Self-heating of bulk high temperature superconductors of finite height subjected to a large alternating magnetic field

Abstract

Abstract. In this work we study, both experimentally and numerically, the self-heating of a bulk, large YBCO pellet of aspect ratio (thickness / diameter) ~ 0.4 subjected to a large AC magnetic field. To ensure accurate temperature measurements, the sample was placed in an experimental vacuum chamber to achieve a small and reproducible heat transfer coefficient between the superconductor and the cryogenic fluid. The temperature was measured at several locations on the sample surface during the self-heating process. The experimentally determined temperature gradients are found to be very small in this arrangement (< 0.2 K across the radius of the superconductor). The time-dependence of the average temperature T(t) is found to agree well with a theoretical prediction based on the one-dimensional (1-D) Bean model, assuming a uniform temperature in the sample. A 2-D magneto-thermal model was also used to determine the space and time-dependent temperature distribution T (r, z, t) during the application of the AC field. The losses in the bulk pellet were determined using an algorithm based on the numerical method of Brandt, which was combined with a heat diffusion algorithm implemented using a finite-difference method. The model is shown to be able to reproduce the main trends of the observed temperature evolution of the bulk sample during a self-heating process. Finally, the 2-D model is used to study the effect of a non-uniform distribution of critical current density J c (r, z) on the losses within the bulk superconductor.

Determination of <inline-formula> <tex-math notation="LaTeX">$J_C$</tex-math></inline-formula> and <inline-formula> <tex-math notation="LaTeX">$n$</tex-math></inline-formula>-Value of HTS Pellets by Measurement and Simulation of Magnetic Field Penetration

“…Some authors already used Hall probes to determine the magnetic field distribution in a superconducting sample [31]- [33]. In order to use the results derived earlier by simulations, one needs to measure the complete penetration magnetic field B P in the pellets.…”

A New Direct Magnetic Method for Determining ${\rm J}_{\rm C}$ in Bulk Superconductors From Magnetic Field Diffusion Measurements

“…Some authors already used Hall probes to determine magnetic field distribution in superconducting sample [9]- [13]. In our method the complete penetration magnetic field is detected with an axial Hall probe placed between two HTS pellets ( is neglected.…”

Study of buoyancy driven heat transport in silicone oils and in liquid nitrogen in view of cooling applications

“…The governing equations (1)(2)(3)(4)(5) are solved by a finite-volume method using a general purpose computational fluid dynamics (CFD) software [23]. The pressure-velocity coupling between the incompressible Navier-Stokes and the continuity equations are solved using the SIMPLE algorithm [24].…”

“…First, the material must be maintained below its critical temperature, T c , above which it becomes a normal material (typically, T c ~ 90 K for YBCO, so that liquid nitrogen is a genuine cryofluid). Second, under time-varying fields, the material may undergo losses and self-heating, resulting in a reduction of the trapped magnetic flux [4]. To avoid a performance loss, the cryogenic system must then rapidly extract the generated heat and transfer it away from the superconducting material.…”