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Magneto-optical (MO) imaging, Hall probe measurements and the magnetic knife method were used for sample quality studies of multi-core Bi-2223/Ag tape with a columnar 60-filament structure made by the TIRT (tape in rectangular tube) process. While the magneto-optical imaging was made at 25 K and without transport current, Hall probe and magnetic knife measurements were performed for the sample carrying a direct current at 77 K. The MO image recorded at 25 K and external field 30 mT has shown non-uniform flux distribution in the three filament columns. 2D field profiles were analysed by the finite element method assuming ideal filament homogeneity. Bz(x) was computed analytically with an assumption that current density flows straight through the cross-section of the filament columns. The best agreement of computed profiles with measured ones is achieved for equal currents flowing in all filament zones. According to the magnetic knife measurements, current density is lowest in the centre column, but in this method the magnetic self-field is hardly influencing the current distribution. A better understanding of such experimental differences is obtained with 3D modelling. Three different cases of meandering transport current were studied computationally in order to explain the measured results. Finally, we discuss the advantages and drawbacks of the methods used and present how they can together produce the most complex information.

The magnetic field dependence of the critical current in (Bi,Pb)/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub 10+x/ tapes is compared with the magnetization response of isolated grains extracted from the tapes. Special attention is paid to the low-field behavior. The goal of the experiment is to test the widely-used hypothesis that current paths in these tapes contain both weak- and strong- linked branches, which in low field act in parallel. The data agree with this hypothesis; at temperatures above 50 K the powder magnetization drops off exponentially from the self-field to the irreversibility field, while the transport and magnetization currents in the intact tapes show an extra low-field component. Below 50 K the powder behavior becomes less straightforward, but the parallel-path picture in the tapes still holds.

We report on the preparation of dense monofilamentary MgB2/Ni and MgB2/Fe tapes with high critical current densities. In annealed MgB2/Ni tapes, we obtained transport critical current densities as high as 2.3*105 A/cm2 at 4.2 K and 1.5 T, and for MgB2/Fe tapes 104 A/cm2 at 4.2 K and 6.5 T. To the best of our knowledge, these are the highest transport jc values at 4.2 K reported for MgB2 based tapes so far. An extrapolation to zero field of the MgB2/Fe data gives a critical current value of ~ 1 MA/cm2, corresponding to an critical current value well above 1000 A. The high jc values obtained after annealing are a consequence of sintering densification and grain reconnection. Fe does not react with MgB2 and is thus an excellent sheath material candidate for tapes with self-field jc values at 4.2 K in excess of 1 MA/cm2., Comment: 11 pages, 3 figures

The critical current density of four MgB2 samples was measured inductively and for one of them also by transport. Pure phased and dense bulk samples yield a critical current density which in value, as well as in its magnetic field and temperature dependence, is essentially the same as the intra-granular current density measured in a dispersed powder. Also the correspondence between the inductive and transport data indicates that the grain boundaries in the bulk samples are totally transparent for the current. The current-voltage relation becomes rapidly shallow in the vicinity of a depinning line, well below the second critical field. Between the depinning line and the second critical field the material is ohmic and shows a marked magneto-resisitivity, indicative of a flux-flow regime., Comment: 15 pages, 14 figures, submitted to Physica C

The influence of the geometrical parameters on the behaviour of the AC losses in multifilamentary Ag-BSCCO(2223) tapes is being more and more evident. Recently, the effects of configurational parameters like the ratio between the overall width and thickness of tapes without barriers as well as their precise arrangements has been put in evidence either theoretically or experimentally. With the goal to go deeper into the comprehension of this relation, we characterized a serie of multifilamentary tapes with gradually different filament arrangements. They were measured in a self-field AC transport system at different frequencies, for current values up to the critical state. The results so obtained are discussed taking into account of some theoretical models.

The transverse electrical resistivity in multi-filamentary Ag/Bi,Pb(2223) tapes is considerably enhanced after introducing inert “ oxide barriers “, a new concept in which each single filament is surrounded by a highly resistive BaZrO3 or SrZrO3 layer. With these oxide barriers, we have so far obtained a shift of the coupling loss maximum from 5 Hz to 116 Hz. This corresponds to a marked lowering of the AC losses at 50 Hz, making promising their use in transformers and cables. The typical Jc value of these tapes is actually 15′000 A/cm2 at 77K, 0T. The fabrication processes leading to these Bi,Pb(2223) tapes with oxide barriers is described.

Jc values of multifilamentary Ag-sheathed Bi(2223) up to about 35’000 A/cm2 have been achieved at 77K and in self field for lengths of several meters using an improved route: Periodic Pressing. This corresponds to an increase by 30–40 % compared to the values obtained for conventionally rolled tapes. In contrast to earlier attempts by pressing techniques, Periodic Pressing is as practical and scaleable a process for the fabrication of long lengths of Bi(2223) conductor as the standard intermediate rolling step. Engineering critical current densities of 8’000 A/cm2 have successfully been obtained for tapes with high filling factor (35%).

We report on three different and complementary measurements, namely magnetisation measurements, positron annihilation spectroscopy and NMR measurements, which give evidence that the formation of oxygen vacancy clusters is on the origin of the fishtail anomaly in YBa2Cu3O7-d. While in the case of YBa2Cu3O7.0 the anomaly is intrinsically absent, it can be suppressed in the optimally doped state where vacancies are present. We therefore conclude that the single vacancies or point defects can not be responsible for this anomaly but that clusters of oxygen vacancies are on its origin., 10 pages, 4 figures, submitted to PRL

Multifilamentary Bi(2223)/Ag tapes often exhibit AC loss levels comparable to those measured in monofilamentary samples, which is partly due to the large coupling currents induced in the low resistive sheath material. Surrounding the individual filaments by electrically insulating barrier layers suppresses these currents and strongly reduces the coupling. We demonstrate this effect with various types of magnetic and self-field AC loss measurements on a series of Bi(2223)/BaZrO3/Ag tapes. We also discuss the influence of barrier thickness, twist pitch length and filament arrangement on the measured losses in these composite conductors.

TEM studies of a Bi-2223 tape have shown that amorphous layers from about 10 nm to more than 100 nm in width were present at c-axis twist boundaries in the highly textured Bi-2223 tape which showed a limited residual weak-link field dependence at low fields ( mu 0H

The grain-to-grain connectivity in Bi2Sr2Ca2Cu3Ox tapes is still poorly understood, even though they have been commercially available in long lengths for several years. This letter explains the effects of tensile strain on the grain-to-grain connectivity in Bi2Sr2Ca2Cu3Ox tapes. The different length scales at which damage to the grain structure occurs are studied with magneto-optical imaging, scanning-electron microscopy, and transport current. These data show that the initial degradation in critical current when strain exceeds the irreversible strain limit is caused by microcracks ( ∼ 100–500 nm in width) that form mainly at high-angle grain boundaries. Filament-wide cracks ( ∼ 5–10 μm in width) form at locations of lower grain density in the filaments at strains far exceeding the irreversible strain limit. However, in contrast to previous reports, a careful analysis of the pinning force as a function of tensile strain, taking into account current sharing with the normal matrix by using the offset criterion, shows that intragranular flux pinning is not affected by strain in any significant way.

Columnar defects are known to provide excellent flux pinning sites in high Tc superconductors. The changes in pinning properties of Bi2Sr2Ca2Cu3O10/Ag tapes due to perfectly splayed amorphous tracks were examined measuring the DC magnetisation versus temperature T and magnetic field H. We compare the hysteretic magnetisation ΔM(T,H) and the magnetic relaxation rate S(T,H) before and after irradiation. The data show how played columnar defects indeed increase the pinning strength at high temperature, but enhance vortex motion at low temperature. We attribute the cross-over between these two regimes to a change in vortex dimensionality.

We report on the absence of the so-called fishtail or peak effect in the magnetisation curves in twinned high purity (5N) single crystals of YBa 2 Cu 3 O 7−δ . The suppression of this effect can be achieved with or without a change of the overall oxygen content. We conclude that the local oxygen vacancy distribution must be correlated with this effect.

We report on the quasi DC magnetisation of various polycrystalline (Bi,Pb) 2 Sr 2 C 2 Cu 3 O 10 systems. Using a vibrating sample magnetometer the magnetic response of PIT tapes, hot rolled bars and sintered pellets was measured as a function of temperature and electric field. Comparing measurements on both bulk and powdered samples, we can distinguish between the inter- and intra-granular contribution to the magnetisation. Differences and similarities in the extracted critical current density and magnetic relaxation rate of these systems are discussed in terms of grain-to-grain connectivity and intra-granular pinning properties.

The DC magnetic moment of BSCCO/Ag 2212 ribbons is shown to scale with the ribbon size, indicating that the screening currents flow coherently throughout the simple. Comparison of these magnetisation loops with the loops taken from 2212 powder extracted from the ribbons shows that the dominant dissipation occurs at an intra granular level. Transport measurements in external fields applied at different angles with respect to the plane of the ribbon show how the j c (H) anisotropy by the ‘effective’ orientational distribution of well connected crystallites.

Current voltage characteristics well below T c were measured in a set of non-random granular YBCO/Ag composites. The addition of 23% vol. Ag increases the transport J c by an order of magnitude, reduces the exponent of the J c (T) dependence, and changes the power in the power-like I–V curves. The DC magnetization indicates a degradation of the intragranular properties by the Ag.

Temperature dependence of resistance, R(T), has been measured for different perpendicular magnetic fields in mesoscopic Al squares. An anomalous oscillatory resistive transition into the superconducting state has been found. This transition is characterized by the appearance of several well defined maxima on the R(T) curve in the vicinity of the superconducting transition temperature. We suggest that these R(T) maxima are caused by destructive quantum interference processes, implying the formation of superconducting but non-current-carrying states in the mesoscopic Al samples.

Measurementsof the quasi-adiabatic normal zone propagation velocity and quench energies of a Superpower SCS4050 copper stabilised ReBCO superconducting tape are presented over a temperature range of 23 − 47K; in parallel applied magnetic fields of 6, 10 and 14 T; and over a current range from 50% to 100% of Ic. The data are compared to results of analytic predictions and to one-dimensional numerical simulations. The availability of long lengths of ReBCO coated conductor makes the material interesting for many HTS applications operating well below the boiling point of liquid nitrogen, such as magnets and motors. One of the main issues in the design of such devices is quench detection and protection. At higher temperatures, the quench velocities in these materials are known to be about two orders of magnitude lower compared to low temperature superconductors, resulting in significantly smaller normal zones and the risk of higher peak temperatures. To investigate whether the same also holds for lower temperatures more extended data sets are needed, both as input and as validation for numerical design tools.

The technological importance of the Bi2223/Ag tapes has been widely recognised and yet their electrical transport properties, particularly in an applied magnetic field, remain inconclusively. We present analysis of magnetisation loops and transport current-voltage characteristics taken on the same sample. Using the dynamic relaxation technique and the transport data we can extend our E - J curves over 5 orders of magnitude in electric field. The resultant E - J characteristics reveal clues as to the current limitation between the grains and the vortex dissipation within the grains. In an applied magnetic field J c behaves hysteretically giving further insight as to the granularity of these samples. We offer a comparative study of tapes with differing values of J c .

The quench protection of the several meter long, large aperture high-field Nb3Sn quadrupoles that the LARP collaboration is developing for the LHC interaction region upgrade, requires efficient protection heaters to quickly generate large resistive segments across the windings. To support the protection design, experiments in the recently tested LARP R&D magnets are aimed to characterize the coil response to different protection schemes. In particular, the delay to quench and the final hotspot temperatures are evaluated after firing the heaters at different powering regimes and coverage. Also, the contribution of external energy extraction is investigated. Based on the performed studies and computer simulations, it seems that if the same protection efficiency per unit length that is measured in a 1 m long model magnet can be scaled to a 3.6 m long magnet, and the heater coverage can be improved, about 1 MJ/m of stored energy can be absorbed in the magnet after a quench. However, significant technology dev...

Superconducting samples of Y-Ba-Cu-O compounds have been prepared by sintering the material at different temperatures and under various mechanical pressures. Using pulsed magnetic fields up to 27 Tesla, we have performed detailed measurements of the upper critical field H c 2 as a function of temperature for different annealing treatments. While the H c 2 values are high in all the samples, the critical current densities are considerably enhanced by the annealing treatment.