Protection of high current density superconducting magnets

Theoretical studies of the protection of a high current density magnet (Javerage= 540 A/mm²at B = 6.5T and T = 4.2 K) have shown that following a quench, a fast removal of the stored energy from the magnet is needed in order to limit the maximum temperature rise in the winding to below 150K. A protection system using a superconducting switch as a current breaker and a secondary copper winding for the extraction of the stored energy was developed, This system enabled us to obtain a short reaction time and a fast current decrease without overvoltage across the magnet. The theoretical study of the protection, the analysis of the characteristics of the switch and the secondary winding and their effects on the behaviour of the protection system are discussed. Results of tests of the superconducting switch with a NbTi dipole and tests of the complete system with a Nb3Sn dipole are presented.