Showing papers in "IEEE Transactions on Applied Superconductivity in 1997"

Prospects for quantum coherent computation using superconducting electronics

Abstract: We discuss the prospects and challenges for implementing a quantum computer using superconducting electronics. It appears that Josephson junction devices operating at milli-Kelvin temperatures can achieve a quantum dephasing time of milliseconds, allowing quantum coherent computations of 10/sup 10/ or more steps. This figure of merit is comparable to that of atomic systems currently being studied for quantum computation.

Analysis of critical-state problems in type-II superconductivity

Abstract: An efficient numerical scheme is proposed for modeling the hysteretic magnetization of type-II superconductors. Numerical examples are presented for the Bean and Kim critical state models. It is shown that Bean's model is a Hele-Shaw type problem.

SQUID based remanence measurements for immunoassays

Abstract: The use of fine magnetic particles as labels for antibodies and the measurement of their remanent magnetization for the preparation of immunoassays is presented. Antibodies were coupled with magnetic nanoparticles and samples were prepared by reaction of the magnetically labeled antibodies with their solid phase adsorbed antigen. After exposing the samples to a field of some mT a dc-SQUID system measures the remanent sample magnetization in the absence of an external field. The combination of high moment labels and SQUIDs yields ultrasensitive immunoassays with a wide range of detectable analyte concentrations. In contrast to most standard techniques in our method the detected magnetic signal is specific only for bound reaction partners, thus eliminating the need for separation of unbound components.

Physics and technology of high temperature superconducting Josephson junctions

Abstract: The controllable fabrication of reliable HTS Josephson junctions with sufficiently small spread of their characteristic parameters has not yet been achieved and prevents the successful use of HTS Josephson junctions in complex integrated circuits. The problems in HTS junction fabrication certainly are related to the specific properties of the cuprate superconductors, which make the fabrication of high quality interfaces in HTS junctions employing artificial barrier layers extremely difficult. Therefore, several types of HTS Josephson junctions make use of so-called intrinsic interfaces originating from grain boundaries or the intrinsic layer structure of the cuprates. Beyond the fabrication technology, the physics of HTS Josephson junctions is not well understood. In particular, the detailed mechanisms of charge transport in the various junctions types and the impact of an unconventional symmetry of the superconducting order parameter are unsettled issues. We summarize the key issues regarding the physics and technology of HTS Josephson junctions and discuss possible routes to a useful HTS junction technology.

Resistive current limiters with YBCO films

Abstract: The aim of this work is to develop a function model of a resistive HTSC fault current limiter. The switching elements of the fault current limiter are bath-cooled in a liquid nitrogen cryostat. The elements consist of YBaCuO films which are deposited on different substrates with critical current densities of up to 3/spl middot/10/sup 6/ A/cm/sup 2/. For the most effective limiting samples so far the peak let-through current was twice tile nominal current and the steady short-circuit current below nominal current. The concept of a 100 kVA function model is introduced.

Performance of a high-Tc superconducting fault current limiter-design of a 6.6 kV magnetic shielding type superconducting fault current limiter

Abstract: Superconducting fault current limiters for electric power systems have been researched. A magnetic shielding type superconducting fault current limiter is developed in the authors' research on superconducting fault current limiters. This limiter consists of a copper primary winding, a superconducting cylinder, an iron core and a control coil. The superconducting cylinder has a Bi2212 thick film on a MgO substrate. The control coil consists of some metallic rings, and the fault level can be adjusted by changing the number of the rings. To design a prototype limiter, the AC magnetic shielding and loss characteristics of small models were measured. The prototype limiter is 6600 V in rated voltage and 400 A in rated current. The superconducting cylinder is 0.45 m in diameter and about 1 m in height. Only the superconducting cylinder was designed to be cooled by liquid nitrogen. The experimentally manufactured limiter is about 1.3 m in width, about 0.6 m in depth and about 2 m in height.

Deposition of YBCO and NBCO films on areas of 9 inches in diameter

Abstract: High quality YBa/sub 2/Cu/sub 3/O/sub 7/ thin films can be grown by reactive thermal co-evaporation. Combined with a rotating disk heater this method allows to fabricate even double sided homogeneous films on substrate areas up to 9" diameter. A scanning inductive j/sub c/ probe is used to monitor the homogeneity of critical current densities of typically >2 MA/cm/sup 2/. Surface resistance values are found to be below 500 /spl mu//spl Omega/ at 10 GHz. On biaxially aligned buffer layers on polycrystalline substrates j/sub c/ values of 1.3 MA/cm/sup 2/ are achieved as well. The properties of Nd/sub 1-x/Ba/sub 2+x/Cu/sub 3/O/sub 7/ films grown with the same system are presented and evaluated.

Data-driven self-timed RSFQ digital integrated circuit and system

Abstract: A novel asynchronous timing scheme, data-driven self-timing (DDST) is proposed and implemented in Rapid Single-Flux-Quantum (RSFQ) superconductive integrated circuits In this asynchronous approach, the timing signals are generated from the data and no global clock is needed to drive the RSFQ circuit and system The essence of the self-timing scheme is to localize the system timing in order to avoid the overhead of global clock distribution, and to minimize the timing uncertainty The DDST scheme has been applied to the design of a shift register, a demultiplexor, and a self-timed high speed digital test system In this paper, test results of a 4-bit DDST shift register and a high speed on-chip clock generator will be presented to demonstrate the successful DDST operation of RSFQ integrated circuits at a rate of 20 Gb/s

Octopux: an advanced automated setup for testing superconductor circuits

Abstract: An integrated multipurpose setup for the automated testing of superconductor devices and circuits has been designed, implemented, and installed in the RSFQ Laboratory of the State University of New York at Stony Brook. The extendable and modular design of the setup allows a wide variety of low-frequency superconductor experiments to be carried out including those that require immediate interaction between the setup and the researcher.

PSCAN'96: new software for simulation and optimization of complex RSFQ circuits

Abstract: The first version of PSCAN program (Personal Superconductor Circuit ANalyzer) was introduced in 1991. The program is a general purpose superconductor circuit simulator with an emphasis on the design of Rapid Single-Flux-Quantum (RSFQ) circuits. In the intervening years a number of new features were gradually added to the program. In particular, verification of the correct circuit behavior was enhanced using a special hierarchical Single-Flux-Quantum Hardware Description Language (SFQHDL). Next, a fast heuristic algorithm for margin optimization was introduced, which increased the number of parameters that can be simultaneously optimized in reasonable CPU times. Finally, recently we improved the numerical algorithm used for the simulation by using sparse symmetric positive definite matrices (instead of general structure band matrices as before). As a result, simulation speed has increased almost tenfold. Now it takes about 30 seconds of a CPU time on HP716/100 workstation to run a 2000 ps simulation of a 120-Josephson-junction circuit, and about a week to optimize all parameters of a two hundred Josephson junction circuit. We have merged all these improvements in a new version of our simulator, PSCAN'96.

Josephson output interfaces for RSFQ circuits

Abstract: We have developed and demonstrated high bandwidth Josephson circuits to interface the output of RSFQ circuits to room temperature electronics. Asynchronous dc powered voltage driver circuits have been designed to amplify RSFQ signal levels to voltage outputs in the 2-4 mV range, in a wide bandwidth. These driver circuits have been characterized and tested for data rates up to 8 Gb/s. The bit error rate for one of these drivers has been measured up to 7 Gb/s for a (2/sup 31/-1) bit long pseudo-random bit sequence (PRBS). In order to match the data rate of Josephson circuits to slower room temperature electronics, we have developed clock-controlled shift registers which allow shift-in and shift-out of data at different frequencies. Complete functionality of these circuits integrated with the drivers has been demonstrated at low speed. Shift registers integrated with the voltage driver circuits have been tested at high-speed for data rates up to 6 Gb/s.

Electrical application of high T/sub c/ superconducting saturable magnetic core fault current limiter

Abstract: A saturable magnetic core electrical fault current limiter (FCT) designed with a high T/sub c/ superconducting (HTS) DC bias winding is described. The HTS winding is prepared by using a Ag-clad (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub O10+x/ HTS wire. The limiting behaviour of this FCL is investigated, and its electrical application is considered in an electrical power transmission system. The results show that this FCL limits fault currents effectively and is a possible solution for reducing power system fault currents.

Small and repetitive axial strain reducing the critical current in BSCCO/Ag superconductors

Abstract: The critical current in two types of axially deformed BSCCO/Ag tape conductors is investigated. An I/sub c/ reduction is observed for small axial strains (ranging from 0 to 0.3%) with a characteristic slope di/sub c//d/spl epsiv/=-5/spl plusmn/1 (relative I/sub c/, change per relative change in length). In the case of an axial compression there is a more pronounced I/sub c/ reduction. For small axial strains ( 10000) and has a similar negative slope for both compressive and tensile strains. It is proposed that the reversible I/sub c/ change is correlated to a non-hydrostatic lattice deformation. The I/sub c/ versus strain behaviour is in good agreement with an earlier proposed model.

Numerical analysis of AC losses in high T/sub c/ superconductors based on E-j characteristics represented with n-value

Abstract: A numerical code for the electromagnetic analysis of high T/sub c/ superconductors by finite element method has been developed. The E-j characteristics of superconductor are represented with the n-value. The equivalent conductivity of superconductor is determined as a function of electric field, and Ohm's law is used as the constitutive equation. First, the current and magnetic flux distributions in the infinite slabs of superconductor exposed to a parallel external magnetic field are analyzed. The influence of n-value on AC loss and the frequency dependence of AC loss are studied. The AC loss in the infinitely-long superconductor tapes exposed to the external magnetic field is calculated numerically to study the influence of n-value.

Improved sensitivity of NMR spectroscopy probes by use of high-temperature superconductive detection coils

Abstract: High resolution nuclear magnetic resonance (NMR) spectroscopy is an important technique for the determination of molecular structures and physical properties of liquid samples. A major drawback is its low signal-to-noise ratio compared with other spectroscopic methods. The dominant source of noise in a spectrometer is often Johnson noise in the detection coils. Conductus, a leading manufacturer of superconductive electronics, and Varian, a leading manufacturer of NMR spectrometers, have successfully developed NMR sample probes using detection coils fabricated from thin films of the high temperature superconductor, YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) which reduce that noise and provide a sensitivity enhancement of a factor of four compared with conventional coils. The detection coils are maintained at about 25 K while the sample, contained in a 5 mm diameter tube, is at room temperature. The coils must carry high radio frequency currents during the excitation phase of a pulsed NMR experiment and, in order to maintain spectral quality, should cause little perturbation of the polarizing magnetic field. Characteristics of these probes and examples of spectra obtained are shown.

Application of Nb/sub 3/Sn superconductors in high-field accelerator magnets

Abstract: Last year a record central field of 11 T at first excitation at 4.4 K has been achieved with the experimental LHC model dipole magnet MSUT by utilising a high J/sub c/ powder-in-tube Nb/sub 3/Sn conductor. This is the first real breakthrough towards fields well above 10 T at 4 K. The clear influence of magnetisation and coupling currents on the field quality, the quench behaviour and the temperature development in the coils has been measured and is discussed. For application in high-field accelerator magnets (10-15 T dipoles, 300-400 T/m quadrupoles) these experimental results clearly reveal the potential, the present limitations and the necessary improvements of Nb/sub 3/Sn technology with respect to strand, cable and coil design and manufacturing. A brief review of developments in this field is presented. The focus is on accelerator dipole magnets but the key issues for quadrupole magnets are quite similar.

Power handling capability improvement of high-temperature superconducting microwave circuits

Abstract: The focus of High Temperature Superconducting (HTS) microwave circuit development has now shifted to improving the high power handling capability. This paper reviews our latest results in this area including: HTS material improvement; new concepts and rules for HTS high power circuit design; and the latest high power filter test results. For HTS materials, the surface resistance versus rf magnetic field test results and the third harmonic test results for both YBa/sub 2/Cu/sub 3/O/sub 7/ and Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ thin films are presented. For HTS microwave circuits, novel design concepts and rules for high power HTS filters are discussed. Several compact planar HTS filters were designed, fabricated, packaged and tested. The performance of a 3-pole, 1.3% equal-ripple bandwidth planar HTS filter showed no measurable degradation for transmitted power levels up to 74 watts at 77 K. The performance of a 2-pole, 1% equal-ripple bandwidth planar HTS filter showed no measurable degradation for transmitted power levels up to 115 watts at 77 K. To our knowledge, these test results represent the highest power handling of compact planar HTS filters operating at 77 K, sufficient for use in telecommunication transmitters.

HTS/ferroelectric devices for microwave applications

Abstract: High Temperature Superconducting (HTS, e.g. YBCO) microwave devices based on bulk or thin film ferroelectrics (e.g. Strontium Titanate-STO) are studied theoretically and experimentally. YBCO/STO/YBCO parallel-plate resonators and based on bulk single crystal STO are for electrically tunable high power filters, phase shifters and other devices in the frequency band 0.5-2.0 GHz of advanced microwave communication systems. Thin film YBCO/STO Inter-digital Capacitors (IDC), Coplanar Waveguides (CPW) phase shifters/delay lines are also designed and studied experimentally for low microwave power applications. Modelling problems of these devices and microwave losses in ferroelectrics are also discussed.

Stochastic simulation of SFQ logic

Abstract: The high speed and low power of Single Flux Quantum logic (SFQ) are extremely attractive, and significant capabilities have been demonstrated in Nb technology. However the burden of cooling to 4.2K has been a barrier to its widespread implementation. The advent of High Temperature Superconductors (HTS), raises the prospect of more accessible temperatures. This paper examines some theoretical constraints on the implementation of SFQ in HTS, and derives some ideas about the parameters required of any HTS SFQ technology.

Development of flux-lock-type fault current limiter with high-T/sub c/ superconducting element

Abstract: The present paper proposes a new the fault current limiter (FCL), which consists of a high T/sub c/ superconducting (HTS) element, three coils wound on the same core, and a magnetic field coil covering the HTS element. In this FCL, the initial limiting current level can be controlled by adjusting the inductances of the coils. Furthermore, an external AC magnetic field is applied to the HTS element to get higher resistance in the current-limiting phase. A current-limiting experiment by a model FCL was carried out, and the limiting performance of the FCL was observed. Both the initial limiting current level and the limiting impedance could be increased in this FCL compared with the corresponding values of the HTS element itself.

Conceptual system design of a 5 MWh/100 MW superconducting flywheel energy storage plant for power utility applications

Abstract: The authors have designed a 5 MWh/100 MW superconducting flywheel energy storage plant. The plant consists of 10 flywheel modules rated at 0.5 MWh/10 MW each. Module weight is 30 t, size is /spl phi/ 3.5 m/spl times/6.5 m high. A synchronous type motor-generator is used for power input/output. Each flywheel system consists of four disk modules made from a carbon fibre composite material, rated at 125 kWh and weighing 3 t. Passive superconducting bearings, integrated into the disk modules, are used for frictionless levitation. The operating speed will be in the 2250...4500 RPM range. Maximum rim velocity is 600 m/s with a maximum tensile stress of 810 MPa. Details of the system layout are discussed, addressing thermal design and subsystems such as power conditioning system and cooling facilities. A preliminary cost analysis has been worked out. In the Western European UCPTE (Union for the Coordination of Production and Transport of Electricity) high power system, the plant could be used to provide spinning reserve for about 30 s while conventional thermal generating units ramp up to meet the increased load during peak power periods. Other applications in the electric utility field include power conditioning and stabilization in portions of a high-voltage power system.

HTS SMES magnet design and test results

Abstract: This paper describes design, construction, and testing of a 5 kJ superconducting magnetic energy storage (SMES) magnet. This magnet was built by American Superconductor Corporation (ASC) for Gesellschaft fur Innovative Energieumwandlung und Speicherung (EUS) of Germany. The magnet consists of a solenoidal coil constructed from a silver-sheathed BiPb/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 2/O (Bi-2223) conductor which was reacted before winding. The coil is epoxy impregnated and cooled with single stage Gifford McMahon (G-M) cryocoolers for operation at 100 A (DC) with substantial AC components due to the frequent variation of current (ramp-up and ramp-down.) The magnet was tested in early spring of 1996 and was shipped to EUS in mid June. The successful operation of this magnet illustrates that the technology of cooling HTS magnets with G-M type cryocooler is now fully established. The long-term operation of this magnet at EUS will verify the reliability of HTS magnet system in critical applications and will open future applications for HTS in the area of SMES and other magnets.

Optimization of SrTiO/sub 3/ for applications in tunable resonant circuits

Abstract: A series of SrTiO/sub 3/ (STO) films have been grown at a wide range of deposition conditions in order to determine the optimal growth parameters to maximize ferroelectric tuning while maintaining the lowest dielectric losses. The deposition pressure of the ambient O/sub 2/ (40 mTorr a(bulk) at higher pressures (P(dep)>65 mTorr). The dielectric constant (/spl epsiv//sub r/) and loss tangent (tan/spl delta/) were determined as a function of applied field at room temperature (300 K) and at liquid nitrogen temperature (77 K). The low frequency dielectric properties of the STO films were found to be a weak function of the strain of the ferroelectric film.

Josephson voltage standard-a review

Abstract: The unique ability of a Josephson junction to convert a microwave frequency f into a voltage Nhf/2e with high accuracy and the adoption of this phenomenon as the basis for the SI Volt Realization have created a market for Josephson voltage standards that is unassailable from any other technology. This paper reviews the development of Josephson voltage standards including the junction and array design, the microwave circuit, and the system integration. With the dc Josephson standard largely transferred to the commercial sector, NIST is developing a new class of devices in which the output voltage can be rapidly programmed either by digitally selecting the quantum number N or by driving the Josephson array with a variable frequency pulse train. These new devices will make possible fast, high-accuracy characterizations of A/D and D/A converters and the synthesis of ac waveforms.

Superconductivity and electric power: promises, promises ... past, present and future

Abstract: The long-awaited marriage of superconductivity with electric power has undergone a lengthy engagement to say the least. Whether those nuptials will indeed ever take plate is a question we here dare answer, recognizing full well the pitfalls entailed, Almost immediately after its 1911 discovery, superconductivity was popularly touted as the key to the lossless delivery of electricity ... at least until the type I nature of these early materials was appreciated ... a cycle of excitement and disillusionment that unfortunately has typified the field throughout its history. With the emergence and exploitation of Type II superconductors in the middle decades of the century, tremendous technical progress was made toward power application embodiments, resulting in operating prototypes of transmission cables and rotating machinery by the early 1980s, nonetheless, these achievements did not mature into commercial power products, primarily because of economic and social factors that had evolved by that time ... successful conservation efforts had lowered expected electricity load growth such that, ironically, the incremental efficiencies offered by superconductivity were no longer required at the cost involved ... an important lesson in that the successful deployment of a technology often rests on factors unforeseen and outside its internal development. The years from 1986 to the present have witnessed the discovery of the copper oxide perovskite high temperature superconductors and their coming-of-age in practical wire form. These events, plus a renewed and growing world-wide demand for electric energy, give hope that the final vows will actually take place during the first quarter of the coming century.

Advanced on-chip test technology for RSFQ circuits

Abstract: We have developed an advanced version of on-chip test system with new high-speed clock generation and control. For high-speed clock generation, a novel wide-bandwidth ring generator is designed using circular Josephson transmission lines with an inductively coupled trigger. The generator is capable of producing SFQ clock pulses in the range of from 15 to 55 GHz using a 1 kA/cm/sup 2/ Nb fabrication process. For clock control, we have designed two different types of clock-controller circuits based on programmable shift-register and counter. Using the on-chip test system, we have successfully tested a parallel multiplier module up to 15 GHz with 16% dc bias margins.

Formation of edge dislocations in thin epitaxial YBCO films

Abstract: Transmission electron microscopy (TEM) of thin YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) films on different single-crystalline substrates revealed (by Moire patterns) edge dislocations having non-superconducting cores normal to the substrate. The dislocations are in small-angle boundaries with the average density as high as 10/sup 11/ cm/sup -2/. An extremely high density of dislocations is thought to be the cause of the high critical current density in YBCO epitaxial films. The mechanism for dislocation formation is considered in the framework of a computer model. Computer modeling provided the details of the dislocation arrangement either in domain boundaries or in twist boundaries, depending on the angle of the in-plane misorientation between film and substrate lattices. The model is found to be in goad agreement with experimental data on dislocations in YBCO superconducting films.

YBaCuO thick films on planar and curved technical substrates

Abstract: For high-current applications, homogeneous well-textured high-temperature-superconducting Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBaCuO) films on technical ceramic or metallic substrates are required. The ion-beam-assisted deposition (IBAD) of yttria-stabilized zirconia (YSZ) buffer layers, which serve as templates for the YBaCuO, was extended to large planar areas (20 cm/spl times/20 cm) as well as curved or cylindrical (diameter /spl les/15 mm) substrates. For both types of substrates a pronounced in-plane alignment is observed. A deposition equipment is developed to grow high-quality YBaCuO films also on tubes by pulsed laser deposition (PLD), which is characterized by a high long-term stability both of the deposition rate and of the surface temperature of the growing film. Current densities up to 0.9/spl times/10/sup 6/ A cm/sup -2/ at 77 K in self fields are observed in films on planar Ni foils with biaxially aligned YSZ buffers (bi-YSZ). For the coating of long lengths of tubes and tapes a translation and a rotation of the samples are incorporated both in the buffer as well as in the YBaCuO-deposition process. Furthermore, the dependence of the critical transport current densities, J/sub c/, of the HTS layer on magnetic fields and on mechanical stresses are investigated.

Preliminary tests of a 500 kVA-class oxide superconducting transformer cooled by subcooled nitrogen

Abstract: The authors have designed and fabricated a 500 kVA-class oxide superconducting power transformer operated in sub-cooled nitrogen. The primary and secondary windings are three- and six-strand parallel conductors of Bi-2223 Ag-sheathed multifilamentary tapes, respectively. In the parallel conductors, the strands are transposed several times for uniform current distribution among them. A transformer, cooled by liquid nitrogen of 77 K, was steadily operated with a 500 kVA secondary inductive load. The efficiency in full operation at 77 K was 99.1 %, even with the refrigeration penalty of liquid nitrogen, 20, for the thermal load to the coolant. They installed the transformer in a continuous flow system of sub-cooled nitrogen as a fundamental step for compact superconducting transformers operating in sub-cooled nitrogen with a single-stage refrigerator. Short-circuit tests of the transformer were also performed in a region of temperature below 70 K. The transformer was operated with no quenching up to a level of critical current at 66 K, that is equivalent to 800 kVA. The efficiency estimated was improved to 99.3 % in the sub-cooled nitrogen.

Design centering methods for yield optimization of cryoelectronic circuits

Abstract: We present the results of comparison of different design centering methods, e.g. simplicial approximation method and centers-of-gravity method. The effectiveness of the proposed yield optimization strategies is demonstrated by application to various RSFQ circuits and analytical test functions. A SPICE-type program which includes the possibilities of analog behavior modeling and transient noise simulation was used for circuit simulation. Based on these methods, an interactive yield optimization framework for cryo-electronic circuits was developed and tested.