Normal Zone Propagation in High-Current Density Nb₃Sn Conductors for Accelerator Magnets

Self-absorbing quench protection schemes for accelerator magnets mainly rely on longitudinal and turn-to-turn normal zone propagation (NZP) immediately after the occurrence of a quench and subsequently on the effectiveness of protection heaters. Especially for impregnated Nb₃Sn coils the protection should not only aim at limitation of the hot spot temperature and internal voltages but also at avoidance of large temperature gradients and local stress accumulation. Considering Rutherford types of cable based on present high current density Nb₃Sn wires with a relatively low stabilizer content, a priori knowledge about their NZP properties is mandatory. Especially the medium and low-field properties appear to be critical for coil protection. The longitudinal NZP velocity of PIT-type Nb₃Sn conductors are investigated both experimentally and numerically in nearly adiabatic conditions typical for impregnated coils. Numerical simulations are extended to extremely high current density Nb₃Sn conductors and protection heater performance.