Novel, internally stranded Bi cuprate conductor concept for ac applications: ring-bundled barrier (RBB) tapes produced by the powder-in-tube assemble and react (PITAR) method

Novel, ring-bundled barrier (RBB) Bi-2223 tapes for high-current ac applications are prepared by the new powder-in-tube assemble and react route (PITAR), where multifilament wires are bundled around a central SrCO3 core within an outermost tube. Wire twisting, rolling and thermomechanical reaction leads to a tape similar to the common Rutherford cable. PITAR avoids the limitations of assembling the too stress-sensitive, reacted Bi-2223 tapes. Applying standard tape deformation routes and reaction parameters, critical-current densities of the order of 0.5-1 kA cm-2 are achieved. Size-scalability is demonstrated by preparing tapes of various filament/strand configurations with comparable critical current densities. Ag98Mg2-alloy outer sheathing enlarges the tape's tensile stress up to 160 MPa at 77 K. Dc magnetic-field and angular dependences of transport current densities at 77 K indicate a texture quality slightly below standard tapes, due to the mechanical behaviour of the SrCO3 barrier. Magneto-optical images confirm a successful realization of the conductor concept and indicate a homogeneous current distribution on the critical-current density level reached so far. Twist pitches of 95 mm, 47 mm and 23 mm do not systematically degrade the critical current density, yet a clear strand-decoupling is observed in external, perpendicular ac fields. This demonstrates the possibilities inherent to the conductor design with regard to future ac application. (author)