Suppression of flux jumps in marginally stable niobium-tin superconductors

Niobium-tin superconductor wires with coalesced filaments may have reduced adiabatic stability. Magnetization measurements on such marginally stable conductors exhibit flux jumps, which appear as a sudden decrease in magnetization as the applied field is changed, caused by the unpinning of flux vortices and resistive heat generation. Flux jumps preclude estimation of the hysteresis loss from the area of the magnetization-versus-field loop. Here, we show that flux jumps can be minimized or suppressed during the measurement of hysteresis loss by immersing the specimen in helium liquid instead of helium gas. The better thermal conductivity of the liquid affords additional dynamic stability against flux jumps. This allows one to determine the loss upon field cycling and to calculate an effective filament diameter, often used to gauge losses and the extent of metallurgical interfilament coupling.