Stability and quench of a HTS magnet with a hot spot

Stability and quench experiments were performed on a cryocooler refrigerate d 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 t he bottom pancake coil of the magnet. According to the experiments the magn et could tolerate considerable power dissipation at the hot spot for long t imes still avoiding a thermal runaway. Processes preceding the quench occur red much slower if compared to quenches in typical LTS windings. For a deep er 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 diffe rences exist between different parts of the magnet, despite the slowness of the processes. Before a quench there were no large temperature gradients a round the hot spot. During the quench the hot spot temperature increased ra pidly but the temperature of other parts of the magnet increased only with a delay.