Quench energies and stagnant normal zones in cryostable magnets

Abstract

In a cryostable magnet cooled by pool boiling helium, an area with deteriorated heat transfer, such as a vapor lock, can locally destroy stability in the winding. When a thermal disturbance occurs in this area, depending on its energy and the size of the area, it can: (a) lead to recovery of the superconducting state; (b) develop into a local steady normal zone; or (c) create an ever-growing stagnant zone. The stagnant zone is characterized by an uncontrollable temperature growth in the center, reaching hundreds of K in several minutes, while its fronts slowly advance into cryostable regions. It represents the most probable quench scenario in a cryostable magnet and should be considered as a main target for a quench protection system. Classification of different normal zones in a cryostable winding with a vapor lock is presented. Criteria are derived which determine regions of existence of steady and stagnant zones in conductors with nonlinear properties and heat transfer. Critical sizes of the uncooled area are defined, and quench energies are calculated for copper and aluminum stabilized conductors.