Thermal effects in bulk high-temperature superconductors subjected to AC magnetic fields

We have carried out a theoretical and an experimental study of thermal effectsarising in bulk high-Tc superconductors. The theoretical study has allowed us to predict the self-heating behaviour. We have calculated the temperature evolution. We have shown the existence of a forbidden temperature window, and we have determined the analytical expression of a threshold field (Htr2) separating the « middle» and the «high» dissipation state . From a numerical modelling of a short cylinder, we have determined the time and spatial dependance of dissipated power and temperature within the sample. We have shown that the temperature rise is the highest along the corner location where the dissipated power is maximum. We have designed and constructed a susceptometer for characterizing large bulk superconductors (f &#8594;32 mm). The susceptometer allows a small temperature gradient (< 0.1K) to be achieved in the presence of large heating rates. It allows large AC and DC fields to be applied simultaneously, and was upgraded to measure simultaneously local temperatures andmagnetic inductions. We have determined the heat transfer occuring in the susceptometer chamber.Magneto-thermal measurements with this system can be carried out with a highsensitivity and are found to be in very good agreement with the theoreticalpredictions. This work underlines the importance of the cooling conditions that can affect the distribution of the magneto-thermal properties within the superconductor.