Flux Jumps Research Articles

Critical State Stability and Flux Jumps' Dynamics in a Single Crystal of YBa2Cu3O7-δ

Critical State Stability and Flux Jumps' Dynamics in a Single Crystal of YBa₂Cu₃O_7-δ

Spark plasma sintering of MgB2 in the two-temperature route

Superconducting bulks of MgB2 were obtained by an ex-situ two-temperature route applied to spark plasma sintering (SPS). Processing of samples was performed at lower temperatures than previously reported. Samples produced by the two-temperature route show a higher morphological uniformity, a higher density (above 98%), a higher Vickers hardness, and undesirable stronger microscale flux jumps, as indicated by magnetic relaxation measurements when compared to a sample obtained by the one-temperature route (95.3% relative density). At the same time, all sintered samples show approximately constant crystallite size, critical current density, irreversibility field, critical temperature, weight fraction of impurity phases (MgB4 and MgO), and the amount of carbon accidentally introduced during SPS processing.

The influence of crystal anisotropy on the critical state stability and flux jump dynamics of a single crystal of La1.85Sr0.15CuO4

We studied the critical state stability of a large cubic sample of single-crystalline La1.85Sr0.15CuO4 for different sample orientations with respect to the external magnetic field as well as the dynamics of the flux jumps. It is shown that thermomagnetic avalanches develop under dynamic conditions, which are characterized by the magnetic diffusivity being significantly lower than the thermal case. In this case, the critical state stability depends strongly on the cooling conditions. We compared predictions from the isothermal model and from the model for a weakly cooled sample with experimental results. In both models, the field of the first flux jump decreases with increase of the sweep rate of the external magnetic field. We also investigated the influence of the external magnetic field on the dynamics of the following stages of the thermomagnetic avalanche. It is shown that the dynamics of the flux jumps is correlated with the magnetic diffusivity, which is proportional to the flux flow resistivity.

Superconducting properties of Y2O3/SiC Co-doped bulk MgB2

The effects of Y2O3/SiC co-doping on impurities and superconducting properties of bulk MgB2 sample were investigated. The temperature range for the formation of YB4 was determined through thermal analysis of the sintering process. Y2O3 and nano-SiC addition improved the critical current density (J c) of bulk MgB2 sample at high field without significant degradation of superconducting transition temperature (T c), and the T c value was about 38 K. Besides boron dislocation caused by C substitution, Mg2Si and YB4 inclusions also contribute to the optimization of J c. In addition, minor flux jumps caused by low stability of magnetic flux occurred at a low field of 20 K and, together with weak intergranular connectivity, were responsible for inferior J c performance.

Stability of the current-carrying state in nonhomogeneous MgB2films

Superconducting thin films can get unstable against magnetic flux jumping. At low temperatures, the probability of an unstable critical state increases since a high critical current density leads to large gradients in the flux density distribution. This phenomenon is often referred to as magnetic flux avalanches. The formation and propagation of these avalanches can be extensively described using a thermomagnetic model. The description of the avalanche process gets more complex when the superconducting film is not homogeneous. Quantitative magneto-optical imaging at tailored MgB${}_{2}$ films revealed that the stability of the current-carrying state is substantially reduced in inhomogeneous superconductors. This is closely related to flux focusing and increased electric fields.

Erratum to: Effect of Flux Jump on Current Density Distributions in Type-II Superconductors

Erratum to: Effect of Flux Jump on Current Density Distributions in Type-II Superconductors

Giant magnetic flux jumps in single crystals of Ba0.6K0.4Fe2As2

Giant magnetic flux jumps are observed in magnetic hysteresis loops of Ba0.6K0.4Fe2As2 single crystals. The size of the flux jumps, which appear only at low temperatures (T<4 K), is so large that it can transform the whole superconducting state into the normal state. The recovery rate to the superconducting state is rather slow, although the superconducting state is almost fully recovered. We show that theoretical predictions based on the adiabatic approach with nonlocal electrodynamics give a good explanation of the flux jumps observed in the single crystals of Ba0.6K0.4Fe2As2.

Observation of Flux Jump in (MgB2)0.96Ni0.04 Superconductor Doped with Milled Ni powders

Bulk (MgB2)0.96Ni0.04 samples doping with premilled Ni powders were sintered at 750 °C for 30 min. During sintering, liquid Mg–Ni phase prompts solid–solid reaction between Mg and B and the size of milled Ni powder determines the final distribution of the secondary MgNi2.5B2 phase in the sintered samples. A flux jump was observed in the (MgB2)0.96Ni0.04 samples doping with Ni powders. Recognized from the measured superconductive properties, smaller-sized Ni powders can provide more effective flux pinning centers and thus improve the performance of the critical current density.

Magnetoresistance, critical current density, and magnetic flux pinning mechanism in nickel doped BaFe2As2 single crystals

The critical current density, Jc, flux pinning behavior and magneto- resistance results of BaFe2-xNixAs2 single crystal have been investigated in fields up to 13 T over a temperature range of 2 to 20 K. The magnetoresistance below the superconducting transition temperature (Tc) shows Arrhenius thermally activated behavior: ρ = ρoexp(-Uo(T,H)/kBT), where Uo is the thermally activated energy. BaFe2-xNixAs2 exhibits high thermally activated flux flow energy with a very weak field dependence. Jc is as high as 2 × 105 A/cm2 for zero magnetic field at 2 K. Jc was found to decrease for B < 1 T, but showed a very weak field dependence and remained nearly constant with increasing magnetic field for B > 1 T at T = 2, 5, and 10 K. Flux jumping was also observed in magnetization loops at very low temperature for large sample, which is related to the high Jc in the single crystal. A peak effect was observed at 10 K.

Erratum to: Effect of magnetic relaxation on flux jumps in high temperature superconductors

The contents of the article ‘Effect of magnetic relaxation on flux jump in high temperature superconductors’, Eur. Phys. J. B 77, 363 (2010) are retracted from publication by the authors due to admitted plagiarism of unpublished work. T.H. He wishes to state that he was not involved in the publication process and his name was used on the paper without his knowledge. The work reported was performed by Xiaobin Yang and You-He Zhou at the Key Laboratory of Mechanics on Disaster and Environment in Western China Attached to the Ministry of Education of China. Xiaobin Yang and You-He Zhou retain the right to resubmit the work for publication in a journal of their choice.

An algorithm for correcting CoRoT raw light curves

We introduce the CoRoT detrend algorithm (CDA) for detrending CoRoT stellar light curves. The algorithm CDA has the capability to remove random jumps and systematic trends encountered in typical CoRoT data in a fully automatic fashion. Since enormous jumps in flux can destroy the information content of a light curve, such an algorithm is essential. From a study of 1030 light curves in the CoRoT IRa01 field, we developed three simple assumptions which upon CDA is based. We describe the algorithm analytically and provide some examples of how it works. We demonstrate the functionality of the algorithm in the cases of CoRoT0102702789, CoRoT0102874481, CoRoT0102741994, and CoRoT0102729260. Using CDA in the specific case of CoRoT0102729260, we detect a candidate exoplanet around the host star of spectral type G5, which remains undetected in the raw light curve, and estimate the planetary parameters to be Rp=6.27Re and P=1.6986 days.

Preparation and Properties of MPMG YBCO Bulk with Y2O3 Layer

In this study, four kinds of melt-processed YBCO samples were fabricated with the MPMG procedure. The compacted powders were located on a crucible with a buffer layer of Y2O3 to avoid liquid to spread on the furnace plate. Their microstructures were defined by XRD analysis and polarized light optical microscopy. The microstructure investigations indicated that the 123 grains were very big and fine and dispersed 211 particles remained in the samples. Resistivities of the samples were measured by a standard continuous dc four-probe method. Magnetization measurements were made and flux jumps were observed at a relatively higher temperature for Y1060. The critical current density, Jc, values of the samples, measured by VSM in 5 T magnetic field, exceeded 0.6×103 A⋅cm−2 at 77 K and 4 T.

Magnetic shielding capability ofMgB2 cylinders

The shielding of a DC magnetic field has been demonstrated to becomplete at 4.2 K, at least up to 2 T, inside the 18 mm bore of a thickMgB2 bulk cylinder, 70 mm tall. No hysteresis has been detected along the cycle of thefield application and removal, and no flux jumps have been detected either. TheMgB2 cylinderwas manufactured by the Reactive Mg Liquid Infiltration process, an ‘in situ’ technology able to give verydense MgB2 manufacts. According to the previously measuredJc(B, T) characteristicsof a MgB2 bulk superconductor of the same origin and quality, a critical current density of about30 kA cm − 2 can be assumed at the fields of 2–3 T and at the temperature of 4.2 K of thereported shielding experiment. Accordingly, we estimate that the thickness of theMgB2 cylinder filled by the shielding currents is about 5.75 mm, lower than the present cylinderthickness of 8.77 mm.

Robust residual‐ and recovery‐based a posteriori error estimators for interface problems with flux jumps

AbstractFor elliptic interface problems with flux jumps, this article studies robust residual‐ and recovery‐based a posteriori error estimators for the conforming finite element approximation. The residual estimator is a natural extension of that developed in [Bernardi and Verfürth, Numer Math 85 (2000), 579–608; Petzoldt, Adv Comp Math 16 (2002), 47–75], and the recovery estimator is a nontrivial extension of our method developed in Cai and Zhang, SIAM J Numer Anal 47 (2009) 2132–2156. It is shown theoretically that reliability and efficiency bounds of these error estimators are independent of the jumps provided that the distribution of the coefficients is locally monotone. © 2010 Wiley Periodicals, Inc. Numer Methods Partial Differential Eq 28:476–491, 2012

Multi‐scale modeling of moving interface problems with flux and field jumps: Application to oxidative degradation of ceramic matrix composites

AbstractProblems involving reaction and species diffusion involve field and flux jumps at a moving reaction front. In multi‐scale problems such as carbon fiber composite oxidation, these effects need to be tracked at the microscopic scale of individual carbon fibers. A multi‐scale model is derived in this paper for predicting species distribution in such problems using a fully coupled multi‐scale homogenization approach. The homogenized fluxes from the micro‐scale are derived using Hill's macro‐homogeneity condition accounting for both flux jumps and species density field jumps at the reacting interface in the micro‐scale unit cell. At the macro‐scale, the competition between the transport of reacting species (oxygen) and the reaction product (carbon dioxide) is modeled using homogenized mass conservation equations. The moving reaction front in carbon fibers at the micro‐scale is tracked using level set method and an adaptive meshing strategy. The macroscopic weight loss of the composite when exposed to oxygen is simulated as a function of time using a coupled finite element methodology at various locations in a validated macroscopic model. Copyright © 2010 John Wiley & Sons, Ltd.

The Influence of the Magnetic Field on the Dynamics of the Flux Jumps in the Flux Flow Model

We studied the dynamics of the flux jumps in type II superconductors assuming this dynamics to be determined by magnetic diffusion. The conductivity of the sample during the jump was described by the flux flow conductivity, which depends on local magnetic field. We solved this problem numerically for an infinite slab sample. We assumed that superconducting sample was fully penetrated by the magnetic flux at the beginning and that the local induction of the magnetic field, in the whole sample, was equal to B0. Then, the external magnetic field was increased by a value of ∆Ba and in the external magnetic field of B0 + ∆Ba flux jump occurred. In our simulations we used the parameters characteristic of a conventional NbTi superconductor at 4.2 K. In particular, we studied the influence of the parameter B0 on the flux jumps’ dynamics. We have found that this parameter strongly influences initial stage of the diffusion process. With increasing B0, the time during which flux front of the avalanche reaches the center of the sample decreases. The results of numerical simulations were compared with experiments.

Saw-tooth pattern from flux jumps observed by high resolution M-H curves in MgB2 thin films

While flux jumps have been observed in the magnetic hysteresis loops of superconductors, a saw-tooth pattern of the flux jump is known to appear only in a bulk superconductor. But in this study, we were able to observe the saw-tooth pattern in MgB2 thin film with the careful data acquisition method enhancing the data taking capability and report the details of the distribution of the field interval between jumps Bfj, and the size of the flux jump, Mfj. The theory based on Bean’s model in the adiabatic approach was adapted and it was compared with experimental results. In addition, we observe the cross-over between the saw-tooth pattern and a rounded saw-tooth pattern, as a byproduct. A patterns diagram of the vortex jump was drawn on the H-T plane.

The Influence of Magnetic History on the Stability of Critical State and the Dynamics of Flux Jumps in Conventional NbTi Superconductor

We studied the influence of magnetic history on the stability of critical state and the dynamics of flux jumps in conventional NbTi superconductor. We have found that both the stability of critical state and the dynamics of flux jumps are strongly influenced by the magnetic history and by the magnetic field distribution induced within the sample by preceding jumps. This magnetic field distribution can be particularly complex in the region of the hysteresis loop where the external magnetic field changes its sign (region of remagnetization), because of demagnetizing eects. For this reason, the most complex structure of the flux jumps is observed in the third quadrant of magnetization hysteresis loop. Flux jumps observed in the region of remagnetization have dierent dynamics in comparison to the jumps observed in other regions of the magnetization hysteresis loop. Duration of these jumps is usually significantly longer than others, and their structure is more complex.

Improvement of the critical current density of spark plasma sinteredMgB2 byC60 addition

We investigated the influence of fullerene (C60) addition on the superconducting parameters ofMgB2 bulk samples obtained by spark plasma sintering (SPS). It was found that 1.5 wt%C60 addition leads to the reduction of the critical temperature by a few K,whereas the magnetically determined critical current density increasesby approximately one order of magnitude in an applied magnetic fieldH = 70 kOe attemperature T = 10 K. This indicates substitution of B by C inC60 addedMgB2 processed by SPS, contrary to some data reported in the literatureobtained using conventional methods. For our relatively large specimens,the occurrence of macroscale flux jumps was observed over a wide (H, T) domain. The relaxation of the irreversible magnetization suggests that themacroscale flux jumps transform into microscale flux jumps with increasingH andT.

Effect of Flux Jump on Current Density Distributions in Type-II Superconductors

The effect of flux jump on the current density distributions is studied in type-II superconducting slab. The magnetothermal diffusion equations based on the Kim model in the regime of flux creep are presented to estimate the effect of flux jump on the distributions of current density inside the slab and the changes of surface current density in the flux jump process. Numerical results show that the current density distributions are dependent on the flux jump. It is intended that the flux jump analysis presented be useful to researchers interested in flux jump instability characteristics.