Stability and quench of a HTS magnet with a hot spot

Stability and quench experiments were performed on a cryocooler refrigerated Bi-2223/Ag magnet with an artificial hot spot. The hot spot was realized by making a poor, high-resistivity joint between the current terminal and the bottom pancake coil of the magnet. According to the experiments the magnet could tolerate considerable power dissipation at the hot spot for long times still avoiding a thermal runaway. Processes preceding the quench occurred much slower if compared to quenches in typical LTS windings. For a deeper understanding of the measured results computer simulations were done by using a model based on the heat conduction equation coupled with Maxwell's equations and solved using the finite-element method. Simulated temperature distributions inside the magnet showed that considerable temperature differences exist between different parts of the magnet, despite the slowness of the processes. Before a quench there were no large temperature gradients around the hot spot. During the quench the hot spot temperature increased rapidly but the temperature of other parts of the magnet increased only with a delay.