The frequency dependence of a.c. losses in type II superconductors

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Abstract
Energy losses occurring under the application of an alternating magnetic field were examined in type II superconductors representative of three different classes of material: (1) single-crystal niobium (elemental) in the Meissner state, (2) molybdenum-25 at.% rhenium (alloy) in fields both below and well above H C1, and (3) Nb3Sn (compound) principally in the mixed state. Losses were measured as a function of field amplitude and frequency in order to determine whether losses of other than hysteretic variety were present. The total loss per unit surface area per cycle associated with the superconductor was found to have the form W T(h 0, v) = W 0(h 0) + vF(h 0) where W 0(h 0) is identified as an hysteretic term, v is the frequency and h 0 the a.c. field amplitude. Only in the Mo-25 at.% Re alloy did the hysteretic term completely mask the frequency-dependent term. Hysteretic losses in the single-crystal niobium and two Nb3Sn specimens prepared by different solid state diffusion methods were strongly dependent on the degree of surface roughness on a scale of ∼ 1 μm. The magnitude of the non-hysteretic losses showed only a mild dependence on the topology of the surface. However, their functional dependence on field amplitude was strongly influenced by surface condition. It is suggested that the frequency-dependent loss per cycle originates from either (i) surface fluxoid motion, or (ii) scattering of normal excitations within a surface layer.

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