Comparative analysis on twisting characteristics of HTS quasi-isotropic strand and stacked-tape strand

Abstract

With the gradual maturity of high temperature superconducting (HTS) technology, several HTS conductors with large critical current like quasi-isotropic strand (Q-IS) and twisted-stacked-tape cable (TSTC) were proposed, which can be assembled into cable-in-conduit conductor (CICC) and applied in power system and fusion magnets. The purpose of this paper is to study the twisting characteristics of Q-IS and stacked-tape strand (STS) with different stacking patterns under self-field at 77 K. The critical currents of two Q-IS samples and two STS samples are measured under increasing twist angle until they break. Each sample is composed of copper tapes and second generation (2G) REBa2Cu3Ox (REBCO, RE = rare earth) HTS tapes. For each type of conductor, one sample has a structure where REBCO tapes are stacked near the outside, while the REBCO tapes are stacked close to the inside in another sample. Simulation based on finite element method (FEM) and H-formulation is carried out for comparison. Numerical calculation of the normalized critical current versus the twist pitch is also conducted. Results show that each kind of strand with REBCO tapes stacked close to the inside have smaller critical twist pitches. Besides, self-field effect is dominant while the twist angle is small, but the effect of longitudinal strain gradually becomes the main factor as the twist angle increases. In general, the critical current degrading rates of STS samples are smaller than that of Q-IS samples. However, when the twist angle is relatively small, Q-IS samples show better critical current characteristics than STS samples.