Time constant of the transverse-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 µm), and number of tapes (up to 20). This article also provides a systemic study of the relaxation time constant τ , 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.

Modeling cross-field demagnetization of superconducting stacks and bulks for up to 100 tapes and 2 million cycles

“…Then, we can apply the effective E(J) relation of (4) as local for each element, now being J the average current density a the element, and hence it can vary along the tape width. As already shown for a single tape 39,45 , this approach can efficiently model cross-field demagnetization effects in a single tape, as long as it is initially fully saturated due to a perpendicular applied field. Next, we present a more general constitutive relation to also describe demagnetization after partial magnetization or the effect of transverse applied fields simultaneously to the parallel ripple field, as long as their characteristic frequency is much smaller than the ripple field frequency.…”

“…This method has been extensively benchmarked against conventional H-formulation method (COMSOL) and experiments, and have produced excellent results for the given case 38,49,52,55 . We also have seen that, whereas the 3D method is important for short samples, the 2D cross-sectional simplification can be used when the tape length is 3 times or more than the tape width, producing practically the same results as the full 3D method, but with much less computing time 45 .…”

“…Cross-field demagnetization depends on various parameters, such as the tape geometry and the applied ripple field properties. It increases with the applied field amplitude and frequency, tape thickness, and tape length, and decreases with the critical current density and the width of the tape 43 – 45 . The geometry of the HTS stacks also plays an important role.…”

“…The geometry of the HTS stacks also plays an important role. Firstly, the cross-field demagnetization rate is roughly inversely proportional to the number of tapes 36,44,45 . Secondly, ferromagnetic materials can also decrease the demagnetization rate, either by intercalating layers between the tapes 36 , or by placing the stack between ferromagnetic plates 45 .…”

“…Firstly, the cross-field demagnetization rate is roughly inversely proportional to the number of tapes 36,44,45 . Secondly, ferromagnetic materials can also decrease the demagnetization rate, either by intercalating layers between the tapes 36 , or by placing the stack between ferromagnetic plates 45 . More sophisticated structures, such as buried C-shaped stacks in the rotor iron, are also expected to present low cross-field demagnetization 46 .…”