Exploiting flux jumps for pulsed field magnetisation

Abstract

Magnetisation is one of the main barriers to practical use of bulk superconductors as high field magnets. Recently several authors have reported a flux jump effect that allows penetration of magnetic flux into a bulk superconductor during pulsed field magnetisation (PFM) at lower fields than that would be predicted on the basis of the Bean model. We have systematically investigated macroscopic flux jumps in single grain GdBa 2 Cu 3 O 7−δ -Ag (GdBCO-Ag) bulk superconductors with diameters of up to 30 mm when subjected to pulsed magnetic fields. Flux jumps were observed at temperatures between 30 and 77 K and in applied magnetic fields of up to 7 T. The applied pulsed field required to trigger the instability or flux jump field, B j , was determined experimentally and found to increase with decreasing temperature. An extended instability criterion based on a 2D axisymmetric model was used to predict B j at various temperatures and the results are in good agreement with experiments. A significant temperature rise has been measured experimentally during the magnetisation process which indicates that local heat generation due to the sharp rise of the applied field in the PFM process is the primary cause of the flux jumps. The experimental results suggest further that the critical current density reduces to almost zero in the warm part of the sample during the short period of non-equilibrium. A peak trapped field of 4.1 T at the surface and 5.3 T between a stack of two GdBCO-Ag bulk superconductors was achieved at 30 K by means of an optimised two-step pulse sequence with the assistance of the flux jumps, which is extremely promising for potential applications of these technologically important materials.

Flux Dynamics and Thermal Behavior of a GdBaCuO Bulk Magnetized by Single- and Double-Pulse Techniques Using a Split-Type Coil

“…Here, the applied field, at which the Bt value begins to increase, is defined as the rise field, Br. The Bt value for S-sp(65 K) increases from Br = 3.06 T by the flux jump and the highest Bt value of 2.79 T was achieved at Bex = 3.76 T. The Bt value for S-sp(25 K) increased from Br = 4.17 T and a highest Bt of 3.96 T was achieved at Bex1 = 5.43 T. These rapid increases in Bt above Br result from flux jumps [11], [12] (or so-called giant flux leaps (GFLs) in other works [13]), which are a characteristic behavior when using the split-type coil. The rise field, Br, increased and the trapped field, Bt, was usually enhanced when lowering the operating temperature during single-PFM [14].…”

Reliable 4.8 T trapped magnetic fields in Gd–Ba–Cu–O bulk superconductors using pulsed field magnetization

“…An avalanche-like flux jump occurs when the reduction in shielding ability exceeds the increment of Ba, and terminates when a critical state is re-established at the field penetration front. This indicates that a flux jump is prone to occur in a strong pinning scenario.In this paper, we report the PFM of a relatively larger GdBCO-Ag bulk sample than has been achieved previously [27] and an optimised magnetization strategy with a two-step multi-pulse process to routinely achieve a trapped field of 4.8 T.…”

“…Previous reports indicate that a flux jump is an essential condition for obtaining such high trapped fields. We have simulated flux jumps numerically during the PFM process [26] and have extended the classical criterion for a flux jump to occur [27]. Exploiting this phenomenon, we obtained BT of 4.1 T measured at the surface and 5.3 T measured in-between a stack of two GdBCO-Ag samples of diameter 30 mm.…”

Effects of edge cracks on the thermomagnetic instabilities of type-II superconducting thin films

“…TMI is usually considered unfavorable for the application of SC thin films, which degrades the current carrying capacity due to the suppression of superconductivity in the hotspot area [ 9 ], introduces electromagnetic noises to SC devices during sudden entry or exit of vortices [ 10 ], and induces thermal strain or even damage to the material [ 11 , 12 ] due to the sharp temperature rises at hotspots. On the other hand, it was reported that TMI can be utilized to design flux injectors [ 13 ] or to magnetize bulk superconductors [ 14 ]. Experimental observations [ 15 , 16 ] and numerical simulations based on the cellular automaton [ 17 ] show that the distribution of vortex avalanche sizes, the duration and temporal power spectra follow power laws and demonstrate finite-size scaling which are indications of self-organized criticality (SOC) [ 18 ].…”