Intensive research work has been carried out in order to develop industrially available HTS
REBCO-coated conductors under the NEDO project in Japan. Recently, several groups in
the project succeeded in the development of high performance coated conductors. Their
characteristic features have been evaluated in terms of mechanical properties
and their influence on critical currents. The mechanical properties at RT and
77 K were analyzed on the basis of the rule of mixtures. The force-free strain
(Aff) was analytically deduced, which indicates the strain at which the residual stress exerted
on the superconducting layer becomes zero. Tensile strain dependence on critical
currents could be divided into elastic and brittle regions. The reversible strain limit
(Arev) was defined as a strain at which the critical current recovers elastically to the level of 99%
Ico. Within the elastic region, the critical current showed a convex strain dependence,
which is explained as Ekin’s intrinsic strain effect. The degradation beyond the
reversible strain limit was attributed to a fracture of the superconducting layer. As a
whole, the present study made clear quantitatively the tensile strain behavior of
critical currents and proposed a reasonable definition for the reversible strain limit.
The fabrication of Bi:2212/Ag-based Rutherford-type accelerator cables, their transport critical current testing and ac loss measurement are described. Multifilamentary strands were used to form several cables with 18-19 strands and a lay pitch, (half the transposition pitch) of 27.5 mm. The inclusion of a metallic alloy core was proposed for mechanical strength, the limiting of cable winding damage and ac loss/residual magnetization mitigation. Single-strand measurements of `poisoning' from several candidate core materials are reported, as well as the influences of winding pitch and thermal shock degradation on . Nichrome 80 was selected as a core material on the basis of ready availability in strip form and its initially observed inertness from a poisoning standpoint. In-cable single-strand studies indicated that winding damage could degrade edge-measured by about 16%. Full cable measurements demonstrated that core-induced poisoning could reduce by a further 35-40%. Ac loss measurements on a series of specially designed cables showed that the core (either bare or coated) effectively insulated the strands against crossover contact. This, together with the fact that the cable had been only lightly compacted (thereby ensuring moderate but not strong side-by-side contact) allowed the effective interstrand contact resistance of a projected Bi:2212/Ag-wound core-type LHC Rutherford cable to fall close to the acceptability range for the windings of accelerator magnets.
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