Table of contents

This special issue of Superconductor Science and Technology contains papers presented at the 6th European Conference on Applied Superconductivity (EUCAS), which was held in Sorrento, Italy, 14--18 September 2003.

This important biennial event followed previous successful meetings held in Gottingen, Germany; Edinburgh, Scotland; Eindhoven, the Netherlands; Sitges (Barcelona), Spain; and Copenhagen, Denmark.

Following tradition, this EUCAS conference focused on the role of superconductivity in bridging various aspects of research with a variety of concrete advanced applications.

EUCAS 2003 attracted about 1000 participants from all around the world with large participation from non-European countries. This conference benefited the worldwide superconductivity community tremendously as scientists operating internationally were able to share their knowledge and experience with one another.

We are grateful to all those who submitted papers to the Conference Proceedings, which will be published in an Institute of Physics Conference Series, and also to those who contributed to this special issue.

Unfortunately we could not consider every one of the large number of papers submitted to this issue and we express our regret to those whose work could not be included.

In this paper we highlight the advantages of using point-contact spectroscopy (PCS) in multigap superconductors like MgB₂, both as a fundamental research tool and as a non-destructive diagnostic technique for the optimization of thin-film characteristics. We first present some results of crucial fundamental interest obtained by directional PCS in MgB₂ single crystals, for example the temperature dependence of the gaps and of the critical fields, and the effect of a magnetic field on the gap amplitudes. Then, we show how PCS can provide useful information about the surface properties of MgB₂ thin films (e.g., T_c, gap amplitude(s), clean- or dirty-limit conditions) in view of their optimization for the fabrication of tunnel and Josephson junctions for applications in superconducting electronics.

A superconducting cable based on Bi-2223 tape technology has been developed, installed and operated in the public network of Copenhagen Energy in a two-year period between May 2001 and May 2003. This paper gives a brief overview of the system and analyses some of the operation experiences. The aim of this demonstration project is to gain experience with HTS cables under realistic conditions in a live distribution network. Approximately 50 000 utility customers have their electric power supplied through the HTS cable. The cable system has delivered 226 GW h of energy and reached a maximum operating current of 1157 A. The operation experiences include over-currents of 6 kA due to faults on peripheral lines, commissioning, servicing and failure responses on the cooling system, continuous 24 h, 7 day per week monitoring and performance of the alarm system. The implications of these experiences for the future applications of HTS cable systems are analysed.

A very peculiar feature of the recently discovered superconductor MgB₂ is the multigap nature of the superconducting state, which is now commonly accepted in the scientific community and confirmed by a large number of experiments. We report a systematic scanning tunnelling spectroscopy (STS) study performed on high quality thin films of MgB₂. Electron microscopy images and STM topography together with the STS investigation allow a direct correlation between tunnelling direction and the observed tunnelling spectra, confirming that the two-gap state is intrinsic to MgB₂.

In the design of electronic applications (for example, rapid single flux quantum (RSFQ) circuits), the properties of Josephson junctions (JJs) provided by technology are the major requirement. The characteristic voltage I_cR_n and the dynamic parameter β_c are non-negligibly important, but the external shunted junctions are only completely described by including the parasitic inductance between the junction area and the shunt resistor. The first publications of the RSFQ circuit technique were closely followed by many studies on the dynamic influence of this inductance. It has been pointed out that no practical value of this parameter has to be taken into account in the circuit design and optimization process. The operation of RSFQ logic circuits made of high-temperature superconductors (HTS) are strongly influenced by thermal noise. Our paper contains the determination of the digital bit-error rate for all different types of JJs by using a Fokker–Planck equation for describing the influence of thermal fluctuations. Our analysis shows that the parasitic inductance cannot be neglected in terms of noise. The intrinsic shunted junctions like high current density-, SINIS- and all kinds of HTS-JJs provide the best noise immunity for a fixed temperature.

We consider a system of stacked Josephson junctions and investigate the different fluxon modes. Some of these modes can be used to construct an oscillator with a frequency in the range 200 GHz–1 THz. We discuss the output that can be expected from the different fluxon modes, and we calculate the theoretical maximum output numerically.

The German government funded resistive superconducting fault current limiter project CURL 10 is based on bulk material and aims at the development of a three-phase prototype for the medium voltage level (10 kV, 10 MVA). The key element of the project is the development of suitable robust superconducting components. On the basis of the well known melt cast process (MCP) for BSCCO-2212, these are manufactured by cutting superconducting tubes to bifilar coils. At the operation temperature of 65 K a current density of 4000 A cm⁻² was achieved. In order to protect the superconductor during limitation the component was equipped with an electrical shunt contacted on its entire length.

Single-phase tests with nine of these components in series, corresponding to a protected load of 1.2 MVA, have been successfully accomplished and are described in detail. These tests include different types of short circuits as specified by the utilities within the project and included lightning surge loads up to 75 kV.

Comprehensive measurements of the flux flow oscillator (FFO) radiation linewidth are performed using an integrated harmonic SIS mixer; the FFO linewidth and spectral line profile are compared to a theory. An essential dependence of the FFO linewidth on frequency is found; a possible explanation is proposed. The results of the numerical solution of the perturbed sine–Gordon equation qualitatively confirm this assumption. To optimize the FFO design, the influence of the FFO parameters on the radiation linewidth is studied. A novel FFO design at a moderate current density has resulted in a free-running FFO linewidth of about 10 MHz in the flux flow regime up to 712 GHz, limited only by the gap frequency of Nb. This relatively narrow free-running linewidth (along with implementation of a wide-band phase locking loop system) allows continuous phase locking of the FFO in the wide frequency range of 500–710 GHz. These results are the basis for the development of a 550–650 GHz integrated receiver for the terahertz limb sounder (TELIS) intended for atmosphere study and scheduled to fly on a balloon in 2005. We report here also on the design of the second generation of the phase-locked superconducting integrated receiver chip for TELIS.

We report on our efforts in developing and scaling-up the systems for IBAD-MgO based coated conductor fabrication. The overall fabrication process involves a number of different processes including: electropolishing of the substrates; barrier-layer, seed-layer, and IBAD-MgO deposition by e-beam evaporation; and pulsed laser deposition of buffer and YBCO layers. All processes are realized in reel-to-reel processing systems. Latest results have shown that the IBAD-MgO approach yields coated conductor performance comparable to the best results achieved elsewhere to date.

We report on the design and testing of a cryogen-free superconducting magnet that is conductively cooled by means of a two-stage pulse tube cryocooler (PTC). The cooler provides a cooling power of 0.4 W at 4.2 K, when operated on a helium compressor with a nominal input power of 6 kW. The 5 T NbTi magnet is equipped with a persistent current switch with a resistive heater. The thermal coupling between the switch and the 4 K stage is chosen so as to allow operation of the switch without a marked heat input to the 4 K stage during the charging process. The PTC is capable of cooling the complete magnet system to a stationary temperature of 2.7 K, which was found to be stable for a period of more than two months. The magnet was charged to a field of 5.5 T, and persistent mode operation was monitored successfully for a period of three weeks.

We have studied the local critical current density, j_c, in the superconductor thin film of bilayer structures consisting of YBa₂Cu₃O₇ and the ferromagnets La_2/3Ca_1/3MnO₃ and SrRuO₃, respectively, by means of quantitative magneto-optics. A pronounced hysteresis of j_c was observed which is ascribed to the magnetization state of the ferromagnetic layer. The results are discussed within the frame of magnetic vortex–wall interactions.

We report on the investigation of the effects of the nonlinear inductance of huge Josephson junction arrays on high-quality RF circuits. The inductance of a single junction is far too small for use in many RF applications; however, Josephson junction stacks and especially intrinsic Josephson junctions in crystalline-layered superconductors open new opportunities. The stack inductance can be altered by changing a DC Josephson current either using an external bias current or an external magnetic field applied to a closed superconducting loop containing two stacks connected in parallel. Using stacks of Josephson junctions in the LC resonators, a tunable 4-pole, 15 MHz band-pass filter with a centre frequency at 1.1 GHz was designed. The filter was simulated in the time domain and the response was converted to the frequency domain. It was found that using an input power level of −62 dBm, corresponding to 0.5% of the junctions' critical current, a linear approximation for the stack nonlinear inductance can be used. By changing the applied magnetic field, a tuning range of 20% was demonstrated with a tuning speed of 2 µs or better. An estimate of the losses is also given. In addition, power handling up to −26 dBm is reported using increasing circuit complexity.

State-of-the-art deforming techniques allow the preparation of composite Bi-2223/Ag tapes with well controlled filament arrangement. This would allow tailoring of the tape's electromagnetic properties according to specific applications. We studied the possibility to reduce, through a suitable filament ordering, the hysteresis losses caused by an external magnetic field perpendicular to the tape's wide face. For this purpose, the distribution of shielding electrical currents in multifilamentary superconducting arrays, as a response to a perpendicular AC magnetic field, was investigated both theoretically and experimentally. We show that due to optimizing of filament arrangement, the hysteresis loss can be suppressed by one order of magnitude.

Within the framework of the R&D studies for the International Thermonuclear Experimental Reactor (ITER) project, the first full size NbTi conductor sample was fabricated industrially and tested in the SULTAN facility (Villigen, Switzerland). This sample (PF-FSJS), which is relevant to the poloidal field coils of ITER, is composed of two parallel straight bars of conductor, connected at the bottom through a joint designed according to the CEA twin-box concept. The two conductor legs are identical except for the use of different strands: a nickel plated NbTi strand with a pure copper matrix in one leg, and a bare NbTi strand with a copper matrix and internal CuNi barrier in the other leg. The two conductors and the joint were extensively tested as regards DC and AC properties. This paper reports on the test results and analysis, stressing the differences between the two conductor legs and discussing the impact of the test results on the ITER design criteria for the conductor and joint. While joint DC resistance, conductors and joint AC losses fulfilled the ITER requirements, neither conductor could reach its current sharing temperature at relevant ITER currents, due to instabilities. Although the drop in temperature is slight for the CuNi strand cable, it is more significant for the Ni plated strand cable.

The objective of our work is to achieve the fabrication of fully integrated monolithic semiconducting preamplifier/superconducting YBa₂Cu₃O_7−δ (YBCO) microbolometers in order to enhance overall sensor performance. To our knowledge this paper reports the first cofabrication of a c-axis-oriented YBCO film with a critical temperature of 86 K and an active semiconducting device on the same silicon substrate. We proposed a process fabrication where the p-MOS field effect transistors (p-MOSFET) with a Pt-based metallization are fabricated first and the YBCO and buffer layers are deposited as the final step. After YBCO deposition the Pt-based multilayers present interconnect sheet resistivity of 0.45 Ω per square and specific contact resistivity of 2 × 10⁻⁴ Ω cm². The drain current versus gate voltage (V_gs) and versus drain–source voltage (V_ds) characteristics of a 30 µm wide 10 µm long transistor are finally given. It presents a transconductance g_m = 8.7 × 10⁻⁵ A V⁻¹ (at V_ds = 10 Vand V_gs = −20 V) and a threshold voltage V_t = −6.53 V at 300 K.

An integrated gradiometer system is presented, consisting of a single-layer high-temperature superconductor dc superconducting quantum interference device (SQUID) and two coils, realized on printed circuit boards. The coils compensate the homogeneous background field, thus allowing the gradiometer to be freely moved in the Earth's magnetic field. The gradiometer signal is not harmed by any additional noise introduced by the homogeneous field compensation. Therefore, a noise-limited magnetic field gradient resolution of 15 pT m⁻¹ Hz^−1/2 with a 1/f corner frequency of 2 Hz was achieved. This is preserved even after motion in the Earth's magnetic field. Because of the small dimensions of the compensation coils, the gradiometer system can also be used for non-destructive evaluation. Preliminary measurements were performed by moving the SQUID across a workpiece, without any shielding.

We have been developing a magnetic levitating device with two-dimensional movement, namely a 'levitating X–Y transporter'. For the real design of a levitating X–Y transporter, it is necessary to clarify the levitation characteristics, such as the lift, the levitation height and the stability against mechanical disturbances. Furthermore various kinds of force may be applied to the levitating part and cause mechanical oscillation. Therefore the characteristics of oscillation are also important factors in the dynamic stability of such a levitation system. In this paper, we examine experimentally the lift and the restoring force and develop a new simulation code based on the three-dimensional hybrid finite and boundary element method to analyse the dynamic electromagnetic behaviour of the HTS bulk. We have investigated a suitable permanent-magnet arrangement to enhance the levitation characteristics through experiment and numerical simulation. We can then determine the suitable conditions for stable levitation from those results.

We have been developing a new type of active-maglev system composed of field-cooled disc-shaped YBCO bulk and multiple ring-shaped electromagnets, which are vertically piled up. One of the useful features of the active-maglev system is that the levitation height is adjustable by varying the operating current through an electromagnet. Maximum levitation height in stable levitation, however, is restricted by the magnetic field distribution produced by the electromagnet. To enhance the levitation height, we have fabricated and tested an active-maglev system with multiple ring-shaped electromagnets instead of using a larger single electromagnet. Up to now, we have reported the experimental results in a model active-maglev system composed of five electromagnets and showed that the levitation height, as well as stability, can be remarkably improved by adjusting the operating current of each electromagnet individually. In this study, we constructed a model system operated as a tubular linear synchronous actuator in the vertical direction in expectation of it being applied in factory automation upgrading. The primary consists of six ring-shaped copper-winding coils without iron core and is supplied with three-phase sinusoidal excitation. A disc-shaped YBCO bulk, which was magnetized by a field-cooling process, was adopted as a secondary (mover). We carried out preliminary experiments to confirm the feasibility of synchronous operation and also numerically investigated the electromagnetic phenomenon within the bulk superconductor by a developed computer program based on the finite element method.

The correlation of the outputs of two dc-SQUIDs connected to the same solid washer was studied. A dedicated electronic system was used in order to operate both SQUIDs at the same time. It was found that the temporal correlation of the SQUID outputs is strongly dependent on the geometry of the SQUID loop. For a solid washer, the flux noise is mainly due to vortices located close to the SQUIDs. However, by surrounding the SQUID loop with a hole patterned in the superconducting solid washer, it is possible to drastically reduce the flux penetration around the loop. The resulting flux noise becomes dominated by vortices located inside the washer far from the SQUIDs. Magneto-optical images were also used to visualize the flux penetration inside the washer around the slit of such SQUIDs. These confirm previous descriptions of vortex penetration deduced from the electrical measurements.

Owing to recent progress in high temperature superconductor (HTS) technology, Bi2223/Ag-sheathed tapes are produced on mass production scales and a 100 m long YBCO sample with a significant value of the transport critical current has been developed. Research into and development of various HTS power apparatus are ongoing in many countries. The technical feasibility of the HTS apparatus is currently being demonstrated and now the economic feasibility needs to be demonstrated. In the paper, costs of HTS conductors are projected and the cost targets to be met by HTS apparatus if they are to be competitive as regards cost with conventional or alternative apparatus are estimated. On the basis of these studies, ways for the HTS power apparatus to go into the real world are described.

A scanning SQUID microscope based on high-temperature superconductor (HTS) dc-SQUIDs was developed. An extremely soft magnetic amorphous foil was used to guide the flux from room temperature samples to the liquid-nitrogen-cooled SQUID sensor and back. The flux guide passes through the pick-up loop of the HTS SQUID, providing an improved coupling of magnetic flux of the object to the SQUID. The device measures the z component (direction perpendicular to the sample surface) of the stray field of the sample, which is rastered with submicron precision in the x–y direction by a motorized computer-controlled scanning stage. A lateral resolution better than 10 µm, with a field resolution of about 0.6 nT Hz^−1/2 was achieved for the determination of the position of the current carrying thin wires. The presence of the soft magnetic foil did not significantly increase the flux noise of the SQUID.

An MgB₂ thin film deposition technology is the first critical step in the development of superconducting electronics utilizing the 39 K superconductor. It turned out to be a challenging task due to the volatility of Mg and phase stability of MgB₂, the low sticking coefficients of Mg at elevated temperatures, and the reactivity of Mg with oxygen. A brief overview of current deposition techniques is provided here from a thermodynamic perspective, with an emphasis on a very successful technique for high quality in situ epitaxial MgB₂ films, the hybrid physical–chemical vapour deposition. Examples of heterostructures of MgB₂ with other materials are also presented.

The quality of Y Ba₂Cu₃O_7−δ biepitaxial grain boundary Josephson junctions has been improved and 'tunnel-like' characteristics are reproducibly obtained. The consequent low barrier transmission probabilities are apparently favourable to isolate intrinsic d-wave induced effects. Extrinsic and intrinsic d-wave effects are discussed within the framework of novel designs for π-circuitry and qubits.

Ac-loss and coupling current time constant measurements were performed with a large variety of Bi-2223 tape conductors at 4.2 and at 77 K. Single tapes in a perpendicular field as well as stacks of tapes in perpendicular and parallel fields were investigated. The frequency dependence of the coupling loss curve has a maximum which is set in correlation to the separately measured time constant. The theoretical treatment of the time constant and loss measurements using a slab model shows a very good agreement between the position of loss maxima and measured time constants. Eddy currents in the silver matrix, which are described by a second smaller time constant, contribute substantially to the losses in the higher frequency range and may lead to the appearance of a second maximum in the loss curve.

Fermilab is developing a new generation of high-field superconducting magnets for future accelerators based on Nb₃Sn. Rutherford-type cables of 27 and 28 strands of various structures, packing factors, with and without a stainless steel core, were fabricated at Fermilab out of Cu, NbTi and various Nb₃Sn strands. The effect of cabling degradation was measured. A method was developed to simulate cabling and possibly understand the strains applied during the process. This paper summarizes the results of such R&D efforts at Fermilab.

The SQUID microscope has the advantages of excellent field sensitivity, small interaction between the sensor and the sample, and a linear, easily calibrated response. It has the disadvantages of modest spatial resolution and the requirement of a cooled sensor. In this paper we will present results from two applications of the SQUID microscope, chosen with these advantages and disadvantages in mind. First, we have found that the distribution of final fluxoid states of quenched superconducting rings can be accounted for by using a mechanism of the freeze-out of thermally activated fluxoids. This mechanism is complementary to one proposed by Kibble and Zurek in connection with tests of models of the generation of topological singularities in the early development of the universe, and which relies only on causality to produce a freeze-out of the order of parameter fluctuations. Second, we have studied Pearl vortices in [BaCuO_x]_n/[CaCuO₂]_m (CBCO) artificial superlattice structures, with as few as three superconducting CuO₂ layers. The Pearl penetration depths of vortices trapped in these films, which should be inversely proportional to the areal superfluid density, are very long (up to  mm), as expected. In both cases it would be difficult to image fluxoids that generate such weak magnetic fields using any other technique.

NbN phonon-cooled hot electron bolometer mixers (HEBs) have been realized with negligible contact resistance between the bolometer itself and the contact structure. Using a combination of in situ cleaning of the NbN film and the use of an additional superconducting interlayer of a 10 nm NbTiN layer between the Au of the contact structure and the NbN film superior noise temperatures have been obtained as low as 950 K at 2.5 THz and 750 K at 1.9 THz. Here we address in detail the DC characterization of these devices, the interface transparencies between the bolometers and the contacts and the consequences of these factors on the mixer performance.

Single-domain crack-free CeO₂-doped YBCO monoliths synthesized in a conventional box furnace using a top-seeded-melt-growth process with an optimized time–temperature profile exhibit trapped magnetic fields of up to 1.4 T and self-field critical current densities at 77 K of up to 1.3 × 10⁵ A cm⁻². The observed magnetic properties of the samples correlate with the microstructure, which was studied using optical and scanning electron microscopy. With these high quality high temperature superconductor samples a superconducting magnetic bearing for a 2 MW/10 kW h class flywheel energy storage system was designed and has been tested in a model arrangement.

We report measurements of the flux hysteresis curves and trapped flux distribution in an YBa₂Cu₃O₇ rf SQUID containing two closely spaced grain boundary Josephson junctions in parallel. Broadening of the flux distribution from T = 15 K to 30 K is followed by a bifurcation at T = 35 K which corresponds to a degenerate ground state. Above  K the bifurcation disappears, the flux distribution narrows significantly and small secondary loops appear in the hysteresis curves. This behaviour can be modelled qualitatively if we assume a temperature dependent second harmonic term in the current–phase relationship of the junctions.

In a phase II experiment on the International Thermonuclear Experimental Reactor (ITER) toroidal field model coil (TFMC) the operation limits of its 80 kA Nb₃Sn conductor were explored.

To increase the magnetic field on the conductor, the TFMC was tested in the presence of another large coil: the Euratom LCT coil. Under these conditions the maximum field reached on the conductor was around 10 T.

This exploration has been performed at constant current, by progressively increasing the coil temperature and monitoring the coil voltage drop in the current sharing regime.

Such an operation was made possible thanks to the very high stability of the conductor.

The aim of these tests was to compare the critical properties of the conductor with expectations and to assess the ITER TF conductor design. These expectations are based on the documented critical field and temperature dependent properties of the 720 superconducting strands which compose the conductor.

In addition the conductor properties are highly dependent on the strain, due to the compression appearing on Nb₃Sn during the heat treatment of the pancakes and related to the difference in thermal compression between Nb₃Sn and the stainless steel jacket. No precise model exists to predict this strain, which is therefore the main information which is expected from these tests.

The method to deduce this strain from the different tests is presented, including a thermohydraulic analysis to identify the temperature of the critical point and a careful estimation of the field map across the conductor.

The measured strain has been estimated in the range −0.75% to −0.79%. This information will be taken into account for ITER design and some adjustment of the ITER conductor design is under examination.

We present the first results of point-contact spectroscopy (PCS) in single crystals of the boride superconductor ZrB₁₂. The material was first characterized by AC susceptibility and resistivity measurements that allowed us to determine several quantities of physical interest. Then we performed point-contact measurements that gave conductance curves with clear Andreev-reflection features typical of an s-wave single-gap superconductor. By measuring these conductance curves at various temperatures and in the presence of magnetic fields, we obtained the temperature dependence of the gap and of the upper critical field of ZrB₁₂, as well as an estimate of the Fermi energy. All the results indicate that ZrB₁₂ is a conventional s-wave weak coupling superconductor.

We have proposed a novel type of tunable superconducting microwave filter consisting of planar lumped elements which can independently change their centre frequency, bandwidth and skirt characteristics. The independent control is realized by applying the technique of mechanical tuning to the interdigital capacitors C which constitute the filter. We design a three-pole Chebyshev band-pass filter (BPF) with a centre frequency of 6 GHz, 3 dB fractional bandwidth (FBW) of 1.67% and passband ripple of 0.1 dB. The values of C and the meander line inductor L element are calculated using an electromagnetic simulator. The designed filter is fabricated using Y Ba₂Cu₃O_7−δ (YBCO) film deposited on a MgO(100) substrate (10 mm  × 10 mm  × 0.5 mm). The measured transmission characteristic shows the typical BPF-like frequency dependence. This result is the first report on the planar lumped-element BPF with a high frequency of 6 GHz. Furthermore, the large shift of the centre frequency of the order of a gigahertz is obtained by an experiment in the mechanical tuning method.

Promising results of static and dynamic investigations on various journal type test bearings encouraged us to develop a scaled-up HTS bearing, able to carry the HTS rotor of a 400 kW superconducting motor. The stator, a YBCO hollow cylinder of 203 mm inner diameter and 250 mm length, is cooled by liquid nitrogen. Permanent magnet rings with a diameter of 200 mm were mounted on a shaft with alternating polarity. Characterization of the bearing capacity was performed with three different YBCO stators at temperatures between 66 and 86 K in a test set-up. A significant influence of the temperature was found. At a stator temperature of 72 K and a rotation frequency of 25 Hz (corresponding to nominal motor speed) a radial bearing force of 2700 N was measured for the shaft at centre position. Under rotation of the shaft the bearing capacity is reduced. At present our results range within the highest radial bearing capacities reported world-wide.

100 m class Y-123 coated conductors were produced by using reel-to-reel vacuum ion-beam-assisted-deposition (IBAD) and pulsed-laser-deposition (PLD) apparatuses. 100 m long IBAD-Gd₂Zr₂O₇ template films were uniformly and routinely obtained on non-textured Ni-alloy tapes, with FWHM of Δϕ below 10°. Y-123 films with FWHM of Δϕ below 7° were formed on them by PLD. End-to-end I_c of 38 A and J_c of 0.76 MA cm⁻² (77 K, self-field) were obtained in a 100 m long sample. For further texture improvement, secondary buffer layers of CeO₂ and Gd₂Zr₂O₇ grown by PLD on IBAD-Gd₂Zr₂O₇ template films were studied. Δϕ of 3° and J_c of 2.9 MA cm⁻² were obtained in a 0.1 m long Y-123 film and J_c improved to 1.6 MA cm⁻² for an 80 m long Y-123 tape by using the CeO₂ buffer layers.

The magneto-optical imaging technique is applied to YBa₂Cu₃O_7−δ (YBCO) thin film coated conductors, providing a visual investigation of sub-millimetre aberrations and fracture defects. The further application of post-processing techniques provides a quantitative analysis of varying patterns resulting from field penetration in the YBCO coated conductor investigations. Application of theory provides critical current information for geometries where the conduction path is perpendicular to the applied field in micro-bridge samples. A Maxwell curl calculation is performed on the intensity distribution of various images. These calculations reveal boundaries for current paths via relative screening current determination and striations.

Using a magnetic compression method a 2.3 mm diameter superconducting MgB₂ rod was synthesized in a stainless steel tube. After the compression the MgB₂ density inside the tube reached d = 2.45 g cm⁻³, i.e. very near to the theoretical value 2.62 g cm⁻³. At T = 4.2 K and B = 0.5 T the quench current reached 870 A. The critical current density versus temperature was determined from electric and magnetic measurements.

Superconducting fault current limiters (SCFCLs) can reduce fault current; thus SCFCLs allow the capacity of circuit breakers to be reduced and lend flexibility to power system network designs. Furthermore, SCFCLs are expected to improve power system stability, as has been confirmed by some experiments. Therefore, various kinds of SCFCLs were proposed and have been studied. For introducing SCFCLs in power systems, there should be some specifications, such as the impedance, the trigger current level, and the recovery time. Fault analyses pointed out that accuracy of the trigger current level is required. A transformer type SCFCL with an adjustable trigger current level was proposed, designed, made, and tested. Then, limiting tests on three-phase SCFCLs by use of an artificial transmission line with a small generator were carried out. The test results indicated that at phase-to-phase faults, the SCFCLs for the fault phases do not always turn into the limiting mode. From two points of view, it is important to study characteristics of the series connection for SCFCLs: one is as mentioned above, and the other is that a series connection of some SCFCLs of small impedance may be installed in place of an SCFCL of larger impedance. This paper describes the results of limiting tests on two SCFCLs connected in series. The tests were carried out under some different conditions of fault currents, fault phases, and trigger current levels of SCFCLs. Some discussion on characteristics of series connection of SCFCLs appears also.

We introduce an apparatus for non-destructive, contact free, reel-to-reel critical current measurement of coated conductor tape. Up to 100 m of tape can be passed in front of a sensor head for the measurement in liquid nitrogen. The tape is transferred and characterized at a speed of 10 mm s⁻¹. After the liquid nitrogen passage it is actively heated up to room temperature to avoid moisture condensation. Two different sensors in the probe head can be optionally activated: a linear Hall probe array in a field coil and an ac-measurement coil picking up third harmonic signals from the film. Once calibrated by transport measurement this arrangement enables characterization of HTS films on magnetic as well as on non-magnetic metal tape substrates. In a 10 mm wide coated conductor, up to 400 A of critical current can be measured.

We have developed a set of dc SQUIDs intended for frequency-domain multiplexed readout of transition edge sensor (TES) arrays. The first design is based on the Ketchen-style washer, and has exhibited a 1.2 × 10⁻⁷ Φ₀ Hz^1/2 flux noise both at 4.2 and 0.43 K. The second design is based on the multiloop structure and utilizes the spoke-terminating resistors which double as junction shunts in order to minimize the excess noise due to the resonance damping. The multiloop device has exhibited a 3.5 × 10⁻⁷ Φ₀ Hz^1/2 flux noise level at 4.2 K. The mutual inductances are chosen such that a typical ac-biased TES-based x-ray calorimeter would need only a moderate amount of negative feedback in order to handle the dynamic range of the signal.

Measurements of the intermodulation distortion (IMD) as a function of temperature from 1.7 K to T_c in Y Ba₂Cu₃O_7−δ are presented. Films of the highest quality show an increase in IMD as the temperature decreases for temperatures below approximately 30 K. Films of lower quality do not show the increase in the temperature range measured. The increase is in qualitative agreement with that predicted by the nonlinear Meissner effect in d-wave superconductors. The temperature dependence of the measurements is compared with a new calculation of the nonlinear penetration depth.

For the optimum design of integrated receivers operating above the gap frequency of Nb, we have designed, fabricated and tested NbN-based quasi-optical superconductor/insulator/superconductor (SIS) mixers. The mixer chip incorporates a resonant half-wavelength epitaxial NbN/AlN/NbN junction, a twin-slot antenna and their coupling circuits. We adopted two kinds of coupling circuit between the antenna and the SIS junction: one is an in-phase feed with a length of 95 µm and the other is an anti-phase feed of 30 µm length. It was found that the anti-phase mixer reveals a 3 dB bandwidth of 43% of the centre frequency; the uncorrected double-sideband receiver noise temperature T_RX = 691 K at 0.91 THz and T_RX = 844 K at 0.80 THz, while 17% and T_RX = 1250 K at 0.79 THz for the in-phase version. Possible reasons for this difference are discussed, which could be transmission loss and its robustness with respect to the variation of junction parameters. These experimental results suggest the NbN-based distributed mixer with the anti-phase feed is a better candidate for wide-band integrated receivers operating above 0.7 THz.

We have developed a high temperature superconductor (HTS) dc SQUID gradiometric measurement system for routine applications in non-destructive evaluations. Low values of the white flux noise and white field gradient noise were measured for gradiometers operating at 77 K. Low frequency noise was suppressed using an ac bias technique, even in a magnetically unshielded environment. First-order planar dc SQUID flip-chip gradiometers were used in the measurement system for stable operation under electromagnetically noisy conditions. A synchronous filter was employed to remove all harmonic components of parasitic line frequency interference. This improved the resolution of the system in a typical laboratory or industrial environment showing strong magnetic gradients of the line frequency signal due to nearby metallic constructions. Test scans of contaminations with magnetic particles were performed.

Numerical methods for calculating the current and field distribution in high-temperature superconductors under non-uniform time-varying fields are being investigated. The highly non-linear behaviour of superconductors makes them difficult to analyse and computationally expensive. This non-linear behaviour is often accounted for through a non-linear E–J constitutive law. This paper proposes a fast method based on the finite element method to solve 2D and axially symmetric problems that contain superconducting materials. An E–J power law together with an A–V formulation is used to calculate the induction of currents in the superconductor due to time-varying external magnetic fields or forced transport current. Experimental data of a magnet-above-superconductor system is obtained in order to validate the model. In the experimental set-up a magnet is brought towards a superconducting puck at different speed rates and is also vibrated on top of it. The force between the magnet and the superconductor is measured and is found to vary with both time and frequency of excitation.

Nb₃Ga wires fabricated by using RHQ (rapid heating and quenching) treatment show T_c = 29.7 K and B_{c 2}(4.2 K) = 32 T. In addition, J_c for the Nb₃Ga phase is very high, comparable to those of practical (Nb,Ti)₃Sn and RHQT (rapid heating, quenching, and transforming)-processed Nb₃Al wires in high fields. We measured their B_{c 2} and J_c at temperatures between 4.2 and 29.7 K. At 10 K and 16 T, the Nb₃Ga wire showed J_c above 100 A mm⁻², which indicates that the wire is potentially very attractive as a superconductor for fabricating a high field magnet cooled by a cryocooler. However, the overall J_c of the Nb₃Ga wire is relatively small because of the small volume fraction of Nb₃Ga phase in the wire. We tried performing the RHQ treatment at higher temperatures; this resulted in a twofold increase in the overall J_c.

Grain boundaries in oxide superconductors are of central importance both for engineering applications and for issues in fundamental science. Although they have been closely studied for more than a decade, there is still by no means a consensus on the properties of individual boundaries, or their collective behaviour when current percolates in a granular material. A quantitative description of current and flux percolation is essential for the practical specification of conductors and for charting routes to conductor optimization. There are three components to the problem: (i) the properties of individual grains and grain boundaries, (ii) microstructure, texture and grain morphology, and (iii) the macroscopic geometrical constraints such as conductor size and shape. On the basis of new results on the angular variation in an applied magnetic field of the grain boundary critical current, it will be argued that grain morphology as well as texture is important for the critical current of a coated conductor. Progress in characterizing and modelling current and flux percolation will be surveyed and particular gaps in understanding and areas of neglect discussed.

We investigated the effect of various impurity additions for MgB₂ tapes fabricated by the in situ process of the powder-in-tube method. As impurities, we tried silicides, such as WSi₂, ZrSi₂, Mg₂Si, SiO₂ and SiC. These silicide powders were added to a powder mixture of MgH₂ and B. The content of the additives was changed from 2.5 to 20 at.%. The powder mixture was stuffed into an Fe tube of 6.2 mm outer diameter and cold-worked into a tape with 0.5 mm thickness by the combination of grove rolling and flat rolling. Heat treatment was performed at a relatively low temperature of 600 °C for 1 h under an argon gas atmosphere. An XRD observation of each tape indicated that Mg₂Si was formed, except for the WSi₂-added tape. The T_c of MgB₂ decreased by all silicide additions. We obtained J_c enhancement for all of these additives, except for SiO₂. The addition of SiC increased J_c and decreased the field dependence of J_c. A high J_c value of 2.5 × 10⁴ A cm⁻² at 4.2 K and 10 T was obtained for the MgH₂+B powder mixture and SiC addition. The irreversibility field of the MgB₂ tape was enhanced from 17 to 22 T by the SiC addition. However, at low fields of  T, the J_c enhancement by SiC addition seemed to be very small.

We have developed a scanning magnetic microscope (SMM) with 25 µm resolution in spatial position for the magnetic features of room temperature objects. The microscope consists of a high-temperature superconductor (HTS) dc SQUID sensor, suspended in vacuum with a self-adjusting standoff, close spaced liquid nitrogen Dewar, X–Y scanning stage and a computer control system. The HTS SQUIDs were optimized for better spatial and field resolutions for operation at liquid nitrogen temperature. Measured inside a magnetic shield, the 10 pT Hz^−1/2 typical noise of the SQUIDs is white down to frequencies of about 10 Hz, increasing up to about 20 pT Hz^−1/2 at 1 Hz. The microscope is mounted on actively damped platforms, which negate vibrations from the environment as well as damping internal stepper motor noises. A high-resolution video telescope and a 1 µm precision z-axis positioning system allow a close positioning of the sample under the sensor. The ability of the sensors to operate in unshielded environmental conditions with magnetic fields up to about 15 G allowed us to perform 2D mapping of the local ac and dc susceptibility of the objects.

For a long coated conductor fabrication, SRL-Nagoya Coated Conductor Center (NCCC) has installed reel-to-reel equipment for IBAD and PLD systems. A 60 m long IBAD and a 55 m CeO₂ capped substrate have been successfully produced. The PLD-CeO₂, which we call the self-epitaxial method, is found to be effective to increase the in-plane alignment and processing rate of buffer layers. PLD-YBCO layers on the self-epitaxial PLD-CeO₂ cap layer exhibited a high degree of in-plane texturing about 3° and thus high J_c up to 4.4 MA cm⁻² and high I_c of 276 A cm⁻¹ width. Long YBCO conductor fabrication using reel-to-reel PLD equipment is now underway.

A project for a 30 m, 35 kV /2 kA_rms, 3 phase, warm dielectric HTS power cable system is underway in China. This system will be installed in the China Southern Power Grid at the Puji substation in Kunming, Yunan province, in 2004. We have carried out a series of experiments to investigate the possible winding angles and layer configurations of the cable conductor with Ag sheathed Bi-2223 tapes, different LN₂ cooling mechanisms, termination configurations, and fabrication techniques. For better understanding of the basics of an HTS cable system and practising new fabrication techniques, a 4 m superconductor cable with terminations and a closed cycle cryogenic system was built and tested. In this paper, we give the detailed parameters of the 30 m cable system and the work plan of the project.

Ultrafast voltage transients from optically thick YBa₂Cu₃O_7−x (YBCO) microbridges, dc-biased and triggered with femtosecond optical pulses at temperatures below the YBCO critical temperature were used to excite radiation of a transmitting gigahertz-frequency antenna. The shape of the power spectrum of the antenna radiation depended on the time evolution and amplitude of the YBCO photoresponse transient, which was controlled by the incident laser fluence and the microbridge temperature and bias current. The main contribution to the above 10 GHz radiation was attributed to the photoresponse rising edge, generated due to the initial supercurrent redistribution and the kinetic-inductive effect. Low-frequency antenna radiation was excited by the photoresponse component associated with heating and cooling effects generated in the superconducting microbridge. Our antenna radiation approach can be used for time-resolved investigation of electric current distributions in photoactivated YBCO thin films.

A novel buffer layer architecture consisting of LaMnO₃/MgO/TiN is proposed as a suitable structural and chemical template for the epitaxial growth of high-T_c superconductors on Cu metal surfaces. For the first time, high J_c values are reported for YBCO films grown by laser ablation on (001) Cu single crystals, textured Cu surfaces and textured Cu–48% Ni–1% Al alloys, without intervening metal coatings. The J_c for single-crystal-like substrates is as high as 3.5 MA cm⁻² and values of 2 MA cm⁻² were obtained on the Cu–alloy tape substrates.

We report on Josephson effects in Josephson junctions fabricated from magnesium diboride (MgB₂) thin films using a focused ion beam (FIB) milling technique. The films were deposited on SrTiO₃(100) and substrates at room temperature using e-beam evaporation of MgB₂ with a post-annealing stage in Mg vapour. Conventional photolithography and Ar-ion-beam milling were used to pattern the films into 4 and 8 µm wide microbridges. A focused ion beam was used to narrow the microbridges to 2 µm. The narrowed microbridges were thinned by making 50 nm wide cuts across them. The depth of each cut was calibrated to remove 75% of the film thickness. A thin MgB₂ layer remained on the substrate after this process. The current–voltage (I–V) characteristics of junctions made using this technique show that the junctions carry excess current. The first Shapiro step was observed when one of the junctions was irradiated with a microwave field of frequency f = 8.92 GHz. The Shapiro step appeared at a voltage value V = hf/2e = 18.445 µV.

We have performed a comparative study of two thin films of magnesium diboride (MgB₂) grown by different techniques. The critical current density at different temperatures and magnetic fields was evaluated from magnetization curves, the structure of the superconducting order parameter was obtained from point-contact spectroscopy and the scattering rates were evaluated by fitting the temperature dependent normal-state resistivity to the two-band model. The films have similar critical temperatures close to 39 K, but the upper critical fields were different by a factor of 2 (5.2 and 2.5 T at 20 K). We have found that the film with higher H_c2 also had stronger scattering in the σ band and a smaller value of the superconducting gap in this band. As the scattering in the σ band is primarily due to the defects in the boron plane, our results are consistent with the assumption that disordering the boron planes leads to enhanced H_c2 and better pinning properties in a magnetic field.

Both experimental and numerical studies of a self-frustrated triangular array of π-junctions are reported. The array of SFS Josephson junctions shows a transition to the π-state and self-frustration with a decrease in temperature. This manifests itself in a half-period shift of the bias critical current versus applied magnetic field. At temperatures close to the 0–π transition this dependence shows a doubling of its periodicity frequency that can be explained by 0–π bistability of the SFS junctions. The change in the array behaviour with number of unit cells has been studied by means of numerical simulation.

In this work we predict the nonlinear behaviour of an eight-pole quasi-elliptic bandpass high temperature superconducting (HTS) filter with an equivalent circuit extracted from intermodulation measurements performed at the centre of the filter passband. We present measurements that show that the equivalent circuit is able to predict the intermodulation products produced by the filter when driven by two in-band or out-of-band sinusoidal signals. Numerical techniques based on harmonic balance are used to extract the elements of the equivalent circuit and to simulate its nonlinear performance.

In mono- and multifilamentary MgB₂ wires with small filament diameters a homogeneous and fine-grained MgB₂ phase is required for high transport currents and reactions with the sheath have to be avoided. Taking this into account we developed very thin steel reinforced MgB₂ wires for application as current leads in a research satellite. In situ phase formation in the wire from Mg+B powders was chosen as the most favourable method to achieve good grain connectivity and thus high transport currents. We tested the current lead wires with transport current measurements and investigated the mechanical performance with applied tensile and bending strains. With an improved processing route applying a low-temperature anneal the superconducting properties of standard undoped in situ MgB₂ wires could be further improved and critical current densities of 38 kA cm⁻² at 6 T and 10 kA cm⁻² at 8 T, 4.2 K could be achieved.

First, this paper gives an overview of possible origins and mechanisms that might be responsible for experimentally observed nonlinear effects. Focusing on well-known physical loss mechanisms with nonlinear characteristics, their impact on microwave properties and their relevance in microwave devices are discussed. Secondly, an experimental investigation of the power handling capability of coplanar resonators in additionally applied dc magnetic fields in zero field cooled, field cooled, and field sweep experiments is presented. These experiments demonstrate that the microwave power handling capability of optimized high-T_c thin film devices is limited by a maximum current density, and suggest that the limitation is caused by the dc critical current density J_c of the superconducting material.

The dc characteristics and magnetic field dependences of Y–Ba–Cu–O bicrystal grain boundary junctions (BGBJs) and step edge junctions (SEJs) were investigated for fabrication of rf-SQUIDs. Test junctions with up to 8 µm widths as well as the junctions of the two types of junction-based rf-SQUID were studied. The SEJs typically showed lower J_c and higher ρ_N as compared to the BGBJs, resulting in close I_cR_N products. All the BGBJs showed classical field dependent I_c following their junction width, resembling Fraunhofer patterns. The field sensitivity of the BGBJs' I_c led to low yield submicron BGBJ rf-SQUIDs partially impaired by the Earth's magnetic field. Two major behaviours of low and high field dependences of I_c were observed for the SEJs. Only the low field-sensitive SEJs resulted in micron size junction rf-SQUIDs not impaired by the Earth's magnetic field. The low field-sensitive SEJs led to low 1/f noise magnetically stable rf-SQUIDs appropriate for applications in unshielded environments at 77 K.

Microwave responses of intrinsic Josephson junction stacks in the flux-flow state have been studied to explore phase-locked vortex motion. Remarkable step and zero-crossing structures are induced in the I–V characteristics by microwave irradiations. It is shown that vortex motion phase-locked to external microwaves plays an important role in the responses. Numerical simulations reveal that the alternating magnetic field of microwaves drives vortices into the stack and generates the zero-crossing step.

We present the experimental observation of the effects of macroscopic quantum tunnelling in a SQUID device, consisting of a rf SQUID coupled to a readout system based on a dc SQUID sensor. Data on the decay rate from the metastable flux states of a rf SQUID are reported, both in the classical and quantum regimes. The low dissipation level and the good insulation of the probe from external noise are encouraging in view of building a macroscopic quantum coherent system.

Several high field Nb₃Sn magnets of different design are under development for future particle accelerators. The high levels of stored energy in these magnets and the high current densities in the conductor can cause high peak temperatures during a quench. The thermal gradients generated in the epoxy-impregnated magnet coils during the fast temperature rise can result in high thermo-mechanical stresses. Considering the sensitivity of Nb₃Sn to strain and epoxy cracks, it is important to define a maximum acceptable temperature in the coils during a quench which does not cause degradation of the magnet performance. A program was launched at Fermilab to study the effects of thermo-mechanical stress in Nb₃Sn coils, supported by experiments and by analysis. In collaboration with LBNL, a sub-scaled magnet was built and instrumented to measure the effect of the thermo-mechanical shock during magnet quenches. The magnet consisted of two racetrack coils, assembled in a common coil configuration with a small gap in between. During the test, the magnet reached the maximum field of  T at the short sample current of 9100 A. Temperature excursions up to 400 K did not diminish the magnet quench performance; only after temperature excursions over 430 K did the magnet show detraining effects, which occasionally reduced the quench current to about 6%. Signs of irreversible degradation (reducing the maximum current of about 3%) appeared only after temperature excursions over 550 K.

High-temperature superconducting (HTS) transformers and reactor coils promise decreased weight and volume and higher efficiency. A critical design parameter for such devices is the AC loss in the conductor. The state of the art for AC-loss reduction in HTS power devices is described, starting from the loss in the single HTS tape. Improved tape manufacturing techniques have led to a significant decrease in the magnetization loss. Transport-current loss is decreased by choosing the right operating current and temperature. The role of tape dimensions, filament twist and resistive matrix is discussed and a comparison is made between state-of-the-art BSCCO and YBCO tapes. In transformer and reactor coils the AC loss in the tape is influenced by adjacent tapes in the coil, fields from other coils, overcurrents and higher harmonics. These factors are accounted for by a new AC-loss prediction model. Field components perpendicular to the tape are minimized by optimizing the coil design and by flux guidance pieces. High-current windings are made of Roebel conductors with transposed tapes. The model iteratively finds the temperature distribution in the winding and predicts the onset of thermal instability. We have fabricated and tested several AC windings and used them to validate the model. Now we can confidently use the model as an engineering tool for designing HTS windings and for determining the necessary tape properties.

We report on the optimization of electron cooling by SIN tunnel junctions due to the advanced geometry of superconducting electrodes and very effective normal metal traps for more efficient removal of quasiparticles at temperatures from 25 to 500 mK. The maximum decrease in electron temperature of about 200 mK has been observed at bath temperatures 300–350 mK. We used four-junction geometry with Al–AlO_x–Cr/Cu tunnel junctions and Au traps. Efficient electron cooling was realized due to the improved geometry of the cooling tunnel junctions (quadrant shape of the superconducting electrode) and optimized Au traps just near the junctions () to reduce reabsorption of quasiparticles after removing them from normal metal. The maximum cooling effect was increased from a temperature drop of d T = −56 mK (ordinary cross geometry) to −130 mK (improved geometry of superconducting electrodes) and to d T = −200 mK (improved geometry of superconducting electrodes and effective Au traps).

The heating peak (instead of cooling) near the zero voltage across cooling junctions has been observed in practice for all samples at temperatures below 150 mK. For higher cooling voltages close to the superconducting gap, the heating was converted to cooling with decreased amplitude. The leakage resistance of the tunnel junctions gives a reasonable explanation of the heating peak. The phonon reabsorption due to the recombination of quasiparticles in superconducting electrodes gives an additional improvement in the theoretical fitting but could not explain the heating peak.

An anomalous zero-bias resistance peak has been observed for all tested structures. The peak is explained by Coulomb blockade of tunnelling in transistor-type structures with relatively small tunnel junctions.

The work on electron cooling is devoted to the development of a cold-electron bolometer (CEB) with capacitive coupling by SIN tunnel junctions to the antenna for sensitive detection in the terahertz region. Direct electron cooling of an absorber plays a crucial role in supersensitive detection in the presence of a realistic background power load.

The basic requirements providing the unique properties of a superconducting quantum interference filter (SQIF) are discussed. The results of a numerical simulation of the oscillation linewidth for a parallel SQIF are reported and compared with those for a conventional parallel array and single Josephson junction.

The magnetic systems of superconducting wires and tapes, consisting of a magnesium diboride (MgB₂) superconducting core surrounded by a ferromagnetic iron (Fe) sheath, have been investigated. The interaction of the superconductor with the soft magnetic environment has been experimentally shown to lead to significant enhancement of dissipation-free super-current densities. The maximum densities of the super-currents can exceed the critical current densities obtained in the same 'bare' wire (with its iron sheath removed) by more than one order of magnitude under the same experimental conditions. This current density enhancement is referred to as the overcritical state, which has been observed over a wide range of magnetic fields applied transversely to the wire. No overcritical currents are observed for the longitudinal field orientation. The irreversibility field is shown to be considerably suppressed by the influence of the iron sheath for both field orientations. Different geometries of the wire cross section have been investigated.

Superconducting cables offer the advantages of lower loss, lighter weight and smaller dimensions, as compared to conventional cables. A 4 m, 2 kA, AC, HTS cable system was developed for the purpose of study. The cable conductor was made of Bi-2223 tapes. A space maintained as a vacuum between two corrugated stainless steel tubes functions as a cryostat surrounding the superconductor. A closed cycle liquid nitrogen cooling system was exploited. The cable was matched with two terminations, which connect the cable conductor at cryogenic temperature to an outside common electric conductor at room temperature and offer a path for liquid nitrogen. A series of tests were carried out to verify the functions of the system. Important data were obtained which can be used for making longer HTS cable systems.

We have developed a new multiseeding method, the MUSLE (MUlti-Seeded seamLEss bulk) technique, for alleviating a problem of the conventional multiseeding method, i.e. the existence of excluded non-superconducting phases (liquid phases and segregated RE211 phases) at the grain boundaries orientated by the multiple seeds in the bulk superconductor. The basic concept of this technique is that the precursor is composed of two or more RE–Ba–Cu–O layers with different peritectic temperatures. In order to study the superconducting prospects for MUSLE bulk, RE–Ba–Cu–O bulk superconductors of various sizes up to 100 mm in diameter were produced by the MUSLE technique. All trapped field distributions for the RE–Ba–Cu–O bulk superconductors had a single peak. For the 46 mm diameter MUSLE bulk, J_c(B) across the multiseed-orientated grain boundary was comparable to that in the grain. The misorientation angle of the c-axis between the multiseed-orientated grains was approximately 3°–7° in this sample, whereas each J_c(B) for the specimens across the multiseed-orientated grain boundary has the same level. We also estimated the macro-J_c flow in bulk superconductors through the peak values of the trapped fields. The macro-J_c for the MUSLE bulk was larger than that for the single-seeded bulk at sizes greater than 65 mm diameter.

We have investigated the microwave properties of a set of four identical YBa₂Cu₃O_7−δ (YBCO) films. One of the films has a 30% Ca-substituted YBCO overlayer, whereas the three others are used as references to highlight the effect of the Ca substitution. The microwave characterization was carried out using the stripline-resonator technique. The R_s of the sample that has a Ca-rich overlayer increased slightly in the linear regime, whereas no significant effect on the nonlinear components and the intermodulation distortion has been observed. We conclude that Ca doping does not significantly improve the nonlinear microwave properties of YBCO films, but the small changes in the linear surface resistance that we observed are discussed in terms of the physics of Ca doping of YBCO.

In this paper the performances of superconducting devices based on fractal layout are discussed. Depending on their shape, fractals are characterized by self-similarity and/or space-filling properties. The former allows the design of superconducting filters with highly reduced dimensions and no appreciable loss of performance, whereas the latter is the basis for the fabrication of multi-band antennas which hold promise for very interesting applications. Our attention was focused on the square Koch loop and the Sierpinski gasket geometries. In particular, prototype four pole filters for UMTS (Universal Mobile Telecommunications Systems) applications based on Koch resonators have been designed. Measurements performed on prototype devices fabricated by using double-sided superconducting thin films are reported and compared with simulations.

We propose a novel magnetic sensor to apply a new phenomenon in high T_c superconductors of strongly anisotropic materials. We have found periodic oscillations in the flux-flow resistance of Josephson vortices in intrinsic Josephson junctions of single crystalline Bi₂Sr₂CaCu₂O_8+y, fabricated by using a focused ion beam. The periodic oscillations in the flux-flow resistance have been observed at higher magnetic fields than 70 kOe at low temperatures. The period depends only on the junction width perpendicular to the magnetic fields and the current, independent of temperature and magnetic field. The detailed analyses of the oscillations show that the characteristics of the flux-flow device can be applied as magnetic sensors to detect an absolute magnetic field and to measure the field precisely to an accuracy in the 1 mOe range, even under magnetic fields as high as 70 kOe. We believe that this kind of sensor opens up new applications of high T_c superconductors.

We present recent advances in the development of NbN hot-electron bolometric (HEB) mixers for flying terahertz heterodyne receivers. Three important issues have been addressed: the quality of the source NbN films, the effect of the bolometer size on the spectral properties of different planar feed antennas, and the local oscillator (LO) power required for optimal operation of the mixer. Studies of the NbN films with an atomic force microscope indicated a surface structure that may affect the performance of the smallest mixers. Measured spectral gain and noise temperature suggest that at frequencies above 2.5 THz the spiral feed provides better overall performance than the double-slot feed. Direct measurements of the optimal LO power support earlier estimates made in the framework of the uniform mixer model.

Significant reduction in AC transport self-field losses has been achieved at 77 K for Ag-sheathed Bi2223 multifilamentary tapes with m × n (m,n = 3 or 5) filament arrangements, fabricated via a rectangular deformation process using two-axis rollers. This achievement is not effected by introducing resistive barriers but by changing filament arrangements. Despite the main contribution being the hysteresis loss of the superconductor, the loss values are decreased by approximately 50–70%, compared with the values for multifilamentary tapes obtained by a standard powder-in-tube process. More complicated filament arrangements achieved via a rectangular deformation process lead to further reduction in the loss values. On the basis of numerical calculations, the loss reduction is mainly ascribed to a division of the field-free core in the filament groups near the tape edge under AC current transmission. The study should pave the way to removing an obstacle to loss reduction and to developing tape strands with low loss values satisfying practical demands.

OSWALD Elektromotoren GmbH, Miltenberg, Germany, is experienced in direct linear motors with high power density. For some special applications the requirements are extremely demanding and almost beyond reach, even for state-of-the-art linear motors operated at room temperature. In order to obtain even higher power density, and hence also increased acceleration, we have to develop a new technology for linear motors. In this paper we propose a round superconducting linear motor. The stator windings will be made of YBCO coated conductors in the shape of double pancake coils. The frequency will be limited to approximately 10 Hz for this application. The actuator will be provided with NdFeB permanent magnets. The special cryogenic design will be discussed. The cooling of the stator will be provided by lN2 at 77 K or lower. The calculated force of this SC linear motor is 10 000 N. The force density will be 2–3 times more than the normal conducting counterpart of the same dimensions.

A highly sensitive cryogenic current comparator (CCC) for the measurement of the intensity of faint beams, such as a radioisotope beam, was developed for the RIKEN RI beam factory project. This monitor is composed of a high-temperature superconducting (HTS) magnetic shield and an HTS current sensor including an HTS superconducting quantum interference device (SQUID), which are cooled by a low-vibration pulse-tube refrigerator. Both the HTS magnetic shield and the HTS current sensor based on Bi₂–Sr₂–Ca₂–Cu₃–O_x (Bi-2223) were fabricated by dip-coating on a 99.9% MgO ceramic substrate. Recently, a prototype of the HTS SQUID CCC system was completed and we carried out the first measurements using DC ion beams in the current range of 0.5–20 µA, which were produced by the ECR (electron cyclotron resonance) ion source in the CNS experimental hall. After the measurement was successfully carried out in the CNS, we installed the system into the beam transport line of the RIKEN Ring Cyclotron (RRC) to measure the current of the high-energy heavy-ion beam which has a bunched microstructure. In this paper, we describe the performance and the present status of the prototype of the HTS SQUID CCC system and the results of the ion beam measurement.

The physics of the interaction between coherent light and superconductivity is a very intriguing subject but the alignment of a laser beam with integrated superconducting systems (thin films, striplines or tunnel junctions) has represented so far a real bottleneck for the experimentalists. The integration of both junctions and waveguides on the same chip surely represents a possible path around this difficulty. In this paper we report on the combined integration of optical structures based on hybrid channel waveguides with superconducting Josephson junctions circuits. The optical films are grown on silicon substrates by a spin-coating process using the hybrid organic/inorganic sol–gel route. The channel waveguides are deposited on a buffer layer and realized by photolithographic and ion milling techniques. We find that the optical buffer is a good substrate for SIS Josephson junctions and circuitry based on the Nb/Al–AlO_x/Nb trilayer processing. The junctions produced present good current–voltage characteristics with critical current density of the order of 1 kA cm⁻², high subgap resistance and good uniformity. Our results show that the quality of the Josephson junctions is well preserved even when they are grown on the sol–gel buffer layer.

This work describes the research and development of a cryogenic pump that is intended for the fuel supply of aircraft engines using advanced low temperature fuel. The basic design is that of a 4-pole reluctance motor. The rotor is constructed from soft iron and BSCCO/Ag laminated material; the latter was developed by the All-Russian Scientific Research Institute of Inorganic Materials. The motor was integrated with a centrifugal cryogenic pump for the cryogenic fuel supply system, developed by the TUPOLEV Company. The results of theoretical modelling and experimental investigations are presented.