Showing papers in "IEEE Transactions on Applied Superconductivity in 1993"

New RSFQ circuits (Josephson junction digital devices)

Abstract: Several novel circuits of the rapid single-flux-quantum (RSFQ) family of Josephson-junction digital devices have been designed, fabricated using a 2.5- mu m 1000-A/cm/sup 2/ Nb trilayer technology, and tested at low frequencies. Numerical simulation and measurements have shown that these circuits have considerably wider parameter margins, due to application of several novel design methods. The authors have also carried out an experiment to measure the rate of errors in a simple RSFQ circuit including an inverter, confluence buffer, and Josephson transmission line. Near the middle of the parameter window at 4.2 K, the error probability was definitely lower than 3*10/sup -15/ per logic operation, despite experimentation with rudimentary shielding and filtering. >

Two-stage integrated SQUID amplifier with series array output

Abstract: A two-stage integrated DC superconducting quantum interference device (SQUID) amplifier has been fabricated which uses a compact series array of 100 DC SQUIDs as the readout device for a low-noise single SQUID. The output noise is dominated by the amplified noise of the input SQUID and substantially exceeds the input noise of a good room-temperature preamp. The input stage is a low-inductance double-loop SQUID with energy sensitivity of approximately 30-h (equivalent flux noise phi /sub n/ approximately=0.3 mu phi /sub 0// square root Hz), with an input transformer having input inductance L/sub in/ approximately=0.25 mu H and net coupling to the SQUID of k/sup 2/ approximately=0.1. The bandwidth extends from DC to about 390 kHz. The series array has an output voltage swing of 3-4 mV, providing a dynamic range of over 50 dB at full bandwidth. The results suggest the general utility of series SQUID arrays as readout devices for SQUIDs. >

Surface resistance measurements of HTS films by means of sapphire dielectric resonators

Abstract: A sapphire dielectric resonator with a copper cylindrical shield and two endplates replaced by high-temperature superconducting (HTS) layers was used for very accurate surface resistance measurements on laser-ablated YBCO films. A system using the TE/sub 011/ mode has a resonant frequency of about 18.1 GHz and parasitic-loss Q factor of about 120000. It allows 10 mm*10 mm samples to be measured with sensitivity of +or-30 mu Omega . Individual samples can be measured with somewhat lower accuracy. Using larger HTS samples, one can reduce parasitic losses of the system to an unsignificant level. The exact formulas presented for the resonant system allow for avoiding calibration procedures during the evaluation of the surface resistance. >

Transmission of single-flux-quantum pulses along superconducting microstrip lines

Abstract: The authors analyzed, designed, and tested drivers and receivers which make it possible to connect distant circuits of the rapid single-flux-quantum (RSFQ) logic/memory family by passive superconducting microstrip lines Using these circuits implemented with a niobium trilayer fabrication technology, reliable transmission and reception of the SFQ pulses over distances up to 1 cm, with margins of bias currents as wide as +or-30%, have been demonstrated The pulses can be passed along wide lines (20 mu m for 35 mu Nb-Al/sub 2/O/sub 3/-Nb Josephson junction technology) over such distances and can be picked up by the RSFQ receiver >

Development of technical high-T/sub c/ superconductor wires and tapes

Abstract: High-temperature superconductor (HTSC) BiSrCaCuO-based wires and tapes have been produced using oxide dispersion hardened Ag alloys as sheath material. The use of such sheath material results in superior mechanical properties. The 0.1% yield strength R/sub p0.1/ of wires with 50% HTSC volume fraction exceeds 100 MPa and may reach values up to 150 MPa depending on the respective alloy. The thermal shock resistance also is improved very much. This progress in mechanical properties removes a major obstacle towards application of HTSC in magnet technology. Work concentrating on upscaling of wire production and especially on improving homogeneity is presented. It is shown that filamentization of Bi-2212 wires may lead to improved critical current densities compared to monofilamentary wires. >

Rapid single flux quantum (RSFQ) shift register family

Abstract: The author presents the design and test results for a novel, buffered RSFQ shift register. The register design makes it possible to build a circular 64-b shift register which is insensitive to the clock pulse direction within the experimentally measured DC bias margin of +or-15%. The implementation of a large variety of unidirectional shift registers using either a buffered or a two-Josephson-junction cell design confirms an expected wide DC bias margin of +or-30% (for a 32-b) and high speed, up to 60 GHz (for a 4-b register). Among these circuits is a 256-b shift register. To the author's knowledge, this is the largest RSFQ circuit (533 junctions) reported to date. This shift register was tested to have a DC bias margin of +or-6% and proper high-speed operation up to 12 GHz. >

A sapphire resonator for microwave characterization of superconducting thin films

Abstract: A simple technique has been developed for the high-frequency characterization of superconducting thin films. A microwave resonator is formed by sandwiching a high-purity c-axis-oriented sapphire rod between a pair of superconducting thin films. For the TE/sub 011/ resonant mode, the theoretical treatment allows for the surface resistance and the RF current density to be calculated from the unloaded Q-value and the dissipated power. This technique is especially useful since it has: no sample preparation; no calibration; great sensitivity; great accuracy; great repeatability; great dynamic range; high internal power levels with only moderate input power levels; and broad temperature coverage (4.2 to over 120 K). A round robin experiment using HTS thin films was performed to deconvolute the films' surface resistance and also to provide a statistical analysis of the method's reproducibility. The standard error for a single measurement is better than 2%. A Nb thin film was measured to verify the technique. >

Thin-film HTS probe coils for magnetic-resonance imaging

Abstract: Several planar coil designs, with integral resonating capacitors, are reported using large-area sputtered YBCO films. One design utilizes a distributed interdigital capacitor, while a second exploits capacitive coupling of two films on opposite sides of a dielectric. Devices of the first design made on LaAlO/sub 3/ have achieved Q>10/sup 4/ at 18.5 MHz, possibly limited by the loss tangent of this substrate. Identical coils made with silver films have Q=10. With the second design, Q>10/sup 4/ was achieved at the even lower frequency of 5.7 MHz. >

Sensitive RF-SQUIDs and magnetometers operating at 77 K

Abstract: Large 6-mm*6-mm and 8-mm*8-mm flux-focusing washer structures with inductances L/sub S/ between 25 pH and 500 pH were fabricated from epitaxial, c-axis YBa/sub 2/Cu/sub 3/O/sub 7/ films. Double step-edge junctions having a low 1/f noise were incorporated in these washers to form RF-SQUIDs (superconducting quantum interference devices). A high tank circuit frequency near 150 MHz was chosen to reduce noise and improve the sensitivity of the SQUIDs. At 77 K, a transfer function exceeding 100 mu V/ Phi /sub 0/ and a white flux noise of 3*10/sup -5/ Phi /sub 0// square root Hz down to below 0.3 Hz were attained with L/sub S/=25 pH. A magnetometer with L/sub S/=190 pH was demonstrated. The best magnetic field and energy resolutions were 170 fT/ square root Hz and 5*10/sup -29/ J/Hz respectively, over a signal frequency range to below 1 Hz. The magnetometer was used to record human magnetocardiograms and auditory evoked human brain responses. >

Multifilamentary Bi-2223 composite tapes made by a metallic precursor route

Abstract: A process based on metallic precursors has been developed for manufacturing high filament count oxide superconductor-silver composite tapes with critical current densities of up to 7.5 kA/cm/sup 2/ at 77 K in zero field. A 30-cm prototype multistrand conductor made of these tapes has a critical current of 240 A at 77 K over a 9 cm gauge length, with an average critical current density of 6 kA/cm/sup 2/. The mechanical properties of tapes made from metallic precursors containing up to 10000 Bi-2223 superconducting oxide filaments were investigated. Critical tensile strains average 0.6%, and bend tests show negligible dropoff in current density up to a 0.70% surface strain. The critical current decrease beyond the 0.70% surface bend strain follows a simple model based on extensive filament damage beyond the critical tensile strain. Increased flow stresses of the composite tapes, compared to similarly processed silver, indicate considerable strengthening of the composite by the oxide filaments. >

High-temperature superconducting current leads

Abstract: The author reviews major high-temperature superconductor (NTS) current-lead concepts and presents several new considerations that should affect design of HTS leads. Considerable advantage is achieved by operating these leads with an intermediate-temperature heat sink. The HTS part of the lead can be made from pressed and sintered powder. Powder-in-tube fabrication is also possible, but the normal metal part of the lead acts as a thermal short and cannot provide much stabilization without increasing the refrigeration requirement. For lead stability, designs with low current density are favored. Such leads can be manufactured with today's technology, and refrigeration requirements are lower for the same allowable burnout time. Higher current densities result in lower helium boil-off for the same lead length, but burnout times can be very short. In comparing experiment to theory in open systems, the density of helium vapor must be accounted for in calculating the expected boil-off. For very-low-loss leads, two-dimensional heat transfer and the state of the dewar near the leads may play dominant roles in lead performance. >

RSFQ 1024-bit shift register for acquisition memory

Abstract: Rapid single flux quantum (RSFQ) 512-bit and 1024-bit shift registers have been demonstrated. These are the longest superconducting shift registers reported to date, employing 1045 and 2069 Josephson junctions, respectively. The circuit functionality has been confirmed with dc bias margins of +or-23% and +or-14% for the 512-bit and the 1024-bit shift registers, respectively. The 512-bit shift register has been tested to 20 GHz and 1024-bit register to 19 GHz using an external clock trigger with relative delay measurements at single and double SFQ clock frequencies. The shift registers with the same design have been used for successful implementation of the acquisition shift register (ASR) memory for the projected transient digitizer. These shift registers have the ability to acquire data at high speeds (gigahertz range), statically hold the acquired data, and then read-out the data into conventional room-temperature electronics at low speeds (megahertz range). A 32-bit ASR has been tested up to 18 GHz (the limit of our test setup), and a 1024-bit ASR-up to 16 GHz of acquisition rates, both at 33 MHz read-out frequency. Total power dissipation is about 1 mW for the 1024-bit circuit. The chips are fabricated using Hypres' Nb/AlO/sub x//Nb process with a junction critical current density of 1.0 kA/cm/sup 2/. >

Whole cortex, 64 channel SQUID biomagnetometer system

Abstract: The authors report on the development and testing of a novel, whole cortex, 64-channel superconducting quantum interference device (SQUID) biomagnetometer system operating in an unshielded environment. The essential features of this instrument, including the cryogenics system, the room-temperature digital electronics, and signal processing capabilities, are described. A noise cancellation scheme is incorporated which allows extraction of biomagnetic signals over the surface of the cortex while operating in the presence of large background magnetic interference. A system description and the results of preliminary magneto-encephalographic measurements are presented. Measurements of noise performance and human auditory evoked response in an unshielded industrial environment have largely validated the noise cancellation methods incorporated in the instrument. >

Large shaped parts of melt cast BSCCO for applications in electrical engineering

Abstract: A melt casting process (MCP) that is tailor made for BSCCO, especially for the 2212 phase, is described. It is reported that melt casting offers an easy access to a variety of different shapes and sizes. The properties of the material obtained by MCP have been significantly improved by admixing strontium sulphate. J/sub c/ values superior to those of sintered ceramics of YBCO or BSCCO have been obtained: J/sub c/ values at 55000 A/cm/sup 2/ (4 K) and 2200 A/cm/sup 2/ (77 K) were achieved without external fields; large bulk parts exhibited more than 2000 A (77 K) in their self-field. The peculiarities of MCP allow the integration of noble metal current contacts which exhibit total resistances >

Design and implementation of a scanning SQUID microscope

Abstract: A cryogenic two-dimensional magnetic imaging system that incorporates a sensitive DC superconducting quantum interference device (SQUID) detector is described. In this instrument, which is called the scanning SQUID microscope, a flux transformer coupled to the SQUID is scanned over the surface of a planar sample by a room-temperature, stepping-motor-controlled manipulator with a positional resolution of about 2 mu m. With the present input coil, spatial resolution of 20 mu m and a flux sensitivity at the sample of 10/sup -5/ Phi /sub 0//Hz/sup 1/2/ is attainable. This system has been designed specifically to image the vortex structure and dynamics of weakly coupled superconductor arrays. >

Digital to analog conversion based on processing of the SFQ pulses

Abstract: Two novel circuits of DC voltage multipliers have been designed, fabricated using a 3.5 mu m 1000 A/cm/sup 2/ niobium trilayer technology, and successfully tested at low frequencies. A novel primary digital-to-analog (D/A) converter based on the processing of single-flux-quantum (SFQ) pulses has been suggested. The performance of analog-to-digital (A/D) converters combined from such primary D/A converters and voltage multipliers is discussed. >

In-plane texturing control of Y-Ba-Cu-O thin films on polycrystalline substrates by ion-beam-modified intermediate buffer layers

Abstract: Biaxially aligned YBCO thin films were successfully formed on polycrystalline Ni-based alloy by using ion-beam-modified yttria-stabilized-zirconia (YSZ) intermediate layers. YSZ layers were deposited by ion-beam-assisted deposition (IBAD) with concurrent off-axis ion beam bombardment. The YSZ

Superconducting millimeter wave oscillators and SIS mixers integrated on a chip

Abstract: All-refractory material superconducting millimeter-wave oscillators have been designed and investigated experimentally with different superconductor-insulator-superconductor (SIS) mixers integrated on the same chip. Tested structures include a flux-flow oscillator (FFO) based on a long Josephson junction, a coupling section, and an SIS detector with tuned out junction capacitance. Coupling sections were designed as multistep microstrip quarter-wave impedance transformers. All junctions have been fabricated on the basis of a high-quality trilayer Nb-AlO/sub x/-Nb process. Microwave oscillations in the frequency range 75-500 GHz have been detected experimentally. The level of delivered power was estimated from the pumped I-V curve of the strongly coupled single junction detector. Coupled power levels higher than 0.1 mu W at 256 GHz were achieved. A spectral linewidth of the FFO of less than 1 MHz has been estimated experimentally. The first attempt to create an integrated receiver based on an FFO and an SIS array mixer integrated on the same chip was made in the 2-mm wavelength band. >

Josephson sigma-delta modulator for high dynamic range A/D conversion

Abstract: A Josephson sigma-delta modulator suitable for use in high dynamic range conversion of MHz bandwidth signals was designed and simulated. Input signal voltages were integrated as current in a superconducting inductor. A single junction quantizer provided analog-to-digital (AID) conversion at 40 GHz and fed back single-flux-quantum (SFQ) voltage pulses to balance the input. A JSIM calculation for a 13,110,000-ps interval indicated ideal first-order noise shaping of a 2.44-MHz voltage-source signal. Spur free dynamic range was 102 dB and signal-to-noise-and-distortion was 98 dB on the DC 10-MHz signal band, equivalent to a 16-b, 20-MSample/s, Nyquist-rate A/D. The modulator transfer function was derived for a current-source signal applied to an input resistor. First-order quantization noise suppression was still observed for signal bands wider than the 2 pi R/L modulator cutoff frequency. The superconductive sigma-delta modulator combines the speed of Josephson junctions for GHz sampling and digital filtering with the quantum mechanical accuracy of SFQ feedback to obtain superior A/D performance. >

European project for the development of high T/sub C/ current leads

Abstract: A European BRITE/EURAM project for the development of current leads in the kA range using high-T/sub C/ superconductor parts has been started recently. The partners in this two-year project are two chemical companies, Hoechst and Rhone-Poulenc, and two companies in the field of electrical engineering, Siemens and Alcatel Alsthom. The goal of these four companies is to develop current leads for 4.2-K systems making use of high-T/sub C/ superconducting materials, to manufacture them, to demonstrate their performance in comparison with conventional all-metal current lead, and to develop relevant models. The objective is the realization of a demonstration current lead toward the end of the project. This device will operate at a steady-state current of 1000 A and an insulation voltage of 20 kV. The losses will be reduced to one-third of the losses of a conventional metallic current lead. First results are given on Y- and Bi-based current lead models, including calculations and the characterization of large bulk high-T/sub C/ samples up to a length of 200 mm with a I/sub C/ value reaching 2000 A. >

Passive shimming of MR magnets: algorithm, hardware, and results

Abstract: An improved method for allowing a magnetic resonance (MR) magnet to meet its stringent field homogeneity requirements has been developed. The method is called passive shimming and involves the placement of pieces of ferromagnetic material in the magnet bore at locations which are determined from an initial field map. Software and hardware which facilitate this shimming procedure have been developed at GE and successfully implemented in MR scanners around the world. The software uses a linear programming algorithm to minimize the thicknesses of shims at each of a number of allowable locations which will bring the peak-to-peak field error in the volume of interest to its minimum level. The hardware consists of arc-shaped drawers on each of which shim packages may be built up to a predetermined maximum thickness. This shimming technology represents a major cost reduction from the superconducting or resistive electromagnetic correction coils which are typically used to shim a magnet's field. >

Design analysis of a novel hot-electron microbolometer

Abstract: The authors propose a novel antenna coupled microbolometer which makes use of the weak coupling between electrons and phonons in a metal at low temperatures. The radiation is collected by a planar lithographed antenna and thermalized in a thin metal strip. The resulting temperature rise of the electrons is detected by a tunnel junction, where part of the metal strip forms the normal electrode. The active area of the bolometer is thermally coupled by its small volume, by the thermal resistance between the electrons and phonons in the strip, and by the reflection of quasi-particles at the interface between the strip and the superconducting antenna. Design calculations based on a metal volume of 2 mu m*6 mu m*0.05 mu m at an operating temperature of 100 mK give an NEP of about 3*10/sup -19/ WHz/sup -1/2/, a time constant of about 10 mu s, and a responsivity of about 10/sup 9/ V/W. The calculated sensitivity is almost two orders of magnitude higher than that of the best available direct detectors of millimeter and submillimeter radiation operated at the same temperature. >

Design considerations for an inductive high T/sub c/ superconducting fault current limiter

Abstract: The authors investigate the design feasibility of an inductive iron core superconducting fault current limiter. A simplified thermo-electromagnetic analysis of the operating behavior of the fault current limiter is presented. This design is based on a high temperature superconducting shield surrounding a magnetic circuit with a primary winding generating an AC magnetic field. During fault conditions the field penetrates the core resulting in a large impedance, and thus a limitation of the fault current. In the analysis of this design, for voltages and currents up to 25 kV root-mean-square (RMS) and 2000 A RMS, it is shown that recuperation times for superconductors operating at liquid nitrogen temperatures are acceptable. >

A nonlinear transmission line model for superconducting stripline resonators

Abstract: A nonlinear transmission line model is used to explain the nonlinear frequency response at high input power levels of stripline resonators fabricated with superconducting NbN and YBa/sub 2/Cu/sub 3/O/sub 7-x/ thin films. The resonator is modeled as a transmission line with a nonlinear inductance and resistance. The inductor flux and resistor voltage drop are assumed to be odd polynomial functions of the transmission line current, and the polynomial coefficients are adjusted to match the measured data. Excellent agreement is found between the measured and calculated results. >

An electronic second-order gradiometer for biomagnetic applications in clinical shielded rooms

Abstract: A modular 55-channel magnetometer for biomagnetic studies in clinical shielded rooms is described. Seventy-six superconducting quantum interference device (SQUID) magnetometers in three levels are used to electronically form first-order or second-order gradiometer configurations. The magnetometers are completely integrated on 7.2-mm*7.2-mm chips. Additional positive feedback is used to increase the gradient of the V- Phi (voltage-flux) characteristics to dV/d Phi approximately=1 mV/ Phi /sub 0/, and a novel scheme called bias current feedback reduces the bias current sensitivity to theta Phi / theta I/sub b/ approximately=0.02 Phi /sub 0// mu A. This allows one to couple the SQUIDs directly to simplified feedback electronics without helium-temperature impedance-matching circuits or flux modulation techniques. The magnetometer noise level is typically 1.4 fT/ square root Hz at 1 kHz and 3.2 fT/ square root Hz at 1 Hz, measured inside a Pb/Cryopern shield. A seven-channel prototype module has been tested successfully inside a clinical shielded room. A typical first-order gradiometer noise level of 2.2 fT/ square root THz at 1 kHz and 7 fT/ square root Hz at 2 Hz has been achieved. >

High-temperature superconducting delay lines and filters on sapphire and thinned LaAlO/sub 3/ substrates

Abstract: The very low microwave surface resistance of high-temperature-superconductor (HTS) thin films allows the realization of microwave devices with performance superior to those made by conventional technology. Superconducting delay lines, for example, have very low propagation loss and dispersion. Long, low-loss, superconducting delay lines on both thinned LaAlO/sub 3/ and sapphire substrates are presented. Delay lines with 27- and 44-ns delay have been made, for the first time, on 5-cm-diameter 254- and 127- mu m-thick LaAlO/sub 3/ substrates, respectively. The insertion losses at 77 K and 6 GHz are 6 and 16 dB, respectively. Delay lines with 9-ns delay have, for the first time, been produced on M-plane sapphire substrates and demonstrate, at 77 K, an insertion loss of 1.0 dB at 6 GHz. A 2.5%-bandwidth 10 GHz four pole edge-coupled bandpass filter on M-plane sapphire substrates is also reported. The filter has minimum insertion loss of less than 0.5 dB at 9.75 GHz and 71 K. >

Noise measurements of series SQUID arrays

Abstract: Series arrays of 50 gradiometer superconducting quantum interference devices (SQUIDS) and 100 magnetometer SQUIDs have been fabricated using Nb/AlO/sub x//Nb junctions with a planarized all-refractory technology for superconductivity (PARTS), the white noise of these arrays has been measured. The individual devices are 50-pH, 2-hole and 100-pH, 1-hole SQUIDs with integrated single turn input coils, 1- mu m/sup 2/ and 0.5- mu m/sup 2/ junctions, and PtRh shunt resistors. The input coil inductance of 10 nH will effectively match with a wide bandwidth to miniature pick-up loop structures for various experiments. Ideally, the coupled energy sensitivity should remain constant as devices are added in series, while the output impedance rises to a level practical for direct coupling to room-temperature electronics. The output impedance of the arrays is 250-1000 Omega . The white noise was measured directly with an ultra low noise preamplifier at room temperature. The best result was Phi /sub N/=0.12 mu Phi /sub 0// square root Hz with a corresponding coupled energy sensitivity of 56 h. >

A 6-bit Josephson flash A/D converter with GHz input bandwidth

Abstract: Edge-triggered periodic comparators based on low-inductance superconducting quantum interference devices (SQUIDs) have been combined with a 5-mV step generator used as a clocked power supply to produce an analog-to-digital (A/D) converter with the widest input bandwidth of which the author is aware. The noise and linearity have been measured to be more than adequate for 6 b. Asynchronous beat frequency tests indicate that the current design is capable of about four effective bits resolution at 5-GHz input bandwidth and 3 b at 10 GHz. The aperture jitter at 6-GHz clock rates has been measured to be 100 fs rms indicating that the intrinsic noise is low enough to permit subpicosecond aperture times in faster designs. Further advantages include tolerance of large critical current variations and a sinewave clock as the only high-speed waveform necessary to drive the circuit. The amplitude and spectral purity of the sinewave are not critical as long as it reaches a threshold value, since it is regulated by the step generator. >

Nb/sub 3/Sn multifilamentary wires with CuNb reinforcing stabilizer

Abstract: A bronze processed multifilamentary Nb/sub 3/Sn superconducting wire with CuNb reinforcing stabilizer (CuNb/Nb/sub 3/Sn), which exhibits a residual resistance ratio of 20 and a magnetoresistance of 0.2 mu Omega -cm at 23 T and 4.2 K, has been developed. The yield stress at 4.2 K, was noticeably improved; a value of 22 kg/mm/sup 2/ was obtained even after heat treatment of Nb/sub 3/Sn formation at 700 degrees C for 200 h. The effect of strain on critical current density, J/sub c/, was described well by using the upper critical field in the prestrain state. It was verified that the CuNb/Nb/sub 3/Sn wire mechanical and superconducting properties need to perform under an enormous electromagnetic force in high magnetic fields. >

First results of stress effects on I/sub c/ of Nb/sub 3/Al cable in conduit fusion superconductors

Abstract: The effect of axial strain, epsilon /sub a/, on the critical current, I/sub c/, of Nb/sub 3/Al cable-in-conduit subconductors has been measured for I/sub c/ >