The Reversal of the Local Magnetic Field Profile at the Surface of Superconducting Sample Caused by the Thermomagnetic Avalanche

Puźniak

2022

Materials

Avalanche cascades of magnetic flux have been detected at thermomagnetic instability of the critical state in the plates of Nb-Ti alloy. It was found that, the magnetic flux Φ enters conventional superconductor in screening regime and leaves in trapping regime in the form of a multistage “stairways”, with the structure dependent on the magnetic field strength and magnetic history, with approximately equal successive portions ΔΦ in temporal Φ(t) dependence, and with the width depending almost linearly on the plate thickness. The steady generation of cascades was observed for the full remagnetization cycle in the field of 2–4 T. The structure of inductive signal becomes complex already in the field of 0–2 T and it was shown, on the base of Fourier analysis, that, the avalanche flux dynamic produces, in this field range, multiple harmonics of the electric field. The physical reason of complex spectrum of the low-field avalanche dynamics can be associated with rough structure of moving flux front and with inhomogeneous relief of induction. It was established that the initiation of cascades occurs mainly in the central part of the lateral surface. The mechanism of cascades generation seems to be connected to the “resonator’s properties” of the plates.

Section: Triggering Propagation and Localization Of The Avalanche Flu...supporting

confidence: 85%

Section: Resultsmentioning

confidence: 80%

Section: Introductionmentioning

confidence: 99%

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Multi-Steps Magnetic Flux Entrance/Exit at Thermomagnetic Avalanches in the Plates of Hard Superconductors

Puźniak

2022

Materials

“…Let us consider the process of a thermomagnetic avalanche in the superconductor. The investigation of induction change, occurring as a result of the flux avalanche on the surface of the superconductor, performed with the help of magneto-optical 27–29 or the Hall sensor array measurements 30–32 , in the screening mode (ZFC measurement), revealed local formation of a “dome-like” profile of induction. In such case, the penetration of a flux avalanche results in the appearance of paramagnetic circular current in the diamagnetic region of a superconductor, that determines the local inversion of the magnetic induction profile (Fig.…”

Section: Resultsmentioning

confidence: 99%

“…It should be added that magnetization inversion was registered locally (the local surface self-field) earlier in conventional NbTi superconductors and in textured YBa 2 Cu 3 O 7− δ by a Hall probe 30,41 , a Hall sensor array 31,32 , and a strain gage (magnetostriction) 42 , as a result of partial flux jumps, for the glued sensor located in the region of the avalanche spot. In our experimental conditions, an avalanche spot covers almost the entire sample.…”

Section: Resultsmentioning

confidence: 99%

Magnetic moment inversion at giant flux jump: dynamical property of critical state in type-II superconductors

Puźniak

Kuchuk

et al. 2019

Sci Rep

Experimental evidence of tremendous magnetic moment dynamical inversion, from metastable trapping state to the state with essentially the same moment oriented in the opposite direction, appearing during giant flux jump connected to thermomagnetic avalanche process in superconducting YBa 2 Cu 3 O 7− δ single crystal, is presented. Magnetization inversion takes place in the system, without thermal contact between sample and sample holder, with a tremendous stored energy once the avalanche process is completed in quasi-adiabatic conditions. A model of magnetic moment inversion, caused by the jump between two metastable states of superconductor with the same energy storage, is presented and discussed in terms of the critical state with peculiar evolution of the critical-current spatial distribution. Importantly, knowledge of conditions of the appearance of such a phenomenon is crucial for applications of bulk superconductors as “permanent” magnets, for example, in superconducting levitation devices, etc.

Transformation of the critical state in hard superconductors resulting from thermomagnetic avalanches

Low Temperature Physics

Kuchuk

Rusakov

et al. 2016

The results of experimental studies of magnetic flux dynamics in finite-size superconductors, obtained using integral and local measurements methods, are presented. Local methods were aimed at clarifying the role of the demagnetizing factor in the dynamic formation of a complex magnetic structure of the critical state of hard superconductors. To understand the reasons for drastic transformation of the magnetic induction, we further analyzed the literature data on the visualization of flux dynamics in the presence of avalanches, obtained by magneto-optical methods. New features in the behavior of the magnetic flux during and after an avalanche were revealed and characterized: two stages in the formation of the magnetic induction distribution inside the avalanche region were established—homogeneous and heterogeneous filling with magnetic flux; the mechanism of inversion of the induction profile; velocity oscillations in the propagating magnetic flux front; transformation of the critical state band near the edge of the sample; and the role of the thermal effects and demagnetizing factor in the dissipative flux dynamics. The generalized information allowed us to present, within the framework of the Bean concept, a model of the transformation of the patterns of magnetic induction in the critical state and superconducting currents in a finite superconductor occurring as a result of flux avalanches in two different regimes—shielding and trapping of magnetic flux.