Critical Current Measurements on Round Cables Made With a REBCO Stack Conductor

The development of high-current high-temperature superconducting (HTS) cables is a key technology for high-field accelerator magnets of 16 T or more. Recently, a flexible HTS round cable, consisting of many rare-earth barium copper oxide (REBCO) coated conductors helically wound in multiple layers on a metal core, have been manufactured by Advanced Conductor Technologies LLC as the conductor on round core (CORC). When a commercially available coated conductor with a substrate thickness of 30 µm is applied to the CORC, the minimum core diameter is approximately 2.4 mm without any degradation in the critical current (<italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub>). Because the engineering current density of the CORC is highly dependent on the core diameter, the cable with a smaller core is desirable to achieve compact coil windings for high-field accelerator magnets. In this study, a stack conductor, made by soldering two coated conductors with their REBCO layers facing each other, was applied to the round cable instead of the commercially available single coated conductor. This stack conductor allows the REBCO layer to be closer to the neutral axis of the conductor, thereby reducing the strain induced in the REBCO layer by winding the conductor on the core. As a result, the stack conductor can be wound on a smaller core than the CORC, allowing for higher current densities. This paper presents the cable <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> measured at 77 K and the strain of the REBCO layer calculated as a function of the core diameter. The cable <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> at 20 T and 4.2 K was estimated using the measured magnetic field dependent <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">I</i><sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> of the coated conductor at 4.2 K.