3D Finite Element Models of Stacked Tapes Magnetic Shields

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Measurements on magnetized GdBCO pellets subjected to small transverse ac magnetic fields at very low frequency: Evidence for a slowdown of the magnetization decay

Due to their ability to trap large magnetic inductions, superconducting bulk materials can be used as powerful permanent magnets. The permanent magnetization of such materials, however, can be significantly affected by the application of several cycles of a transverse variable magnetic field. In this work, we study, at T = 77 K, the long term influence of transverse ac magnetic fields of small amplitudes (i.e. much smaller than the full penetration field) on the axial magnetization of a bulk single grain superconducting GdBCO pellet over a wide range of low frequencies (1 mHz–20 Hz). Thermocouples are placed against the pellet surface to probe possible self-heating of the material during the experiments. A high sensitivity cryogenic Hall probe is placed close to the surface to record the local magnetic induction normal to the surface. The results show first that, for a given number of applied triangular transverse cycles, higher values of dBapp/dt induce smaller magnetization decays. An important feature of practical interest is that, after a very large number of cycles which cause the loss of a substantial amount of magnetization (depending on the amplitude and the frequency of the field), the rate of the magnetization decay goes back to its initial value, corresponding to the relaxation of the superconducting currents due to flux creep only. In the amplitude and frequency range investigated, the thermocouples measurements and a 2D magneto-thermal modelling show no evidence of sufficient self-heating to affect the magnetization so that the effect of the transverse magnetic field cycles on the trapped magnetic moment is only attributed to a redistribution of superconducting currents in the volume of the sample and not to a thermal effect

Semi-analytical study of AC losses in an infinitely long superconducting cylinder surrounded by a metallic sheath: magnetic field dependent critical current density and generation of harmonics

We study the AC losses in an infinitely long cylinder made of a superconducting core surrounded by a non-magnetic metallic sheath and subjected to an axial magnetic field. The losses are computed by assuming the Bean–Kim model for the superconductor and Ohmic dissipation for the metal. The time varying magnetic flux crossing the superconductor induces eddy currents in the metal sheath and, due to the nonlinear response of the superconducting material, generates harmonics in the metal current density. In turn, these currents generate distorted magnetic fields acting back on the superconductor. This coupling mechanism is sensitive to the magnetic constitutive law of the superconductor and affects both the waveform of the fields and the total losses. In this paper, we study the importance of the harmonics in the metal on the total losses, as well as their sensitivity to a field dependent critical current density following Kim's law

AC and heat losses in a superconductor cylinder surrounded by a metallic sheath

In this work, we study numerically the magneto-thermal properties of a composite cylinder made of a superconducting core surrounded by a metallic sheath and subjected to an axial AC magnetic field . The metal is ohmic and non-magnetic while the superconductor is characterized by an E(J) power law. The system is studied in a regime for which the temperature is non uniform (and hence the current density is varying spatially), in order to investigate heat draining by the cryofluid. The magneto-thermal problem is solved by the Brandt method for the magnetic part, associated with a standard discretization of the stationary heat equation. The heat loss in the structure is determined as a function of the external parameters (frequency, amplitude of the applied field and convective exchange coefficient). The influence of the spatial step in the metal and in the superconductor is discussed

Numerical modelling of the magneto-thermal effects of a transverse AC magnetic field on the axial magnetization of a bulk superconducting cylinder

We study the decrease of the magnetization of a bulk superconducting sample when it is subjected to a perpendicular AC magnetic field (\crossed field" configuration). The effect of a transverse AC magnetic field on the permanent magnetization of the supercon- ductor are twofold: (i) the superconducting currents are redistributed in the supercon- ductor, and (ii) AC losses are generated, which may locally increase the temperature and degrade the superconducting properties. These two phenomena (current redistribution and losses) are differently affected by the frequency and the amplitude of the transverse magnetic field. The goal of this study is to identify the sets of experimental conditions that accentuate each of these two effects, and more specifically to assess their importance in crossed field experiments being carried in our research group. The cylindrical superconductor is surrounded by a metallic sheath and is initially magnetized along its symmetry axis. The magneto-thermal modelling is carried out by using a finite element method (FEM) implemented in a open source package (GetDP). The validity of these simulations is verified by comparing modelling results with experi- mental data obtained for small amplitudes and large numbers of cycles for the transverse magnetic field.ARC 11/16-0

AC losses of an infinitely long superconductor cylinder surrounded by a metallic sheath

We study the AC losses in a structure composed of an infinitely long superconducting cylinder surrounded by a metallic sheath. The system is subjected to an axial alternating magnetic flux. The AC losses are evaluated locally in each material and compared to the situation where no metallic sheath is used. The superconductor region is described by the Bean-Kim model, while the metal is assumed ohmic and non-magnetic. The variation of the magnetic flux in the superconductor directly affects the eddy currents in the metal sheath. This back-reaction is modelled numerically through a Fourier decomposition of the electrical and magnetic field at the interface between the superconductor and the metal. The resulting losses are evaluated as a function of the frequency and the amplitude of the source field.ARC 11/16-0

3D Finite Element Models of Stacked Tapes Magnetic Shields

International audienc

Finite Element Models for Magnetic Shields Made of Stacked Superconducting Tape Annuli

Stacks of high-temperature superconducting tape annuli can be used as magnetic shields operating efficiently for both axial and transverse fields. However, due to their layered geometry and hybrid electrical and magnetic properties, implementing models of such structures is not straightforward. In this work, we propose two different modelling approaches with the finite element method: layered and homogenized. We compare their accuracy and numerical efficiency for three different formulations (h-phi, h-phi-b, and a-j), in both axial (2D-axisymmetric) and transverse (3D) configurations. We show that both approaches lead to comparable performance in the axial case, but that the homogenized model is considerably harder to use in the transverse case

A Suspected Case of Lyme Borreliosis in a Dog from Belgium

A 5-year-old Jack Russell Spaniel was presented in December 2017 to his veterinarian in Belgium for sudden weakness, reluctance to move, and pain. Blood analysis showed no deviations and serum increased levels of B. burgdorferi s.l. antibodies were detected. The dog recovered a few days after the onset of doxycycline treatment. This case illustrates the possible relationship between tick-borne diseases and orthopedic problems

Magnetic characterisation of large grain, bulk Y–Ba–Cu–O superconductor–soft ferromagnetic alloy hybrid structures

Large grain, bulk Y–Ba–Cu–O (YBCO) high temperature superconductors (HTS) have significant potential for use in a variety of practical applications that incorporate powerful quasi-permanent magnets. In the present work, we investigate how the trapped field of such magnets can be improved by combining bulk YBCO with a soft FeNi, ferromagnetic alloy. This involves machining the alloy into components of various shapes, such as cylinders and rings, which are attached subsequently to the top surface of a solid, bulk HTS cylinder. The effect of these modifications on the magnetic hysteresis curve and trapped field of the bulk superconductor at 77 K are then studied using pick-up coil and Hall probe measurements. The experimental data are compared to finite element modelling of the magnetic flux distribution using Campbell’s algorithm. Initially we establish the validity of the technique involving pick-up coils wrapped around the bulk superconductor to obtain its magnetic hysteresis curve in a non-destructive way and highlight the difference between the measured signal and the true magnetization of the sample. We then consider the properties of hybrid ferromagnet/superconductor (F/S) structures. Hall probe measurements, together with the results of the model, establish that flux lines curve outwards through the ferromagnet, which acts, effectively, like a magnetic short circuit. Magnetic hysteresis curves show that the effects of the superconductor and the ferromagnet simply add when the ferromagnet is saturated fully by the applied field. The trapped field of the hybrid structure is always larger than that of the superconductor alone below this saturation level, and especially when the applied field is removed. The results of the study show further that the beneficial effects on the trapped field are enhanced when the ferromagnet covers the entire surface of the superconductor for different ferromagnetic components of various shapes and fixed volume