. Eddy current dampers could support rotor dynamic stability.

Design of HTS Axial Flux Motor for Aircraft Propulsion

Abstract: Development of all-electric aircraft would enable more efficient, quieter and environmentally friendly vehicles and would contribute to the global reduction of greenhouse gas emissions. However, conventional electric motors do not achieve a power density high enough to be considered in airborne applications. Bulk high temperature superconducting (HTS) materials, such as YBCO pellets, have the capacity of trapping magnetic flux thus behaving as permanent magnets. Experimental data show that one single domain YBCO pellets could trap up to 17 T at 29 K, which enables the design of very high power density motors that could be used in aircraft propulsion. We designed a superconducting motor based on an axial flux configuration and composed of six YBCO plates magnetized by a superconducting coil wound on the outside of the motor. The six-pole homopolar machine uses a conventional air-gap resistive armature. Axial-flux configuration allows several rotors and stators to be stacked together and therefore enables the use of one or several conventional permanent magnet rotors to generate minimum safety torque in case of loss of superconductivity. All-electric aircraft are expected to be powered by fuel cells or turbo-generators fed with pure hydrogen cryogenically stored that would provide the motor with a convenient cooling system at 20 K. This paper presents the design and simulated performance of the motor for an application in aircraft propulsion.

The Development of Second Generation HTS Wire at American Superconductor

Abstract: Development of the second generation (2G) YBCO high temperature superconducting wire has progressed rapidly and its performance is approaching, and in some areas exceeding, that of first generation (1G) HTS wire. American Superconductor's approach to the low-cost manufacturing of 2G wire is based on a wide-strip (4 cm) process using a metal organic deposition (MOD) process for the YBCO layer and the RABiTS (rolling assisted biaxially textured substrate) process for the template. In addition, the wide-strip RABiTS/MOD-YBCO process provides the flexibility to engineer practical 2G HTS wires with architectures and properties tailored for specific applications and operating conditions through slitting to custom widths and laminating with custom metallic stabilizers. This paper will review the status of the 2G manufacturing scale up at AMSC and describe the properties and architecture of the 2G wire being developed and tested for various applications including in cables, coils and fault current limiters. Performance of 100 meter class, 4 mm wide wires at 77 K, self-field has reached 100 A (250 A/cm-width) with single-coat YBCO and 140 A (350 A/cm-width) with double-coat YBCO. A 5 cm inner diameter coil fabricated from the latter wire achieved 1.5 T at 64 K, confirming the capability of the wire for coil applications.

Design and Implementation of a Pipelined Bit-Serial SFQ Microprocessor, ${\rm CORE}1\beta$

Abstract: A pipelined 8-bit-serial single-flux-quantum (SFQ) microprocessor, called CORE1beta, was designed and tested. The CORE1beta has two cascaded arithmetic logic units (ALUs) based on forwarding architecture, which can perform two register operations from one instruction. Pipelining is also extensively adopted to enhance the performance. A new design method, known as one-hot encoding, has been introduced into the design of the control circuit. The 4-stage-pipelined SFQ microprocessors, CORE1beta8, have been implemented using the CONNECT cell library and the SRL 2.5 kA/cm2 Nb process. The frequency for the instruction fetch is 25 GHz, and 20 GHz for the bit-serial data operation. The peak performance and the power consumption of the CORE1beta8 are estimated to be 1400 MOPS (million instructions per second) and 3.4 mW, respectively. We have experimentally demonstrated 4-stage pipelining and all functionalities of the CORE1beta8 microprocessors by on-chip high-speed tests.

An Update High-Temperature Superconducting Maglev Measurement System

Abstract: An update high temperature superconducting Maglev measurement system (SCML-02) was successfully developed. The system includes liquid nitrogen vessel, cylinder permanent magnet (PM) or PM guideway (PMG), data collection and processing, mechanical drive and autocontrol. The liquid nitrogen vessel with high temperature superconductors (HTSCs) can be placed above or under the PM. This measurement system has other characteristics: high measurement precision, instant measurement at movement of the measured HTSCs specimen, synchronous measurement of levitation and guidance, measurement in three dimensions at one time, relaxation measurement of both levitation and guidance force, and so on. In order to calibrate measurement precision and to verify special function of SCML-02, two cylinder PMs are used, which have the same scale and technical specification. The Maglev properties of HTSCs YBCO bulk are measured on the basis of the calibration results of SCML-02. The levitation forces of single YBCO bulk and of seven YBCO bulks above PMG are measured by SCML-02. All functions are validated experimentally. The main specification of the measurement system is: position precision plusmn 0.05 mm; vertical force precision 2%c; horizontal force precision 1%c. The force measurement precision has achieved 0.02 N

The Project Overview of the HTS Magnet for Superconducting Maglev

Abstract: This paper describes the outline of a development project for the HTS magnet for the superconducting Maglev, which commenced in 1999. A very small current decay rate of 0.44%/day was achieved in 2003, using a prototype HTS coil, and a second HTS magnet, consisting of four persistent current HTS coils, was produced in 2005 for vehicle running tests. The second HTS magnet was operated in a persistent current mode at a rated magneto-motive force of 750 kA, and a top speed of 553 km/h was attained on the Yamanashi Maglev Test Line on December 2,2005.

Influence of Temperature and/or Field Dependences of the $E-J$ Power Law on Trapped Magnetic Field in Bulk YBaCuO

Abstract: Temperature, current density and magnetic field distributions in YBCO bulk superconductor during a pulsed-field magnetization (PFM) process are calculated using the finite difference method. Simulations are based on the heat diffusion equation with account of the heat produced by flux motion, and Maxwell's equations. A power law with temperature and magnetic field dependent parameters is used to characterize the electromagnetic behavior of the superconducting material. We analyze how the stored magnetic energy depends on the temperature and field dependences of the power law.

20 ${\hbox{kA/cm}}^{2}$ Process Development for Superconducting Integrated Circuits With 80 GHz Clock Frequency

Abstract: Results of the development of an advanced fabrication process for superconductor integrated circuits (ICs) with 20 kA/cm2 Nb/AlOx/Nb Josephson junctions is presented. The process has 4 niobium superconducting layers, one MoNx resistor layer with 4.0 Ohm per square sheet resistance for the junction shunting and circuit biasing, and employs circular Josephson junctions with the minimum diameter of 1 mum; total 11 photolithography levels. The goal of this process development is the demonstration of the feasibility of 80 GHz clock speeds in superconducting ICs for digital signal processing (DSP) and high performance computing. Basic components of rapid single flux quantum (RSFQ) logic such as DC/SFQ, SFQ/DC converters, Josephson transmission lines (JTLs), and simple digital circuits such as T-flip-flops and 4-bit digital counters have been fabricated and tested. The T-flip-flops were shown to operate up to 400 GHz with the widest margin of operation of plusmn13% at 325 GHz. Digital testing results on the 4-bit counters as well as the junctions, resistors, and other process parameters are also presented. Prospects for yet higher speeds and very large scale integration are discussed.

Modeling High-Temperature Superconducting Tapes by Means of Edge Finite Elements

Abstract: In this paper we present a new 2D numerical model for AC loss computation in high-temperature superconductors, based on the use of edge finite elements. These elements are curl conforming by construction and involve the continuity of the tangential component of the magnetic field between adjacent elements. In this way they do not require the imposition of the zero divergence of the magnetic field at each time step, which, with standard nodal elements, increases the null space of the matrix and the risks of divergence of the algorithm. The model, implemented in the software package COMSOL, uses the two magnetic field components as state variables. It has been tested for computing the losses of YBCO coated conductor tapes in two cases of practical interest, which cannot be investigated by analytical models.

Superconducting Magnetic Energy Storage (SMES) for Energy Cache Control in Modular Distributed Hydrogen-Electric Energy Systems

Abstract: The proposed energy cache control enables fast compensation of stochastic power fluctuations through the use of Superconducting Magnetic Energy Storage (SMES) connected to the DC bus of Distributed Generation (DG). The term energy cache control is chosen to reflect its analogy to the utilization of the data cache in computers. The SMES provides an energy cache for other types of storage with a higher capacity of energy but also longer response times. Together, the two types of storage provide high controllability over diverse time scales. The SMES so enables the creation of modular DG building blocks which can be readily connected to the network in a plug-and-supply mode. In the developed case study, the SMES is part of a DG unit of a model commercial facility that includes renewable-based electric power generation and a hydrogen-electric conversion and storage plant composed of an electrolyser, a fuel cell, and tanks. It is shown how the cache energy control deals with fast fluctuations of power generation and load that are caused by changing wind and sudden power demands of an elevator, respectively. Through its shock-absorbing role, the energy cache control of the SMES prevents the involved disturbances from propagating over the network interface and makes the DG unit behave as a good citizen in terms of network integration.

AC Loss Reduction of Superconducting Power Transmission Cables Composed of Coated Conductors

Abstract: AC losses in power transmission cables composed of coated conductors could be potentially small. A strategy to approach their potentially small AC loss was studied. AC losses in mono-layer conductors for cables were calculated numerically in order to show the principle for AC loss reduction: use of narrower coated conductors and/or decrease in space between conductors reduce the magnetic field component perpendicular to the wide face of coated conductors, and they are effective for AC loss reduction. This principle was confirmed experimentally by using short mono-layer conductors. Based on the principle, 1 kArms-class three-layer conductors were fabricated, and AC loss of 0.054 W/m at 1 kArms was achieved in a three-layer conductor whose outer diameter is 19.6 mm. The influence of a magnetic substrate on the AC losses in a single coated conductor and a mono-layer conductor for a cable was studied numerically.

Design, Construction and Tests of MgB2 Coils for the Development of a Cryogen Free Magnet

Abstract: Since the development of the first MgB2 tapes, ASG Superconductors started an R&D activity aimed to produce magnets using the MgB2 conductor. Reacted and wound magnesium diboride double pancake coils were designed, manufactured (using industrial equipment) and tested, in a cryogen free apparatus, by ASG Superconductors. Each double pancake coil was produced by winding a single length of MgB2, 1600 m long, multifilamentary stabilized tape. The successful achievement of the tests allowed us to produce and test the first coil made with 6 single double pancakes, then the second coil and finally the completion of a cryogen free magnet for MRI application. This paper reports the main features of the coil design and construction, the magnet and the test results.

Design and Analysis of a Prototype Linear Motor Driving System for HTS Maglev Transportation

Abstract: High temperature superconductors (HTSs) can produce a strong magnetic levitation force with self-stabilizing feature and hence have attracted much attention for applications in maglev transportation systems. For the linear motion of transportation, a linear drive is an obvious advantage. This paper presents the design and analysis of a permanent magnet (PM) linear synchronous motor for driving a small-scale prototype vehicle with PM-HTS bulks used for the levitation. The linear motor consists of PMs on the vehicle sides and stator windings in the slots of side tracks. The three phase windings are supplied by a 3-phase inverter, and are controlled with a brushless DC control scheme. Analysis and calculation details of magnetic fields and driving force about the motor prototype are presented.

Dark Counts in Nanostructured NbN Superconducting Single-Photon Detectors and Bridges

Abstract: We present our studies on dark counts, observed as transient voltage pulses, in current-biased NbN superconducting single-photon detectors (SSPDs), as well as in ultrathin (~4 nm), submicrometer-width (100 to 500 nm) NbN nanobridges. The duration of these spontaneous voltage pulses varied from 250 ps to 5 ns, depending on the device geometry, with the longest pulses observed in the large kinetic-inductance SSPD structures. Dark counts were measured while the devices were completely isolated (shielded by a metallic enclosure) from the outside world, in a temperature range between 1.5 and 6 K. Evidence shows that in our two-dimensional structures the dark counts are due to the depairing of vortex-antivortex pairs caused by the applied bias current. Our results shed some light on the vortex dynamics in 2D superconductors and, from the applied point of view, on intrinsic performance of nanostructured SSPDs.

Second Generation High-Temperature Superconducting Wires for Fault Current Limiter Applications

Abstract: In this paper, we report the results from evaluation on the use of 2nd generation high-temperature superconducting wires, or 2G HTS conductors, as elements for superconducting fault current limiter (SFCL). The unique features of 2G HTS conductors such as high N-values, superior electromechanical performance, large surface area available for cooling, and their availability in long-lengths that could be produced by high-throughput and low-cost manufacturing, may provide advantages for SFCL applications. We tested SuperPower's standard ion-beam-assisted-deposition based 2G HTS conductors under various conditions. First, individual conductors 10-20 cm long with dc critical current (Ic) ranging from 180 amps to 277 amps were tested at prospective fault current up to 3 kA (peak). 2G HTS conductors demonstrated good fault current limiting performance, including first peak limitation. Quench current was in the range of 1.8 to 3 times Ic, and the response time was within 1 ms. Secondly, 3-5 conductors in parallel connections demonstrated uniform current sharing and fast recovery under no-load condition. Finally, an assembly consisted of 12 elements in series connection was tested at high-power condition at KEMA PowerTest. Each element had four 40 cm long conductors with Ic 120 A in parallel connection. With 1080 V supply voltage and 90 kA prospective fault peak current, the fault current was limited to 32 kA at the 1st peak with 3.2 kA in the HTS elements. The response time was less than 1 ms. All these testing results indicate that our 2G HTS conductors are promising for practical SFCL applications.

Processing of Long-Length YBCO Coated Conductors Based on Stainless Steel Tapes

Abstract: Recent progress is reported in the development of long lengths (up to 100 m) of YBCO coated conductors with good mechanical stability and improved critical current homogeneity. A non-magnetic CrNi stainless steel tape, 0.1 mm thick, is employed as a substrate tape. Prior to deposition, the tape is coated with yttria stabilized zirconia in the form of a bi-axially textured buffer layer, using a novel "alternating beam assisted deposition" technique. After deposition of a ceria cap layer onto the buffer, YBCO films are deposited employing a high-rate pulsed laser deposition method that allows higher critical current densities and incurs a lower YBCO loss (~10%) than other deposition techniques. The last processing step consists of coating the with a thin layer of silver or gold and a 2-40 mum thick copper shunt layer. In 40 m long tapes with a 1 mum thick YBCO film (at 77 K and zero external field), critical current, Jc, up to 235 A per cm-width were obtained. Higher critical currents, up to 574 A per cm, were achieved in short 0.2-7 m sections corresponding to current densities, Jc, of up to 4.4 MA/cm2 . The critical current homogeneity and performance of our improved coated conductors under applied temperature cycling, multiple quench-recovery cycles, and repeated bending are reported.