Liyuan Liu

Effect of Winding Tension, Support Material, and Epoxy Impregnation on the Strain and Critical Current of YBCO Coil

The critical current (Ic) of epoxy impregnated YBCO coil is prone to degrade due to the cumulative radial tensile stress. Winding tension has the advantage to reduce the amount of epoxy resin and decrease the radial tensile stress during cool down process. In this paper, the dry and wet coils wound on copper and polytetrafluoroethylene (PTFE, Teflon) bobbin materials with various winding tensions were manufactured and tested. With increasing the winding tension, the winding strain increased but the thermal strain reduced. The low coefficient of thermal expansion for bobbin and epoxy impregnation could decrease thermal contraction of the innermost layer of coil. The winding tension effectively reduced the degradation of epoxy impregnated YBCO coil. The bobbin materials and winding tension of 10–50 N had no effect on the degradation of Ic. A proper winding tension of 20 N was successfully used to fabricate an epoxy impregnated YBCO coil of 30 turns without degradation

Effect of the Bending Strain on the Inhomogeneity of Critical Current for YBCO Coated Conductors

The inhomogeneity of critical current in long superconducting tape is one of the factors affecting the performance of the superconducting magnet. We investigate the effect of the bending strain on the inhomogeneity of critical current for the YBCO-coated conductor by a Hall sensor array system. Two kinds of the commercial YBCO-coated conductor tapes, which are prepared by rolling-assisted biaxially textured substrate (RABiTS) and ion-beam-assisted deposition (IBAD), are used for measurement. The bending strain is applied to the coated conductor tapes, and the distribution of the perpendicular component of the remanent field in the direction along the tape length after bending is measured by a Hall sensor array. The local critical current of the tape is calculated through the method of calibration and the distribution of the critical current in the long tape is described by the Weibull function. The result shows that the inhomogeneity of critical current in the long tape is significantly increased when bending strain exceeds an irreversible strain. The change of the superconducting layer structure is believed to be the cause of the critical current inhomogeneity growth.

Delamination Properties of YBCO Tapes Under Shear Stress Along the Width Direction

In YBCO coils, the increased transport current density and magnetic field will induce larger electromagnetic force, and YBCO coated conductors (CCs) may suffer interlayer delamination by shear forces, which may give rise to performance degradation of the conductor. The delamination characteristics of YBCO CCs under shear stress should be well understood for better application of YBCO tapes to high-temperature superconducting equipment. This paper describes the experimental results for different delamination strength and critical current Ic degradation behaviors of the YBCO CCs under shear stress at the unslit edge, center, and slit edge of the conductor. The results show that the average delaminated shear strengths of 4.9, 5.9, and 3.7 MPa are recorded for the unslit edge, center, and slit edge at room temperature, whereas 6.8, 7.2, and 5.1 MPa are recorded for the unslit edge, center, and slit edge at liquid nitrogen. The different degradation behaviors of Ic under shear stress are related to the location where delamination occurs in the sample. According to the finite-element method analysis for the stress distribution of the conductor under shear stress, stress concentration leads to initial delamination of the conductor. The decrease of moment arm is suggested to reduce the effect of moment on the delamination test.