Time constant of the cross field demagnetization of superconducting stacks of tapes

Abstract

Stacks of REBCO tapes can trap large amounts of magnetic fields and can stay magnetized for long periods of times. This makes them an interesting option for major engineering applications such as motors, generators and magnetic bearings. When subjected to transverse alternating fields, superconducting tapes face a reduction in the trapped field, and thus it is the goal of this paper to understand the influence of all parameters in cross field demagnetization of stacks of tapes. Major parameter dependencies considered for the scope of this paper are ripple field amplitude, frequency, tape width, tape thickness (from 1 to 20 $\mu$m), and number of tapes (up to 20). This article also provides a systemic study of the relaxation time constant $\tau$, which can be used to estimate the cross-field demagnetization decay for high number of cycles. Modeling is based on the Minimum Electro-Magnetic Entropy Production method, and it is shown that the 2D model gives very accurate results for long samples when compared with 3D model. Analytical formulas for large number of cycles have been devised. The results show that when the ripple field amplitude is above the penetration field of one tape, the stack always fully demagnetizes, roughly in exponential decay. Increasing the number of tapes only increases the relaxation time. The formulas derived also hold when validated against numerical results, and can be used for quick approximation of decay constant. They also show that the cause of the decreases of cross-field demagnetization with number of tapes is the increase in the self-inductance of the magnetization currents. The trends and insights obtained for cross field demagnetization for stacks are thus very beneficial for engineers and scientists working with superconducting magnet design and applications.