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

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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 smalland reproducible heat transfer coefficient between the superconductor and thecryogenic fluid. The temperature was measured at several locations on thesample surface during the self-heating process. The experimentally determinedtemperature 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 temperatureT(t) is found to agree well with a theoretical prediction based on the one-dimensional (1D) Beanmodel, assuming a uniform temperature in the sample. A 2D magneto-thermal model wasalso used to determine the space and time-dependent temperature distributionT(r, z, t) during the application of the AC field. The losses in the bulk pellet were determined usingan algorithm based on the numerical method of Brandt, which was combined with a heatdiffusion algorithm implemented using a finite-difference method. The modelis shown to be able to reproduce the main trends of the observed temperatureevolution of the bulk sample during a self-heating process. Finally, the 2D model isused to study the effect of a non-uniform distribution of critical current densityJc(r, z) on the losses within the bulk superconductor.