The Use of a Winding Model Instead of Effective Continuum Model Can Lead to a Drastic Reduction of MQE in Superconducting Coils

Thermal stability is an essential parameter for coil design. Previous measurements in an MgB2 solenoid have revealed a change in trend in the minimum quench energy (MQE) dependence on j = I/I., showing lower values than expected for operative current close to the critical current (j ~ 0.90). The obtained MQE is comparable to the one predicted by analytical models in a single tape (1D) being of the order of 1-10 mJ, which indicates that propagation across winding and layers (3D) is not taking place. This reduction of MQE is not obtained in the effective continuum model, because the different turns and layers are not reproduced. Obtaining 1D propagation depends on the ratio between the thermal conductivity of the conductor and insulation and on the cooling conditions at the boundary layers. A numerical model of a superconducting solenoid with full representation of the winding is used to study MQE as a function ofj (0.6-0.99). The quench is triggered applying heat pulses of 40 ms in a region of 4 mm of the outer layer of the coil. The effect of changing the cooling boundary conditions of the coil from adiabatic to isothermal is also investigated.