Vortex shaking study of REBCO tape with consideration of anisotropic characteristics

Stacks of superconducting tapes can trap much higher magnetic fields than conventional magnets. This makes them very promising for motors and generators.However, ripple magnetic fields in these machines present a cross-field component that demagnetizes the stacks. At present, there is no quantitative agreement between measurements and modeling of cross-field demagnetization, mainly due to the need of a 3D model that takes the end effects and real micron-thick superconducting layer into account. This article presents 3D modeling and measurements of cross-field demagnetization in stacks of up to 5 tapes and initial magnetization modeling of stacks of up to 15 tapes. 3D modeling of the cross-field demagnetization explicitly shows that the critical current density, J c , in the direction perpendicular to the tape surface does not play a role in cross-field demagnetization. When taking the measured anisotropic magnetic field dependence of J c into account, 3D calculations agree with measurements with less than 4 % deviation, while the error of 2D modeling is much higher. Then, our 3D numerical methods can realistically predict cross-field demaga This article has been published in Supercond. Sci. Technol. with netization. Due to the force-free configuration of part of the current density, J, in the stack, better agreement with experiments will probably require measuring the J c anisotropy for the whole solid angle range, including J parallel to the magnetic field.the probe is at least 10 mV/T. Cross-field demagnetization consists on the following three main steps: magnetization by field cooling (FC) method, relaxation time and cross-field demagnetization. The detailed process is the following:• The sample is placed into the electromagnet at room temperature.• The electromagnet is ramped up to 1 T.• The sample is cooled down in liquid nitrogen bath at 77 K.• The electromagnet is ramped down with ramp rate 10 mT/s. 400 405 410 415 420 425 B t /B 0 [-] t [s] cal 50 mT cal 100 mT cal 150 mT cal 50 mT n(B,θ) cal 100 mT n(B,θ) cal 150 mT n(B,θ) (b) FIG. 19: (a) The n(B, θ) measured data on a 4 mm wide SuperOx tape, measured in Bratislava by the set-up in [46]. (b) The comparison of calculation with constant n=30 and n(B, θ) dependence, both cases use J c (B, θ) dependence. Using n(B, θ) slightly reduces the demagnetization rate for a few number of cycles, but later on it is increased slightly.

Section: G Cross-field Demagnetization: Measurements and Modellingsupporting

confidence: 84%

Section: Introductionmentioning

confidence: 99%

Cross-field demagnetization of stacks of tapes: 3D modelling and measurements

Kapolka¹,

Pardo

Grilli³

et al. 2019

“…As we can see, this increase is simply due to the larger mutual inductance between the whole stack and the current in the magnetization loop of one of the tapes of the stack. 6…”

Section: Time Constant For a Thin Stack Of Tapesmentioning

confidence: 99%

“…Also, for thin tapes, this magnetization decay is very slow [4]. Similarly, the relaxation decay constant is also dependent on different parameters, and it decreases with ripple field amplitude and increases with number of tapes [4,6]. However, measurements in [5] show that for large enough ripple fields (above the parallel penetration field of one tape, according to [2]) the stack fully demagnetizes after many cycles (10 4 or more).…”

Section: Introductionmentioning

confidence: 99%

Time constant of the transverse-field demagnetization of superconducting stacks of tapes

Dadhich¹,

Pardo²,

Kapolka³

2020

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.

“…The main advantage of using this geometry for the numerical modelling of type-II superconductors, relies on the possibility to compare the numerical results with exact or semi-analytical approaches, what allows a direct grasp of the physics involved whilst a phenomenological benchmark for the implementation of practical applications is being developed. In fact, the simple notion of this 2D geometry, i.e., by assuming that the length of the SC wire is much greater than its radius, and the critical state theory for the macroscopic modelling of the electromagnetic properties of type-II SCs [5][6][7][8], has allowed to predict and explain multitude of experimental phenomena including but not limited to, the magnetization and demagnetization of SCs under crossed and rotating magnetic field experiments [9][10][11][12][13], the low pass filtering effect in the magnetic moment of AC SC wires [14], and strong patterns of localization of the density of power losses inside the SC materials under magnetic and electrical stress conditions [15,16].…”

Section: Introductionmentioning

confidence: 99%

Magnetic characteristics and AC losses of DC type-II superconductors under oscillating magnetic fields

Robert

2018

Remarkable features on the magnetic moment of type-II superconducting wires of cylindrical shape, subjected to direct current conditions (DC) and transverse oscillating (AC) magnetic fields, are reported. We show how for relatively low amplitudes of the applied magnetic field, Ba, the superconducting wire rapidly develops a saturation state, |Mp|, characterizing the limits of magnetization loops that exhibit a Boolean-like behaviour. Regardless of the premagnetization state of the superconducting wire, we show how after two cycles of magnetic relaxation, boolean-like ±Mp states can be measured during the entire period of time from which the external magnetic field B0 ranges from 0 to ±Ba, with the signs rule defined by the sign of the slope ∆B0y(t). In addition, for the practical implementation of superconducting DC wires sharing the right of way with AC lines, we report that for relatively low values of magnetic field, Ba ≤ BP /2, being BP the analytical value for the full penetration field in absence of transport current, Itr, the use of semi-analytical approaches for the calculation of AC-losses leads to a significant underestimation of the actual contribution of the induction losses. This phenomena is particularly relevant at dimensionless fields ba < 1 − i 2/3 a , being ba = Ba/BP and, ia = Ia/Ic the amplitude of an AC or DC transport current, due to the local motion of flux front profiles being dominated by the occurrence of transport current. On the other hand, we have found that regardless of the nature of the transport current, either be DC or AC, when a transverse oscillating magnetic field greater than the classical limit ba = (1 − ia) is applied to the SC wire, the difference between the obtained AC losses in both situations results to be negligible indistinctly of the approach used, semi-analytical or numerical. Thus, the actual limits from which the estimation of the AC-losses can be used as an asset for the deployment of DC SC wires sharing the right of way with AC lines, against the sole use of SC wires for the transmission of AC transport current, are established.