Abstract
Degradation of a REBCO coil under external magnetic fields is one of the major technical problems in the field of HTS magnet technology. A possible cause of such degradation is an inhomogeneous hoop stress distribution, or hoop stress modification (both increase and decrease), induced by the screening current. In this work, we investigate such a hoop stress modification by a small coil experiment with a strain measurement and a numerical simulation. An experimental result shows a very high stress increase factor of >4.1, defined by the maximum circumferential stress over BzJR stress, and the simulated result is in qualitative agreements. The strain (stress) shows a hysteresis effect corresponding to the screening current behavior. A large hoop stress modification causes not only a hoop stress increase, but also buckling of the conductor, which induces delamination and micro-clacks of the superconducting layer. We also show the stress modification can be reduced by bonding turns with epoxy.
Highlights
O NE of the most important issues to be solved for high-temperature superconducting (HTS) magnet is Manuscript received September 25, 2019; accepted February 12, 2020
Even in the case of zero current, a circumferential strain as high as 0.16% appears at the upper part of the conductor, while it is nearly zero at the lower part of the conductor; the 0.16% strain corresponds to 281 MPa based on the stress-strain characteristics obtained from the numerical model described. They are caused by the electromagnetic force resulting from the interaction between the axial-magnetic field and the screening current; their directions are outward at the upper part whereas inward at the lower part
After holding the external magnetic field, εθ,up showed a negative drift with time, which was caused by relaxation of the screening currents
Summary
O NE of the most important issues to be solved for high-temperature superconducting (HTS) magnet is Manuscript received September 25, 2019; accepted February 12, 2020. The mechanism of thermal stress-induced degradation has been extensively investigated and some remedies have been proven to be effective [4]–[12]. In the latter case, the REBCO coil still sometimes shows degradation of the performance, the average hoop stress, BzJR, caused by the electro-magnetic force is far below the tolerable strength of the REBCO conductor [3], where Bz is the axial magnetic field, J is the conductor current density and R is the coil radius
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