Superconductor stability and helium heat transfer: the minimum propagating zone relationship in design

Selection of a magnet or superconductor stability criterion is frequently based more on philosophical considerations than on science. Once a stability criterion is selected, the stability must be related to magnet parameters such as mechanical support, field and current, and, to the focus of the present work, helium heat transfer. One technique for relating the stability of the superconducting state of a conductor to the environmental cooling is to use the concept of the minimum propagating zone (MPZ) and to derive the relationship between the length of the zone and the local cooling. This paper describes the application of the MPZ theory to the design of several successful superconducting coils which were, at the time of their conceptualization and design, new and unique concepts. Interpretation of experimental data from the coils' operation or model test programme infers a correlation of MPZ length and stability limit. Each of the coils, the 300 kJ pulsed discharge coil, the Nb 3Sn forced flow, supercritical helium cooled Large Coil and an epoxy potted Nb 3Sn coil utilized a form of the MPZ theory to relate a different cooling technique to the chosen coil stability criterion. Experimental data show that the MPZ concept is a powerful tool and that it is closely related to magnet stability. It has been a major factor in selecting design parameters and operating modes of the above coils.