A. Saggese

Flux creep in Josephson junction arrays

In order to analyze the diamagnetic properties of weakly coupled structures in high-T/sub c/ ceramic superconductors , the Josephson junction array model is used. It is suggested that the coupling is strong enough to allow magnetic flux trapping inside nonsuperconducting regions surrounded by superconducting loops closed by Josephson junctions. It is noted that the presence of currents flowing through the junctions has to be taken explicitly into account in the Hamiltonian. This description leads to a creep model of the Josephson junctions array. As a result, one observes the following: (1) pinning centers generated by nonsuperconducting regions into the loops, (2) pinning potentials determined by fluxon motion barriers due to the Josephson junctions, (3) absence of degeneracy of the states corresponding to a different number of fluxons in the loops, and (4) a reduction of the barrier height due to measuring currents or to diamagnetic shielding currents. The last effect is equivalent to the Lorentz force effect in type-II semiconductors. Thus, it is believed that this picture greatly modifies the usual superconducting glass model.

The transverse resistivity in S/C multifilament wires studied through ac susceptibility measurements

In this paper we discuss an experimental method based on the ac susceptibility for measuring the effective transverse resistivity in multifilament NbTi wires designed for low loss applications. Short samples of wire are involved in the measurements, with lengths comparable with the twist pitch. The measured values of the transverse resistivity are compared to expectations coming from a model based on a numerical approach.

Stability Measurements on YBCO Coated Conductors

The working features of 2nd generation high temperature superconducting tapes related to applications, such as their stability, are becoming of real interest as long tape lengths become available. In this work, we present results on normal zone propagation velocity obtained on a 40 cm long YBCO coated conductor, called 344 superconductors, manufactured by American Superconductor Corp. Although the tape is wound on a 40 mm diameter cylinder support, its transport features are unaffected compared to the straight tape. The quench is induced by an external heater, and the propagation is observed from the increasing voltages along the tape. In the range 75divide80 K, we observed a propagation velocity lower than 2 cm/s at current bias slightly below 80% of the critical current. Our investigation covers different bias current values as well as temperatures, and experimental analysis is also compared to an approximate numerical model.

Further measurements on electrolytic cold fusion with D sub 2 O and palladium at Gran Sasso Laboratory

This paper reports on several experiments performed at the Gran Sasso Laboratory on an 0.8-cm-diam {times} 5-cm-long, hyperpure, high-temperature vacuum-annealed palladium rod used as a cathode for electrolytic infusion of D{sub 2}O and 0.1 M LiOH with regular additions of gaseous CO{sub 2} at a current density of 60 mA/cm{sup 2}. In the very low background radiation environment, several gamma bursts lasting up to 15 min were detected whose intensity, in terms of cold fusion, was {gt}10{sup {minus} 20} fusion/(deuteron pair {center dot} s). Under normal background conditions, none of these burst signals would have been detected with statistical significance. The shape and intensity of these signals are quite similar to those detected previously.

Hot Spot Stimulated Transition in YBCO Coated Conductors: Experiments and Simulations

Extensive applications of YBCO tapes claim for a comprehensive knowledge of their behavior in every possible operating condition. In particular, one of the main topics to explore is the tape stability against heat and current disturbances, in terms of minimum quench energy and normal zone propagation velocity (NZPV). In this work we present the results of NZPV measured in a 35 cm long YBCO coated conductors manufactured by SuperPower Inc. The tape is cooled by means of a cryocooler and kept under vacuum in order to achieve near-adiabatic conditions. Two independent heaters are positioned on the tape: one on the YBCO side and the other on the substrate side. The quench is monitored by recording the voltage and the temperature evolution along the tape. The NZPV, at 77 K, reaches a value larger than 2.5 cm/s for a bias current I=50 A which represents the 56% of the critical current value. The quench dynamics of the tape are discussed with the help of a simulation program based on the finite difference discretization of the tape with the Fouriers heat equation: the model is able to reproduce the experimental behavior.

Torque measurements of textured Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ sintered pellets

The authors present at 4.2 K magnetic torque balance study of a textured sintered YBCO sample produced by pyrolysis using in the thermal treatments an ozone-enriched oxygen atmosphere. The shape of the torque signal as a function of the angle and of the measuring field shows features that are intermediate between those of a standard sintered pellet and those of a single crystal. The anisotropic experimental values of the lower critical field H/sub c1/ compare rather well to those previously found on YBCO single crystals. The intensity of the remaining magnetization, measured after a field cycling procedure, suggests the existence of a strong pinning for H>or=H/sub c1//sup (c)/ along the c-axis. >

Quench Behavior of a Conduction Cooled

Extensive applications of YBa2Cu3O7-x (YBCO) tapes require a comprehensive knowledge of their behavior in every possible operating condition. In this paper, the thermal stability behavior of a conduction-cooled pancake coil wound with commercial YBCO coated conductors is reported from both the experimental and numerical point of view. The coil stability against heat disturbances is studied in terms of minimum quench energy and normal zone propagation velocity (NZPV) for different values of the coil currents at different temperatures. The quench dynamics of the tape is compared between experiment and the numerical analysis, obtaining a reasonable agreement for voltage traces, NZPV, and temperatures. The model allows a wide range of case studies that can be used to suggest appropriate solutions for the design of detection and protection systems.

\hbox{YBa}_{2} \hbox{Cu}_{3}\hbox{O}_{7 - {\rm x}}

In this paper we compare two different experimental methods to estimate the transverse resistivity in superconducting NbTi and Nb3Sn multifilament wires. The first method is based on direct measurements of the resistance through micro wires bonded on the cross section of the strand. The second method exploits the physical meaning of the ac susceptibility of the multifilament wires. Both methods have been previously used: the first on Nb3Sn and NbTi strands, while the second has been applied only to NbTi wires. Here we first compare and analyze results in a NbTi system, then we extend the ac susceptibility method to Nb3Sn wires. The advantages and the limits of both techniques, when used on wires with different layouts, are analyzed. The results of the transverse resistivity obtained by using the two experimental methods are finally compared to the estimation based on a numerical approach.

Tape Pancake Coil

Rare Earth (Nd,Sm) elements, having relatively large ionic radius, have been substituted for Yttrium in Y123 superconducting melt textured bulk samples. In (Nd,Sm)123 a wider solidification range and higher recrystallization rate than Y123 significantly increase the solidification rate, making the whole process much faster. A preliminary comparison among directionally solidified (Y,Sm,Nd)/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7-x/ bulk bars fabricated by the Horizontal Bridgman method has been done. The different microstructural and superconducting features are studied by X-ray, SEM analysis, and ac and dc magnetic measurements. The (Nd,Sm)123 samples, appear comparable to Y123 grown with a pulling rate almost two orders of magnitude lower.

Experimental investigation of the transverse resistivity in Nb3Sn wires through ac susceptibility

The quench behavior of a pancake coil made of a copper-stabilized MgB2 tape is investigated with the aim of collecting fundamental know-how on the quench characteristics, and to provide an experimental benchmark for the validation of numerical models. Comparison between numerical and experiment results are reported. Relevant aspects of the quench mechanism are numerically investigated.