YBa₂Cu₃O_7-x superconducting thin films were prepared using the Excimer laser ablation technique. The current-voltage characteristics of these films was measured under various ramp current waves with rise rates of 50A/s-10kA/s. It was shown that the resistance of the flux flow increased with current rise rate. Critical current density decreased with increasing the current rise rate. The critical current density-temperature characteristics of the YBa₂Cu₃O_7-x superconducting thin films were consistent with a proximity effect junction behavior. The frequency dependence of the critical current density was found to be attributed to the electon diffusion coefficient (electron mobility) in the normal conductor layer of the superconductor-normal conductorsuperconductor (SNS) junction.

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Using the datas reported so far on the mechanical properteis of Bi/Ag oxide superconductings of tape conductors, critical issues on designing a medium size, high field magnet are discussed. The mechanical properties of the conductors limit the scales of asuperconducting magnet into a narrow region of the radius and the generating magnetic field. In order to constract a large scale magnet beyond the limit, an intensive improvement the mechanical properties such as irrevasible strain and matrix yield stregth should be done on Bi/Ag oxide superconducting tape conductors.

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A microwave pulse source using a Y-Ba-Cu-O (YBCO) superconducting thin film switch was studied. The YBCO thin film with a thickness of 0.2μm and a critical temperature of about 90K was fabricated by a laser ablation technique. It was installed in a cylindribcal TE011 mode cavity made of copper with a resonant frequency of 24.0GHz. The cavity was set inside a cryostat and cooled to 20K. The unloaded Q values of the cavity and the CW microwave power transmitted through the film were measured. Microwave pulses were extracted from the cavity by the switching from superconducting to normal states in the film, using a 1.06μm pulsed Q-switched Nd; YAG laser with a duration of 10ns. The peak power of the output pulses obtained below 40K at the incident laser energy density of 76.0mJ/cm2 was larger than the CW input power of 7.0mW.

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We have investigated a correlation between structure and superconducting properties ofTl₁Ba₂Ca₂Cu₃O_x [Tl-(1223)] thin films. The Tl-(1223) thin films were prepared on SrTiO₃, LaAlO₃ and MgO (100) substrates by Metalorganic deposition method. Films grown on SrTiO₃ and LaAlO₃ exhibit a particularly high degree of epitaxial growth, which may be attributed to the remarkably close lattice matching. Then, these films had high Jc>10₅A/cm₂ at 77K in a zero magnetic field. However, in case of MgO, the film was distributed randamly and had low Jc of about 10₃A/cm₂ at 77K in 0T. Moreover, in case of magnetic field applied perpendicular to the a-b plane, we found that the field transport properties of the Tl-(1223) thin films were superior to those of the Tl₁Ba₂Ca₂Cu₃O_x[Tl-(2223)] andBi₂Sr₂Ca₂Cu₃O_x[Bi-(2223)] thin films. On the other hand, grain boundary properties of the Tl-(1223) thin films depended on (001) tilt angles of grain boundaries. We found that low tilt angle (θ_??_10°) grain boundaries did not work as a weak link and retained high Jc However, it is well known that grain boundaries for (θ_??_5°) in Y₁Ba₂Cu₃O_x[Y-(123)] thin films are weak links. It suggests that the Tl-(1223) system is superior to Y-(123), Tl-(2223) and Bi-(2223) systems as a material for high field applications at high temperatures.

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We investigated the formation process, oricntation of grains and surface morphology of CeO₂ films on Al₂O₃ substrates, and YBCO films on CeO₂/Al₂O₃ substrates. CeO₂ films were prepared by electron beam deposition and MOCVD methods. It was found from X-ray analysis that orientation of grains of CeO₂ films was affected by substrate temperature, Ts, and deposition rate, Rd. (100) plane oriented films were prepared as following conditions, Ts-650°C, Td-4A/s, YBCO films on CeO2/Al2O3 substrates were prepared by MOCVD. It was found from X-ray pole figure measurement that the configuration of the crystal axes of YBCO, CeO₂ and Al₂O₃ was as follows; YBCO<110>//CeO₂<100>//Al₂O₃<-110>. Surface morphology of CeO₂ and YBCO films was observcd by Atomic Force Microscope (AFM). It was found that CeO₂ films were smooth surface containing of small projection, andYBCO films were columnar structure which size was about 0.5μm. Tc of these films were about 80K.

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Light irradiation decreases critical current and increases voltage generated in flux flow state in Bi base oxide superconductor. The light of short wave length (<440nm) is effective for acceleration of S-N transition, although light absorption characteristics of specimen is independent of wave length. Voltage deviation observed in V-I characteristics drastically increases under light irradiation. Without light irradiation, even specimen temperature is increased by pressurization of liquid nitrogen, voltage deviation remains small. Magnetic field <200 G gives little change to light irradiation effect on V-I characteristics. However the voltage deviation decreases with magnetic field strength. Temperature rise due to light absorption seems not to be the main cause of acceleration of S-N transition, although it takes certain part. Light irradiation may help magnetic flux to escape from pinning center through electronic mechanism. Proposed model, in which light irradiation causes avalanche of flux and therefore large voltage generation, gives a qualitative explanation to the experimental results.

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Electrical breakdown phenomena have been studied by using electrode systems with different finned surface, which was machined for promoting the heat transfer from the electrode to liquid, in order to find an optimum fin shape for dielectric performance under the quenching condition of superconducting coils. The results show that the breakdown voltage as a function of heat generation in the finned electrode closely relates to the thermal-bubble behavior under an electric field. The bubble behavior depends strongly upon the distribution of electric field in the gap and the combination of electrical gradient force and buoyancy. It is argued in conclusion that if the grooves are formed to guide a bubble trapped in the groove from a region of higher electric field to lower one the reduction in breakdown voltage due to thermal bubble is prevented. The circular groove is very effective to avoid the influence of thermal bubble on the breakdown voltage.

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In multiply-twisted superconducting cables placed in a transverse external magnetic field, the strands of multifilamentary wires are exposed to a complicated field whose amplitude and angle to the wire axis vary spatially. Due to the twisting structure of filaments in the strand, the presence of the parallel field component to the strand axis leads to the occurrence of extra ac loss (longitudinal ac loss), in addition to the hysteresis loss in the filaments and the usual coupling-current loss among the filaments for the transverse component to the strand axis. In order to evaluate the total ac lasses of strands in multiply-twisted cable, we have measured the ac loss of short samples exposed to ac magnetic fields by varying the angle to the sample axis between 0°to90°by an electromagnetical method. The measured longitudinal ac loss for the pure parallel field and coupling-current loss for the pure transverse field could be explained well by theoretical formulations. Furthermore, the measured losses for the 60Hz magnetic field with oblique angle to the wirealso could be explained as a sum of the hysteresis loss, the coupling-current loss for the transverse component and the above-mentioned longitudinal loss for the parallel component.

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High critical current densities, such as 1.61×10¹⁰A/m² at IT, 6.1×10⁹A/m² at 3T and 2.9×10⁹ A/m² at 5T were achieved by optimizing Nb artificial pins in NbTi multifilamentary superconducting wires for AC use. The obtained critical current densities are over two times higher than those in conventional AC wires, because the position of the peak of pinning force density could be shifted in the wide range of the magnetic field by controlling the pinning parameters. The usefulness of increases in critical current densities with regard to wire for AC applications is clarified, furthermore, a 2.5T/100KVA AC superconducting magnet was fabricated by using high-Jc AC wire having designed Nb artificial pins. Compared with conventional versions, this magnet features drastic improvements with regard to compactness as well as reductions in AC losses.

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We have been studying about the superconductig wires for AC use by Powder-Metallugy Process. Nb and Cu-1%Mn powders preparedby PREP (Plasma Rotating Electrode Process) were used, because the PREP powder is not contaminated by oxigen and has good workability. The starting material is the mixture of Nb and Cu-1%Mn powders, and the ratio of mixteure is (Cu-1%Mn)-15 to 50%Nb. We measured and estimated the critical current densities and the hysteresis losses of the wires.
The critical current densities are propotional to the Nb concentration powered by 2. As the Nb concentration is increased, the effective filament diameters and upper critical magnetic field increase.
As the superconducting fiaments are discontinuous in the powder-metallugy processed wires, the critical current density depends on the frequency of bridging among superconducting filaments. Moreover the filaments'bridging causese the increase of the effective filament diameters.

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Voltage response to light pulse was investigated in Bi-based bulk and Y-based thin film specimens immersed in liquid nitrogen. When light pulse is applied to a bulk specimen under constant current flow, voltage rise is observed. The voltage rise increases with light intensity and current. Relationship between voltage rise due to light pulse and electric field due to flux flow before light irradiation shows almost the same straight line in six specimens. In thin film specimen, voltage response is pulse like and large in comparison with bulk specimen. Time-integrated value of voltage response increases with light intensity and current; the same tendency with bulk specimen is observed. Voltage response is delayed from light pulse. Delay time decreases with light intensity. Minimum delay time of 0.02ms was obtained in thin film specimen.

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The distribution and the behavior of vortices on a Bi2212 single crystal were observed by Bitter technique at a decoration temperature of 35-50K. For the sample cooled under a magnetic field, the movement of the vortex during the Ni decoration (5min.) was small, and a vortex lattice and a vortex chain were observed. The vortex lattice was observed when the sample was cooled under high magnetic field (_??_45G), while vortex chain was observed whenthe sample was cooled under low magnetic field (_??_35G). On the other hand, for the sample cooled under no magnetic field, the movement of the vortex during 5 min. was quite large, and striped pattem of Ni decoration was observed.

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Because the superconducting bulk materials have grains of various sizes, its magnetic properties are not uniform. Therefore it is necessary to develop a nondestructive measurement method to characterize the superconducting bulk materials. This paper describes measurement method of magnetic property distribution of the high temperature superconducting bulk materials.
We developed a magnetic field visualization system which produces visual images of the magnetic field on a computer display. The magnetic flux on the surface of YBCO bulk material under the field of -500 to 5000e was measured by this system and visualized as two dimensional graphics. The pattern of the flux distribution roughly agreed with the grain morphology. The experimental results show that this method is useful to assess the property distribution of superconducting material.

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Based on the concept of epitaxy in a solid phase, design and structure determination of new high-Tc superconductor were carried out. Three factors to prepare a cuprate with a CuO₂ and a new blocking layer were elucidated. The first is the difference between an a-axis length between the CuO₂ and the blocking layer. The second is a kind of B site element of the blocking layer, ABO₃. As a B site element, a typical element is more suitable than a transition one. The third is valency of an A site ion of the blocking layer, ABO₃. Based on the concept, a new cuprate, Sr_1.1Nd_0.9GaCuOy was prepared and its structure was determined. Its space group is Ima2 with a lattice parameter of a=16.271A, b=5.5198A and c=5.3301A. This compound is not superconducting but semiconducting, probably resulting from low hole carrier density of the system.

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Magnetically controlled superconducting switches are rectifier elements by use of transition be tween superconducting state and normal ones of superconductors due to changes of magnetic field around the conductors. There are few discussion whether the switches will be applicable or not. The reason of it may be that there are few detailed considerations on the switching behavior. In above mentioned standpoint, we have been studying the switching behavior experimentally and the oretically. The gate wires which have two states of superconducting and normal are requested to have properties of high resistivity in normal state and high thermal conductivity. According to Wiedemann-Franz low, the proper matrix material cannot be chosen. The request can be satisfied by the gate wire on a copper thin cylinder. By use of such improved switches that we made, we carried out experiments of switching characteristics. This paper describes theoretical considerations on the switching characteristics, which can explain the characteristics obtained by the experiments.

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To investigate the unbalanced current distribution problem in a high-current capacity superconducting AC/pulse conductor, we have carried out the measurement of the self-inductance of each element wire and the mutual inductances between each element wires in a sub-size conductor for the DPC coil. Substituting these figures into the circuit equations, we have calculated the distributed current of each element wire. The calculated values agree well with the measured current. The dispersions of the measured self-inductances is very small. And, the difference between the self-inductances and the mutual inductance is also very small. In a conventional situation, this extent of small dispersion of the self-inductances can be regarded to be small enough. However, according to our calculation, in a situation of a high-current superconducting AC/pulse conductor, this small deviation generates large current unbalance up to about factor of 3. This result can be explained by the zero resistance of the element wire and the good magnetic coupling between the element wires (k=1), which generate large current unbalance even for the very small unbalance of self-inductances. Meanwhile, it is estimated that the current unbalance is amplified by the outbreak of an electrical contact between two element wires particularly at a point near the end of the conductor.

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We measured dc and ac breakdown voltages in liquid nitrogen with coaxial cylindrical electrode configuration under thermal bubble condition. Experimental results revealed that the breakdown voltage decreased as the bubble volume VBub increased. We introduced "effective bubble volume: Veff" in the gap space in order to investigate the volume effect on the breakdown voltage. It was verified that the breakdown strength in liquid nitrogen decreased with increasing Veff, and finally got saturated at the breakdown strength in nitrogen gas; the volume effect on breakdown strength appeared.
Moreover, we introduced the effective stressed liquid volume ESSLV. ESSLV was defined by assuming that the increase of weak points in the presence of thermal bubble was equivalent to the increase of the stressed liquid volume. It was concluded that the introduction of ESSLV allowed to quantitatively interpret the degradation of the breakdown strength in liquid nitrogen in terms of the volume effect under thermal bubble condition.

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The critical current density of the highly oriented YBa₂Cu₃O_x bulk superconductors was measured at 77 K as a function of size and volume fraction of dispersed Y₂BaCuO₅ phase. The obtained critical current density increases with a decrease in the Y₂BaCuO₅ particle size and an increase in its volume fraction. Thisfact means that the Y₂BaCuO₅ phase contributes to increasing the pinning force. However, the dependence of the obtained values upon the morphology of the Y₂BaCuO₅ phase suggests that the pinning force by theY₂BaCuO₅ phase itself is comparable to that by other pinning sites which were inevitably formed during processing. The critical current density under the flux creep was tried for estimation using the virtual critical current density in the flux creep-free case calculated from a linear summation and analytical data of morphology of the Y₂BaCuO₅ phase.

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In order to practically use High-Tc current lead using silver-sheathed wires, low heat leak properties and mechanically reinforced structure are needed.
Heat leak of 0.28W/kA/lead (about 1/4 compared with Cu lead) was obtained by reducing the volume of the silver sheath wires by means of reducing the silver ratio, improving the Jc and using a step-like structure that the wires are stacked thick at high temperature region and thin at low temperature region, because the heat leak highly depends on the heat conduction through the silver sheath. Mechanically reinforced structure was obtained by fixing the wire on a FRP using a epoxy resin, the superconducting properties of the wire could be kept under an electromagnetic force and thermal cycles. The current carrying capacities from 350A to 2.5kA were obtained by changing the number of the stacking wires and conecting some units parallely. The current leads were practically used for storing capacity 400kJ, 350A SMES magnet, and superconducting compact syncrotron radiation ring (2kA). Besides, low heat leak of 0. 108W/kA/lead was achieved using Ag alloy with Au (1at%) which shows lower heat conduction ratio than pure Ag.

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Electromagnetic properties of sub-micron filament Nb-Ti superconducting double stack wires with CuNi-Mn alloy matrix for AC use are described. The Nb-Ti superconducting wires are prepared, (1) changing Mn contents (0-1.78wt%) in Cu-30wt%Ni-Mn alloy matrix, (2) the vadation of ratio of filament spacing (dn) to filament diameter (df), (3) due to increasing Nb-Ti ratio (λ), the structufes of wires are varied from the triple stack in the past papers^1)-3) to the double stack, and the copper ratio of wires are deceased.
The addition of Mn to Cu-Ni matrix reduces the hysteresis loss of Nb-Ti filaments and increases the critical current density of Nb-Ti filaments. The minimum hysteresis loss of Nb-Ti wire with Cu-30%Ni-0.9%Mn alloy is 17J/m³ (±0.5T/cycle) and less than 1/16 of that of Nb-Ti wire with Cu-30%Ni alloy. The critical current de1unity Jc of Nb-Ti wire (filament diameter df=0.1μm) with Cu-30%Ni-1.34wt%Mn alloy is 20000A/mm² (λJc=4100A/mm²) at 0.5T and 9200A/mm² (λJc=1900A/mm²) at 1T. AC loss (Qt)/λJc of wires with Cu-30%Ni-Mn alloy is 1-2×10^-8J/A•m and about 1/10 of that of wire with Cu-30%Ni alloy. Qt/λJc of the double stack wires (λ=0.17) decreases to about 1/2.7 of that of the triple slack wire (λ=0.11).

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Behavior of bubbles in non-uniform fields is of great interest primarily to study the electrically enhanced boiling heat transfer, the electrical breakdown of dielectric liquid in the presence of thermally induced bubbles and electrically stabilized boiling within a vessel in the zero-gravity space environment. In the present work, to clarify the bubble behavior in cryogenic and dielectric liquid during quenching period of superconducting coils, the bubble motion is analyzed numerically with a cylinder-to-plane electrode system in liquid nitrogen/helium under d.c. applied voltages.
The main content of this paper may be summarized as the following. A set of differential equations which describes the motion of a spherical bubble in dielectric liquid under non-uniform electrostatic field is given. The equations are solved numerically with three arrangements of the electrode system in liquid nitrogen or helium and the some results are confirmed by experiments. The effects of bubble size, gap parameters and characteristics of liquid on the bubble behavior are newly found and the formation of bubble stagnation is discussed.

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We have constructed a model vehicle, which runs without contact to its magnetic rail, using oxide superconductors. The present paper describes the forces acting between the superconductor and the rail for such a purpose. The attractive and repulsive forces were measured at 77K as a function of distance between the superconducting pellet and the magnetic rail. As superconducting pellets, three kinds of materials MPMG-Y123, BiPb2223, and Hg1223 were investigated for three kinds of permanent magnets Nd-Fe-B, Sm-Co and ferrite.

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Non-linear I-V characteristics have been clearly obtained for the Au/insulator/BKBO tunnel junctions with natural and MgO barriers. Epitaxial BKBO films prepared by rf-magnetron sputtering were used as the base-electrodes of the junctions. Two kinds of barriers were used for an insulation layer. One is a natural barrier existing on BKBO surface, the other is an artificial MgO barrier fabricated using the MBE method. Non-linearity well explained by the BCS theory was clearly observed for the tunnel junctions. The leakage of the Au/MgO/BKBO structure measured at 4.9 K corresponds to the theoretical value predicted by the BCS theory within 5% error, which is less than 1/3 of that using natural barrier. These results may come from the difference of barrier structure between natural and MgO barriers.

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This paper describes the breakdown characteristics of liquid helium (LHe) caused by quench of a superconducting wire exposed to a high electric field. As the thermal energy injected into the superconducting wire after the quench-onset increased, the quench-induced dynamic breakdown of LHe took place at lower stress than the static breakdown limit without the quench. There existed a delay time Td of the order of ms from the quench-onset to the dynamic breakdown. At small gap lengths, Td decreased under the higher stress irrespective of the thermal energy, while at large gap lengths, Td reduced with increasing the thermal energy and the electric field strength as well. Thermal bubble behavior was observed using a fiberscope inserted into the FRP dewar and a high speed video system, and it was also analyzed numerically. By these investigations, it was found that the dynamic breakdown of LHe have two different mechanisms: (1) small gap breakdown and (2) large gap breakdown. The small gap breakdown occurs when thermal bubbles reach the plane electrode above the superconducting wire. The large gap breakdown takes place after thermal bubbles rebound from the plane electrode into the gap space.

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Ag-sheathed Bi-2223 superconducting oxide conductors with a critical current density of 66, 000A/cm² at 77K, and coils that have generated magnetic fields of about 1.5 Teslas at 4.2K had been successfully developed. The conductors were piled-up bundles of four to six sheets of element tapes and had transport currents of around 200Amps. The so-called CSL (Core Size Limit) prevented us from making single conductors which had the capability of transporting a large current of as high as 200Amps at 4.2K. Until now, it was difficult to fabricate a conductor with high critical current density and the capability of transporting a large amount of current simultaneously. The role of CSL to large scale applications of high temperature superconducting oxides is discussed.

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We applied the Bi-based oxide superconducting bulk to the current leads in a cryocooler-cooled superconducting magnet which is free from liquid helium in operation. The magnet is capable of generating a steady magnetic field of 4T at a current of 400A in room-temperature 38mm diameter bore.
Here we have investigated superconducting properties of Bi-based oxide bulk fablicated by intermediate cold isostatic pressing process. The bulk has a composition of (Bi+Pb): Sr:Ca:Cu=2:2:2:3 and is utilized in the form of thin-walled sintered cylindrical tube. The transport critical current density and critical current of the bulk at 77K under self-magnetic field are more than 1000A/cm² and 1100A, respectively.

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