Axial Movement Effect on Voltage and Current Behaviors of Insert No-Insulation REBCO Pancake Coil

In 2017, an insert 12-stacked no-insulation (NI) rare-earth barium copper oxide (REBCO) magnet was used in the generation of a world-record DC magnetic field of 45.5 T. The experiment demonstrated its high thermal stability; however, NI REBCO pancake coils are facing mechanical problems. In such an ultra-high field, strong electromagnetic forces work on insert REBCO coils, in particular during sequential normal-state transitions. Strong forces may cause insert REBCO coils to deform and move. Some research revealed that some deformations and movements affected the coil voltages and the screening currents. Whereas, the coil movement in an ultra-high field has not been studied yet. Voltages and currents are induced by the movement of insert coils, and there is a possibility of degrading the coil stability. In this paper, we have investigated the electromagnetic behaviors during sequential normal-state transition, considering the coil movement due to strong axial forces. The electromagnetic behaviors are simulated using a radially-subdivided partial element equivalent circuit (PEEC) model coupled with mechanical movement analysis.

I. Introduction

Ultra-high magnetic field is strongly desired for nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI). The combination of rare earth barium copper oxide (REBCO) superconductors [1] and a no-insulation (NI) winding technique [2] permits ultra-high magnetic field generation beyond 45 T. In 2017, 12 NI REBCO pancake coils generated 14.4 T under a 31.1 T background field of an outsert magnet (a DC magnetic field of 45.5 T in total) [3]. The 45.5-T generation demonstrated that NI REBCO pancake coils provide thermal stability enough to reach over 45 T. To date, several other magnet-development teams adopted the NI winding or a similar technique as a key feature to achieve an ultra-high field, and they showed some successful results [4], [5], [6], [7], [8], [9], [10]. However, mechanical damages have emerged as a major problem. For instance, small cracks and wavinesses were found on REBCO tapes after the 45.5-T generation [11]. Small cracks and wavinesses have been observed in other high field generations [12]. In addition, REBCO tape deformations have been reported in several cases [14], [15]. Some researches against these problems are underway, showing good progress [16], [17], [18].