Magneto-thermal phenomena in bulk high temperature superconductors subjected to applied AC magnetic fields

TL;DR

This paper combines theoretical and experimental analysis to understand how bulk high-temperature superconductors heat up under large AC magnetic fields, identifying key thermal thresholds and regimes.

Contribution

It introduces an analytical model predicting temperature limits and thresholds for thermal runaway in superconductors under AC fields, validated by experiments.

Findings

Existence of a limit heat transfer coefficient separating thermal regimes

Identification of a forbidden temperature window during self-heating

Thermal runaway occurs above a critical field amplitude Htr2

Abstract

In the present work we study, both theoretically and experimentally, the temperature increase in a bulk high-temperature superconductor subjected to applied AC magnetic fields of large amplitude. We calculate analytically the equilibrium temperatures of the bulk sample as a function of the experimental parameters using a simple critical-state model for an infinitely long type-II superconducting slab or cylinder. The results show the existence of a limit heat transfer coefficient (AUlim) separating two thermal regimes with different characteristics. The theoretical analysis predicts a "forbidden" temperature window within which the temperature of the superconductor can never stabilize when the heat transfer coefficient is small. In addition, we determine an analytical expression of two threshold fields Htr1 and Htr2 characterizing the importance of magneto-thermal effects and show that a thermal runaway always occurs when the field amplitude is larger than Htr2. The theoretical predictions of the temperature evolution of the bulk sample during a self-heating process agree well with the experimental data. The simple analytical study presented in this paper enables order of magnitude thermal effects to be estimated for simple superconductor geometries under applied AC magnetic fields and can be used to predict the influence of experimental parameters on the self-heating characteristics of bulk type-II superconductors.