Meter-length REBCO ultralow resistance joint for the gigahertz NMR magnet

Abstract

In this paper, we present a meter-length REBCO joint using the most conventional soldering method. The joint resistance can attain 7.21 × 10^-11 Ω at 4.2 K with a joint length of 2 m. It essentially displays a linear relationship with the joint length. The microstructures of the joints were investigated systematically using scanning electron microscopy (SEM) and micro-computed tomography (micro-CT). Multiple voids were identified in all the joints. More importantly, the voids were not distributed homogeneously. The internal voids near the innermost and outermost rings were larger than those in the middle ring. Two types of gigahertz magnetic resonance spectroscopy (NMR) magnets were designed: 15 T low temperature superconductor (LTS) + 15 high temperature superconductor (HTS) and whole HTS. The current 10^-11 Ω-level joint can satisfy the magnetic field decay criterion of 1 ×10^-8/h. However, a pure HTS gigahertz NMR magnet requires a joint length of over 20 m, which is not feasible. The current resistance resource of the meter-length joint was also discussed by an equivalent circuit analysis. This paper presents a reliable engineering REBCO joint fabrication method for gigahertz NMR magnets without heat treatment.