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 c onductors exhibit flux jumps, which appear as a sudden decrease in magnetiz ation as the applied field is changed, caused by the unpinning of flux vort ices and resistive heat generation. Flux jumps preclude estimation of the h ysteresis loss from the area of the magnetization-versus-field loop. Here, we show that flux jumps can be minimized or suppressed during the measureme nt of hysteresis loss by immersing the specimen in helium liquid instead of helium gas. The better thermal conductivity of the liquid affords addition al dynamic stability against flux jumps. This allows one to determine the l oss upon field cycling and to calculate an effective filament diameter, oft en used to gauge losses and the extent of metallurgical interfilament coupl ing.