Reduction of ac losses of Nb3Sn by surface treatment

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Abstract
Large reductions in the ac losses of Nb3Sn were obtained by surface treatment of thick layers of Nb3Sn formed by reacting Nb-1% Zr with liquid tin. The surface treatments studied were chemical etching, electropolishing, and mechanical polishing. The significant reduction in the losses is attributed to the removal, by polishing, of a porous outer layer of Nb3Sn which forms at the Nb3Sn–liquid-Sn interface and extends into the material. The porosity of this layer results in low surface and bulk current densities (hence high losses) compared to the material further away from the surface. Low losses (∼1 μW/cm2 at 500 rms A/cm) can be obtained by fine mechanical polishing, electropolishing, or chemical etching of the surface. Mechanical polishing can produce the smoothest surfaces but introduces damage beneath the surface which is detrimental to achieving very low losses. When this damaged layer is either etched away or annealed, however, the losses can be made smaller than 1 μW/cm2 at 500 rms A/cm. The lowest losses were obtained by annealing after rather coarse mechanical polishing. This behavior is attributed to the creation of new pinning centers near the surface after cold working and annealing.