Flux jumps in high-J_cMgB₂bulks during pulsed field magnetization

Abstract

Pulsed field magnetization (PFM) of a high-J c MgB 2 bulk disk has been investigated at 20 K, in which flux jumps frequently occur for high pulsed fields. Using a numerical simulation of the PFM procedure, we estimated the time dependence of the local magnetic field and temperature during PFM. We analyzed the electromagnetic and thermal instability of the high-J c MgB 2 bulk to avoid flux jumps using the time dependence of the critical thickness, d c (t), which shows the upper safety thickness to stabilize the superconductor magnetically, and the minimum propagation zone (MPZ) length, l m (t), to obtain dynamical stability. The values of d c (t) and l m (t) change along the thermally-stabilized direction with increasing temperature below the critical temperature, T c. However, the flux jump can be qualitatively understood by the local temperature, T(t), which exceeds T c in the bulk. Finally, possible solutions to avoid flux jumps in high-J c MgB 2 bulks are discussed.

Trapped field of 1.1 T without flux jumps in an MgB2bulk during pulsed field magnetization using a split coil with a soft iron yoke

“…which T(t) takes a maximum at around 1 s at the center of the bulk surface due to the large C value of 132 J kg −1 K −1 and relatively small κ value along the c-axis of 4 W mK −1 . On the other hand, the C and κ values of the MgB 2 bulk used in the simulation are 2.9 J kg −1 K −1 and 20 W mK −1 , respectively [14]. These results imply that the heat generation and propagation in the present MgB 2 bulk are quite different to those in the GdBaCuO bulk.…”

“…Based on the experimental setup, three types of models were constructed for the numerical simulation of PFM as shown in figure 2, in which the physical phenomena occurring during the magnetization process are described using the fundamental electromagnetic and thermal equations in axisymmetric coordinates. The details of the numerical simulation have been described elsewhere [14]. The power-n model (n=100) was used to describe the nonlinear E-J characteristic of the MgB 2 bulk.…”

“…The flux dynamics and heat generation in the MgB 2 bulk around 20 K are in clear contrast with those in the REBaCuO bulk at around typical operating temperature of 40 K because of the small specific heat, large thermal conductivity, and a narrow temperature margin against T c for the MgB 2 bulk. We have previously analyzed and discussed the electromagnetic and thermal instability of a high-J c MgB 2 bulk using conventional relations, such as the critical thickness, d c , and the minimum propagation zone length, l m [14].…”

Magnetic flux instability in NbN films exposed to fast field sweep rates

“…The instability occurs when vortices escape from pinning centers, locally heating the material, thus promoting additional flux motion and generating a positive feedback that results in largescale flux avalanches. This phenomenon appears as flux jumps in wires and bulk superconductors [7][8][9], and as dendritic flux formations in thin films. The latter has been observed in a large number of superconductors important for practical applications, such as MgB 2 [10,11], Nb [12,13], YBCO [14], and NbN [15].…”

Trapped Field Enhancement of a Thin, High-Jc MgB2 Bulk Without Flux Jumps Using Pulsed Field Magnetization With a Split-Type Coil With a Soft Iron Yoke

“…B z (PFM) = 0.81 T at 14 K had been the highest value in a high-J c MgB 2 bulk pair [14], in which flux jumps took place frequently during PFM due to a small specific heat, a large thermal conductivity, and a narrow temperature margin between the operating temperature, T s , and the superconducting transition temperature, T c = 39 K. The use of a split-type coil was effective to enhance the B z value for the REBaCuO bulks during PFM both experimentally and numerically [15,16,17]. We have also analyzed the electromagnetic and thermal instability of a high-J c MgB 2 bulk using conventional parameters, such as the critical thickness, d c , and the minimum propagation zone (MPZ) length, l m [18]. Recently, we have achieved a trapped field of B z = 1.1 T on a thick, high-J c MgB 2 bulk at 13 K without flux jumps using a split-type coil with a soft iron yoke, which is a record high trapped field by PFM for bulk MgB 2 to date [19].…”