L. Martini

Cryocooler cooled HTS current leads for a 1 MJ/1 MW-Class SMES system

Within the Italian project for the development of 1 MJ/1 MW SMES, Ansaldo-CRIS and CESI have been involved in the design and manufacturing of 1100 A resistive/HTS current leads conduction cooled by a closed cycle refrigerator. Main design and technological problems concern the achievement of electrical insulation between cryocooler and current leads at the voltage level of 6 kV together with a thermal contact good enough to keep the upper part of the HTS material at a temperature below 70 K, when the cryocooler temperature is 60 K and about 100 W is the thermal load. Thermal transport properties of electrical insulating materials as thin sheet placed between copper massive plates have been measured as a function of temperature and hence, according to the measured thermal performances, the area for thermal contact with the cryocooler has been suitably dimensioned. Mechanical structure for the whole current leads has been designed and manufactured. Final assembly of resistive section has been completed. Results about the achieved lead performances are presented and discussed.

AFM multilayered Bi-2223 conductors for 13,000 A current leads for CERN

Abstract Large current carrying capacity multilayered Bi-2223 conductors are reproducibly prepared by means of the “Accordion-Folding Method” and suitably used to manufacture the low temperature stage of 13,000 A hybrid metal-HTS current lead prototypes for CERN. In this work, we report on the electrical characterisation of AFM multilayered Bi-2223 conductors having critical current as high as 400 A at 77 K and on a specific experimental set-up that has been developed to study the thermo-electrical performances of the AFM Bi-2223 composite conductors during the sudden resistive transition of the HTS: quench event.

Strong vortex pinning and anisotropy in Ag/BSCCO-2223 tapes caused by surface columnar defects

This paper is aimed at investigating the vortex dynamics driven by 0.25 GeV Au ion implantation in Ag/BSCCO-2223 tapes. Columnar defects are produced perpendicularly to the tape plane and only affect a surface layer of about 10% of the whole sample volume. The paper shows that this defect topology does not deteriorate the critical temperature significantly. Moreover, the irreversibility line (IL) measured with the applied field perpendicular to the tracks is nearly unaffected. On the contrary, the IL with the applied field either parallel to the tracks or moderately tilted is shifted towards higher fields and temperatures. The amount of the shift is comparable with that obtained for tracks crossing the whole sample. In a large range of magnetic fields near the dose equivalent field the IL anisotropy falls to zero. However, the ILs after surface implantation exhibit very peculiar features.

Preparation of silver-sheathed Bi-Pb-Sr-Ca-Cu-O superconducting wires

The preparation of textured single and multifilamentary wires and tapes of the 110 K phase Si(Pb)-2223 by the powder-in-tube method using a silver sheath is reported. High critical current densities (8000 A/cm2 at 77 K and 0 T) have been obtained by means of a repeated rolling and sintering process. The Jc magnetic field dependence at 77 R up to 2000 Oe has been studied.

Modification of bulk properties of Bi/sub 1.8/Pb/sub 0.33/Sr/sub 1.87/Ca/sub 2/Cu/sub 3/O/sub y/ tapes by means of ion induced surface trenches

This paper demonstrates the possibility to enhance predictably the behavior of the superconducting properties in a bulk material, by introducing linearly correlated defects within a surface layer. Columnar defects, having a depth of about 5% of the total thickness of the sample, were created on Ag/BSCCO-2223 tapes by means of 0.25 GeV Au ions at different fluences, with a dose equivalent field B/sub /spl phi// ranging from 1 T to 5 T. The implantation depth was about 15% of the total thickness of the sample. The shape and position of the irreversibility lines were deeply modified. The vortex dynamics, as investigated by a.c. susceptometry in d.c. magnetic field, exhibits features characteristics of Bose-glass like localization in a range of field between the onset of the localized phase and the dose equivalent field. From all the experimental data it emerges that the strategy of creating surface trenches for vortices is always largely effective in this range of magnetic fields.

Vortex pinning and anisotropy in Ag/(Bi, Pb)2Sr2Ca2Cu3O10 tapes by means of surface columnar defects

Abstract Surface columnar defects (SCDs) are produced in high-quality Ag/(Bi, Pb)2Sr2Ca2Cu3O10 tapes by irradiating them with 0.25 GeV gold ions only on a top layer up to about 10% of the fuli volume. The ion beam is orthogonal to the tape piane. Such defects seem to be able to confine the vortices inside ‘tubes’ having a diameter determined by the SCD average distance. This confinement shows itself in the shift of irreversibility lines (ILs) towards high temperatures and magnetic fields, in the enhancement of critical current densities and in the strong decrease in the dependence of ILs on freauency. In this paper we concentrate on the shutdown of the IL anisotropy (low-current regime) as well as on the decrease in the critical current density anisotropy (high-current regime). IL anisotropy is also studied as a function of the angle between the applied magnetic field and the ion tracks. The following results should be emphasized, because they are peculiar to the SCD topology. Firstly, the enhancement of the superconducting properties when the magnetic field is applied parallel to the ion tracks does not compromise the superconducting properties with the field perpendicular to the ion tracks, which are unchanged. Secondly, a dose-dependent zone of the ILs exists (toward the highest temperature and lowest magnetic field), where SCDs do not affect the intrinsic properties of the material. On the contrary, a Bose-glass like phase starting just at the mentioned dose dependent point phase (T onset, 5 onset) is induced by the defects. A correlation between the two features is discussed on the basis of recent models.

FEASIBILITY OF A 5 T MAGNET BY USING BI-2223/AG TAPE AT 30 K

The feasibility of a 5 T magnet by using Bi-2223/Ag tapes at 30 K has been investigated. Preliminary computation demonstrates that the target is attainable by assembling 20 double pancakes manufactured by co-winding two Bi-2223/Ag tapes having overall cross-section of 0.25 × 3.5 mm2 and 25% fill factor, being the expected operating current about 200 A. The pancakes located at the edges are the most critical because the component of the magnetic field orthogonal to the tape surface is large enough to strongly depress Jc. To overcome this problem the edge pancakes could be fabricated by using Bi-2223/Ag tapes with new concept design (e.g. with oriented filaments).

Bulk Confinement of Fluxons by Means of Surface Patterning of Columnar Defects in BSCCO Tapes

We have studied the effect of surface columnar defects on the vortex dynamics inside the whole sample. Trenches of columnar defects along about 5% of the sample thickness were created by means of 0.25 GeV Au ions in Ag/BSCCO-2223 high-quality tapes. Strong phenomena of vortex localization inside the bulk were revealed by notable shifts of the irreversibility lines (ILs) as well as their after-irradiation shape. The enhanced ILs exhibit their own particular characteristics, such as a Bose-glass-like behavior up to quite high magnetic fields, with a dose-dependent onset point. Moreover, the irreversible regime expands with decreasing defect density. The results are consistent with the setting up of vortex morphologies confined in the bulk. The central achievement of this work is that a surface patterning with nanometric defects provides boundary conditions for mesoscopic domains in the bulk where vortices are confined. In particular, the lateral wandering is allowed inside tubes whose lateral cross section b0 is dose dependent and whose length scale is comparable with the distance between defects for each dose.

Control of the flux regime in BSCCO tapes by means of surface columnar defects.

Surface columnar defects (SCDs) are produced in high quality Ag/BSCCO tapes by irradiating them with 0.25 GeV gold ions only on a top layer up to 10% of the full volume. The ion beam is orthogonal to the tape plane. In the low current regime, the irreversibility lines (ILs) with the applied field either parallel to the tracks or tilted are shifted towards higher fields and temperatures. Moreover, SCDs do not damage the superconducting properties when the magnetic field is applied perpendicularly to the tracks. It can be shown that, as a consequence, the IL anisotropy falls to zero (or is strongly reduced) in a quite controllable range of magnetic fields near the dose equivalent field.

PHASE DIAGRAMS OF SURFACE HEAVY ION IRRADIATED BI-2223/AG TAPES

Surface columnar defects, SCDs, are produced in high quality (Bi,Pb)2Sr2Ca2Cu3Oy/Ag tapes by irradiating them with 0.25 GeV gold ions only on a top layer up to ≈15% of the full volume. The ion beam is orthogonal to the tape plane. In this paper we report experimental results of the surface implantation of columnar defects both in the high-current and in the low-current regime with a particular emphasis on the influence of SCDs on the anisotropy dependent properties. Some characteristic labels of the SCD implantation are pointed out. A comparison between all the results in the two current regimes is made. A hint is given concerning analogous results obtained by means of transport electrical measurements. The experimental data can be interpreted by assuming that a new confined phase driven by SCDs sets up in both the current regimes. Acting on the dose, it is possible to obtain tunable control of the in-field performance in both regimes.