high-T/sub C/ Superconducting Research Articles

THz beam generation from YBCO thin films using a 1.55 μm femtosecond laser

High-T/sub c/ superconductor (HTS) is a promising material for terahertz (THz) electronics. The THz beam generation from YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta// (YBCO) thin film antennas using femtosecond (fs) laser pulses with a wavelength /spl lambda/ of around 1.55 /spl mu/m is demonstrated. The mechanism of the THz beam generation is revealed to be the one photon excitation process corresponding to an intraband transition in the in-gap state. The properties of the THz beam generation excited by the 1.55 /spl mu/m fs laser are similar to that excited by an 800 nm fs laser.

Broadband HTS microstrip antennas for satellite communication

The major limitation of high-T/sub c/ superconducting (HTS) microstrip antennas is the narrow bandwidth due to the high Q and thin substrate. Defining bandwidth as the frequency range over which standing wave ratio (SWR) 2:1 or less, HTS antenna bandwidths are typically 0.85% to 1.1%. Thus considerable effort has been focused on developing antennas for broadband operation. In this work the HTS antenna, which consists of two triangle-radiation patches, was designed and fabricated using a YBa/sub 2/Cu/sub 3/O/sub 7-x/ (YBCO) superconducting thin film on MgO substrate for broadband operation. The input impedance and other characteristics for this antenna have been calculated using the moment method. Experimental results for our HTS antenna were reported in terms of radiation patterns, return losses, bandwidths and other various characteristics. The bandwidth obtained was a significant 6.7%.

Characteristic analysis of coupled HTS interconnects with two-dimensional FDTD

In this work, the frequency-dependent RLGC parameters of high-speed coupled high T/sub c/ superconductor (HTS) interconnects are extracted with a two-dimensional (2-D) FDTD algorithm. The response signals of an HTS interconnect circuit and a normal Al interconnect circuit are simulated and compared, showing that not only the signal dispersion, delay, and magnitude decay of HTS interconnects are smaller than that of Al interconnects, the crosstalk of HTS interconnects is much smaller, too.

A design and tests of HTS power cables and feasibility study of HTS power transmission system in Korea

In this paper we present the results of tests for high-T/sub c/ superconducting (HTS) power cables and the analysis of an HTS power transmission system in Korea. Prototype HTS cables have been constructed using Bi-2223 based Ag-sheathed HTS tapes. The prototype cable has been tested at 77 K with DC currents. Results shows that the cable carried up to 730 A DC and the authors analyzed the feasibility of a HTS power transmission system in the Seoul area in Korea. According to the analysis results, the HTS power system can be reduced by 5.2%/spl sim/9.7% of the underground transmission lines compared to the conventional cable system in Seoul, Korea and the HTS power cable is feasible in future power systems.

Numerical analysis of flux diffusion into high-T/sub c/ superconducting tapes carrying AC transport currents

A numerical approach for analyzing the mechanism of flux motion inside high-T/sub c/ superconducting (HTS) tapes is presented. The approach is based on a 2-dimensional numerical model which simulates Ag-sheathed HTS tapes carrying ac transport currents in terms of a diffusion process. The HTS tape is treated as a highly nonlinear conductor with its conductivity being specified by a power law E-J characteristic. Using this model, a number of cases with different values of ac transport current have been investigated. Results show that the model provides sufficient descriptions and account for the flux creep and flux flow regimes in HTS tapes.

Possibility of Nd/sub 1.9/Ba/sub 1.1/Cu/sub 3/O/sub 7+δ/ and Pr/sub 1.14/Ba/sub 1.86/Cu/sub 3/O/sub 7-δ/ single crystals for insulator in high-speed superconducting circuits

Dielectric properties of both Nd/sub 1.9/Ba/sub 1.1/Cu/sub 3/O/sub 7+/spl delta// (NBCO-213) and Pr/sub 1.14/Ba/sub 1.86/Cu/sub 3/O/sub 7-/spl delta// (Pr-rich PBCO) single crystals have been examined at low temperature. These materials have good lattice matching to high-T/sub c/ superconductors (HTS), but they are conductive at room temperature. Below 80 K, they are insulators with low dielectric constants, /spl epsi//sub /spl tau// below 25, and low dielectric loss tan /spl delta/ below 0.1 at 100 kHz. The value of /spl epsi//sub /spl tau// is suitable for insulators in integrated circuits using strip line widths of 10 /spl mu/m order, providing short delay time, no excitation of surface wave, and low radiation loss. The value of tan /spl delta/ is comparable to loss of superconducting surface resistance above 100 GHz. These results indicate the applicability for the insulator layers in multilayer superconducting electronic devices such as Single Flux Quantum (SFQ) circuits operated at high speed.

Transport current properties of double-pancake coils wound by Ag-sheathed Bi-2223 tapes

In order to apply high T/sub c/ superconducting (HTS) tape to the power apparatus, it is necessary to evaluate the property of the coil shape. In particular, the stability of the coil is important, because it is expected to influence both the machine design and the operating conditions. In order to study the feasibility of HTS power application, they used double-pancake coils wound by Ag-sheathed Bi-2223 multi-filament tapes, and measured the transport current properties. From the experimental and analytical results, it was noted that the HTS double-pancake coils exhibited very stable properties, even beyond the critical current of coils. They believe that they can show the possibility of application of the HTS tape to the winding of power apparatus, although further improvements in superconducting properties of the tapes are needed. The stabilities of HTS and LTS (low T/sub c/ superconductor) are quite different, so it is also necessary to establish an analytical method for determining the stability of HTS winding.

High-resolution measurement by a high-T/sub c/ superconductor sampler

We measured a signal current waveform by using a high-T/sub c/ superconductor (HTS) sampler with a 1-ps delay between every sampling point. The maximum time differential obtained in the measured waveform was 12 /spl mu/A/ps with a 2.5-/spl mu/A current sensitivity at 25 K. This result guarantees that the sampler is able to measure current waveforms correctly when their maximum time differential is less than 12 /spl mu/A/ps. The superior temporal response was achieved by using high-speed single-flux-quantum pulses generated in the HTS circuit. A unique feature of the sampler is that it directly measures the current with picosecond and microampere resolutions. Measurement of current flowing through wiring in a semiconductor large-scale integrated circuits is a promising application for the HTS sampler.

Magnetic characteristics of a high-T/sub c/ superconducting cylinder for magnetic shielding type superconducting fault current limiter

The superconducting fault current limiter (SCFCL) is expected to be the first application of high-T/sub C/ superconductors (HTSs) in power systems. To develop a magnetic shielding type superconducting fault current limiter, we have carried out some fundamental experiments concerning the magnetic shielding characteristics of an HTS bulk cylinder. In this paper, the experimental results of magnetic flux penetration into a Bi(2223) cylinder are shown. AC magnetic field is applied to the sample by a primary winding (copper coil) excited by AC triangular- and sinusoidal-waveform currents in the frequency range of 0.1 Hz to 100 Hz. We also developed a finite element method (FEM) computer program for evaluation of the dynamic electromagnetic behavior of the HTS cylinder in a time-varying external magnetic field. The results of computer simulations considering the voltage-current (E-J) characteristic are compared with experiments.

Design of Bi-based superconducting current lead for SMES

2.1 kA-class hybrid-type current leads for superconducting magnetic energy storage (SMES) magnets, consisting of conventional gas-cooled copper leads and high-T/sub c/ superconducting (HTSC) current leads, were designed. The HTSC parts of the leads are made of Bi-2223/99.8%Ag0.2%Au tape. For the design, numerical analysis was performed to find an optimum shape factor and an optimum varying rate of the cross-sectional area of HTSC lead which result in minimum refrigeration power consumption, and then the minimum evaporation rate of LHe. In the present study, the main results of the design are presented.

Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ and YBa/sub 2/Cu/sub 3/O/sub 7/ films on large area MgO and sapphire substrates for high power microwave and rf applications

This paper describes recent work at DuPont to extend the high power microwave performance of YBa/sub 2/Cu/sub 3/O/sub 7/ and Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ high T/sub c/ superconducting (HTS) films on 2 inch diameter MgO and sapphire substrates. The YBa/sub 2/Cu/sub 3/O/sub 7/ films were produced by in-situ off-axis sputtering with radiant substrate heating. The Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ films were produced by off-axis sputtering followed by annealing in oxygen and Tl/sub 2/O vapor. The microwave surface resistance and power handling of these films was measured over a wide range of temperature (20 K to 85 K) using an HTS-sapphire resonator technique. At 70 K, the surface resistance of YBa/sub 2/Cu/sub 3/O/sub 7/ films on sapphire remained below 250 /spl mu//spl Omega/ (@8.1 GHz) for microwave surface current densities up to 1.7/spl times/10/sup 6/ A/cm/sup 2/. At 70 K, the surface resistance of Tl/sub 2/Ba/sub 2/CaCu/sub 2/O/sub 8/ films on MgO remained below 230 /spl mu//spl Omega/ (@8.1 GHz) with microwave current densities up to 2/spl times/10/sup 6/ A/cm/sup 2/.

Critical current degradation caused by winding process of Bi-2223/Ag HTS wire in the form of a coil

High T/sub c/ superconducting (HTS) (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/ Cu/sub 3/O/sub 10+x/ Ag-clad wire has potential for practical applications in the form of a coil or a winding. This HTS wire has strong magnetic field-dependent and mechanical strain-dependent critical currents, consequently it has a severe problem of critical current degradation when it is used in the form of a coil. To design a winding with this conductor, the critical current degradation has to be identified with relation to the magnetic fields and the winding process. This is important to optimize an appropriate coil winding procedure using this HTS wire. A specially designed noninductive sample has been made with a (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/ Cu/sub 3/O/sub 10+x/ Ag-clad 27-filament wire, and the critical current degradation, which is caused by the magnetic field generated and the mechanical winding procedure used to form a coil, has been separated into two factors accordingly, and discussed in this paper.

Analysis of the operational characteristics of a heater-trigger type high-T/sub c/ superconducting power supply

This paper deals with the design and fabrication of a heater-trigger type high-T/sub c/ superconducting (HTSC) power supply, and its characteristics has been analyzed through experiments. A heater-trigger HTSC power supply consists of two heaters, an electromagnet, and a YBCO superconducting bulk. In this experiment, a 0.6-T magnet and a 2.3-A dc heater current were used and 190 s and 380 s were used for the pumping period. In order to measure the pumping current with respect to the magnet flux changes, hall sensors were installed at the surface of the YBCO bulk and inside of an iron core. The experimental observations have been compared with the theoretical predictions. In this experiment, the pumping current has reached about 12 A. In computer simulation, the maximum pumping current of the system has been predicted to be about 13 A.

Theory of digital phase shifters based on high-T/sub c/ superconducting films

A theory of digital phase shifters based on transition from a superconducting (S) state into a normal (N) state in high-T/sub c/ superconductor (HTS) films is developed. It is shown that the insertion-loss level in the phase shifters is determined by the commutation quality of the HTS film and does not depend on the S-N switching element shape and size. The procedure for designing the S-N phase shifter with minimum insertion loss is equivalent to designing a matching circuit for the termination Z=/spl radic/R/sub S/R/sub N/ (R/sub S/ and R/sub N/ are the resistances of the switching element in the S- and N-states, respectively). Different versions of S-N phase shifters are suggested and discussed. High-quality S-N phase shifters can be successfully designed in a 1-10 GHz frequency range and find application on the receiving end of microwave systems.

E-J characteristic and flux pinning in superconducting PbBi alloys

The characteristics of electric field E vs. current density J at various magnetic fields and temperatures in superconducting PbBi samples with different pinning strength were measured by using a usual resistive method. The obtained E-J curves show a nonlinear behavior similar to that in high T/sub c/ superconductors (HTSC). With a scaling generally used in HTSC, E-J curves collapse onto one of the two universal branches, with the exponents of /spl nu/ and z similar to those reported in HTSC. It was found that the stronger flux pinning strength leads to the higher transition points of T/sub g/ and B/sub g/ in the E-J characteristic. The nonlinear E-J curves and the enhancement of the transition points were explained and reproduced by using the flux creep and flux flow theory.

Processing and characterization of high T/sub c/ magnetic shields and flux transformers

The use of high T/sub c/ SQUID (HTSQUID) magnetometers for magnetocardiography (MCG) and nondestructive testing (NDT) requires effective shielding from magnetic noise fields. The authors have investigated the shielding characteristics of high-T/sub c/ superconducting (HTS) material and developed a compact, portable system consisting of several HTS components. A process for fabricating these components out of Bi-Pb-Sr-Ca-Cu-O has been developed. Tubes and disks of various sizes have been formed by cold isostatic pressing (CIP) and sintered at reduced oxygen pressures. The HTS tubes exhibit shielding factors of up to 10/sup 3/ at low frequencies. A novel technique for fabricating superconducting flux transformers (specifically, first-order gradiometers) has also been developed. The flux transfer efficiencies of these first-order gradiometers have been evaluated. The tubes, gradiometers, and flux focusers have been incorporated into an HTSQUID setup. The authors find that the low-frequency noise rejection obtained in their setup is very promising for potential MCG and NDT applications in an unshielded environment.

Measurement of microwave magnetic properties of superconducting YBa/sub 2/Cu/sub 3/O/sub 7-δ/ thin films

The magnetic property of high-T/sub c/ superconductors (HTS) at microwave frequency has been of increasing interest since their magnetic behavior is related to microwave vortex dynamics. Thus, we recently designed a cavity perturbation system (CPS) for simultaneous measurement of the resonant frequency (f/sub 0/) and the quality factor (Q). The magnetic susceptibilities (/spl chi/'-j /spl chi/") of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta///MgO thin films at microwave frequencies were investigated and could be analyzed by cavity perturbation theory. We expected that the microwave magnetic susceptibility data as a function of temperature measured using the HTS thin film would explain the phenomenological superconducting dispersion relation. The transition temperature is 91 K and the temperature corresponding maximum imaginary part (/spl chi/") is 85 K. However, the transition temperature and surface resistance (R/sub s/) measured by a conventional transmission-line method were 86 K and 180 /spl mu//spl Omega/ at 40 K, respectively. We found absolute differences between the cavity perturbation method and the transmission line ones, and suggested the CPS is one of the most promising technique for measuring the magnetic susceptibility of HTS thin film with a 0.1/spl sim/1 /spl mu/m thickness.

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

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.

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

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.

Stability and oscillation of MTG high-T/sub c/ superconductors in inhomogeneous magnetic field

The dependencies of the oscillation amplitude on the oscillation frequency of high-T/sub c/ superconducting (HTSC) melt-texture growth (MTG) ball-sample suspended or levitated in an inhomogeneous magnetic field have been investigated. The dependence of the oscillation amplitude versus the oscillation frequency has maxima. The rotation of the levitated or suspended MTG ring-sample in the rotating magnetic field have been studied. The curve of the sample rotation frequency versus the magnet rotation frequency has maxima which correlate with maxima of the dependence of the oscillation amplitude of the levitated HTSC MTG ball-sample on the frequency of the external magnetic field.