Surface structure and a.c. losses in superconductive niobium

Abstract

The effect of different surface treatments on flux penetration and dissipation at 50 Hz in bulk niobium samples has been quantitatively investigated. It is shown that for a sufficiently smooth sample surface the onset of a.c. dissipation occurs when the peak applied magnetic field exceeds Hc1+ΔH, in striking contrast to earlier results. A comparison of experimental results with a generalized critical-state model yields good agreement.