Nb/sub 3/Al superconductors

Abstract

Recent progress of Nb 3 Al superconductors is described. A large-current conductor of 46 kA at 13 T, produced from a jelly-roll (JR) Nb/Al composite by a direct diffusion-process, is about to be tested as an insert-coil conductor for fusion magnets. The 1 GHz NMR magnet has given a motivation to do an entirely new approach to Nb 3 Al processing that allows a dramatic improvement in high-field characteristic. The RHQT process produces a stoichiometric Nb 3 Al with fine-grain structures via a bcc supersaturated solid solution and, hence, large over the whole range of magnetic fields. Controlling a heating temperature within a narrow region of extended bcc solid solution ensures the room-temperature ductility of as-quenched wires and homogeneous characteristics of transformed Nb 3 Al along the long piece-length. A solenoid coil fabricated by a wind-and-react method, with Cu clad RHQT Nb 3 Al conductors, could generate an additional 1.3 T at 2.1 K in a bias field of 21.2 T, and was operated in a persistent-current mode without a bias field. The variants of RHQT process (TRUQ, DRHQ, Cu-added RHQO) improved significantly the high-field without sacrificing the strain tolerance.Index Terms-DRHQ, high field improvements, RHQO, RHQT, TRUQ.

Observation of A15 phase transformation in RHQ-Nb3Al wire by neutron diffraction at high-temperature

“…It has been reported that when a RHQ treatment is applied to Nb and Al raw materials, a solid solution of NbAl y with a bcc structure is formed [6][7][8][9][10][11][12]. Upon further heat treatment, this undergoes an A15 phase transformation to form Nb 3 Al.…”

Transformation Heat Treatment of Rapidly Quenched $ \hbox{Nb}_{3}\hbox{Al}$ Precursor Monitored In Situ by High Energy Synchrotron Diffraction

“…The sample temperature accuracy is better than ±2 °C. The ramp rate was either 120 °C/h or 800 °C/h (for comparison, the ramp rate during a transformation HT is typically between 200 and 800 °C/h [4]). Unlike in a standard transformation HT, during the 120 °C/h HT the 800 °C plateau has been omitted, and the maximum temperature was 1100 °C, slightly above the Cu melting temperature of 1085 °C.…”

“…The Rapid Heating and Quenching (RHQ) stages are followed by a transformation heat treatment (HT) at relatively low temperature of typically 800 °C, during which the fine grained A15 phase with high Al content is formed from the Nb(Al) SS solid solution [4].…”

Impact of an ‘inter linked’ central solenoid configuration on the size of tokamak DEMO

“…We analyzed applicability of Nb 3 Sn and Nb 3 Al for a superconducting wire of the IL-CS. It was found that since the critical current density J c of a Nb 3 Sn wire in the high magnetic field environment is smaller than that of a Nb 3 Al wire [8], the operation current density J op normalized to J c of the Nb 3 Sn wire is higher than that of the Nb 3 Al wire and above the design limit of J op /J c for the ITER superconducting magnet design. Accordingly, the IL-CS made of Nb 3 Sn has to be thicker than of Nb 3 Al to decrease J op /J c .…”