Bending Behavior of HTS Stacked Tapes in a Cable-in-Conduit Conductor With Twisted Al-Slotted Core

In this paper, we report on the experimental study of the bending behavior, at 77 K, of a cable-in-conduit conductor composed of a stack of REBa$_2$Cu$_3$O$_{7-\mathrm{x}}$ (REBCO) tapes inserted into a helical duct formed in the extruded aluminum cylindrical core. The investigation was carried out by powering each single tape individually. By the analysis of the single tape $I_c$ dependence as a function of the bending radius, $R_b$ , the effect of bending strain as a function of tape position inside the stack was investigated for each tape. The results evidence the good bending strain tolerance of all the tapes, showing the onset of degradation at $R_b \approx 0.25$m, ascribed to the slippage occurring among tapes within the stack. Interestingly, the decrease in the intertape contact resistance between neighboring tapes with respect to the straight cable condition reveals that the bending stress results in a beneficial additional transverse load on the stacks, which leads to a more uniform compaction of tape stack. The experimental $I_c$ behavior with $R_b$ has been explained considering that the current transfer mechanism among tapes might mitigate the degradation of the tape $I_c$ due to the bending strain. The identification of the mechanisms acting on individual tapes under cable bending conditions is a relevant achievement in perspective of the interpretation of the electromechanical behavior of the conductor with all tapes powered in parallel.