Conduction crisis and quench dynamics in cryocooler-cooled HTS magnets

Analysis of the dynamics of quench development in HTS conductively cooled coils is presented. Thermal instability can occur when an increasing index loss outruns the conduction through the bulk of the winding and the stability parameter becomes /spl lambda/ > 1. In the present study, quench is considered as a slow overheating event initiated by a system failure such as an overcurrent or a cryocooler malfunction. Analytical and numerical modeling of the initial phase of the quench is performed. Major characteristics are obtained in a 1-D homogeneous medium model, the results are compared with numerical modeling of a Bi-2223 coil. It is shown that quench dynamics depends on the failure scenario and on the initial operating temperature profile. The derived adiabatic ignition time offers a conservative estimate for quench protection.