Abstract
This article describes the present status and R&D issues related to the rapid heating/quenching and transformation (RHQT)-processed Nb 3 Al superconducting wires. The RHQT-processed Nb 3 Al wires were first proposed by Y. Iijima, et al. in 1994. Eighteen years have already passed, and this wire is still undergoing R&D towards the realization of practical superconducting wires. The RHQT-processed Nb 3 Al wires are made utilizing three unique fabrication stages: jellyrolled precursor fabrication, rapid heating/quenching treatment and electroplated copper stabilizer fabrication. Recently, even though a RHQT-processed Nb 3Al wire with copper stabilizer of over 1 km in length was fabricated, several difficult problems still remain to be resolved before industrial mass production is possible. In the stage of precursor fabrication, much improvement in the mechanical cold-workability of tantalum raw materials is required in order to avoid wire breakage. Additionally, the homogeneity of the cross-section of the precursor wires in the longitudinal direction needs to be improved for carrying out the homogeneous rapid heating/quenching treatment. The copper stabilizer for the RHQT-processed Nb 3 Al wires is fabricated using high-speed reel-to-reel electroplating. Furthermore, reel-to-reel copper ion-plating with a thickness of 1 μm is performed before the thick electroplating, thereby obtaining good bonding between the Nb 3Al wire and copper stabilizer. The maximum current density during electroplating is currently 40 A/dm2, and approximately one week is needed to electroplate 1 km of Nb 3Al wire with a 1.0 mm outer diameter and 1.0 Cu/non Cu ratio. Future R&D will focus on significantly shortening the electroplating time.
