Heat transfer and stability characteristics of cable bundles in He I and subcooled He II

Results from two experiments aimed at understanding the stability of cabled composite superconductors are reported. The first is a heat transfer study of stainless steel tubular bundles. The peak heat flux from these test sections is investigated as a function of number of bundle tubes and gap between each tube. Experiments are performed in both He I at 4.2 K and subcooled He II at approximately 2 K. Results indicate that the peak heat flux is supressed substantially as the gap between tubes is reduced. With zero gap the surface averaged heat flux is only about 20% of that achieved with large gaps roughly equivalent to an open bath. The second experiment consists of a stability measurement on a bundled composite conductor. The test section is made up of a 66.3 cm length of a 7 wire bundle. The six outer wires are copper-NbTi composites while the core is an insulated resistance wire. Normalization of the conductor, either achieved by exceeding Icor by pulsing the heater, is detected by voltage taps across the inductively wound test section. Stability measurements made in both He I and He II are correlated with the heat transfer results in the first experiment.