the experimental studies performed under this contract comprised investigation of two principal approaches to achieving very low levels of ac loss in nb sub 3 sn produced by the liquid diffusion process. these approaches were inhibition of the formation of coarse structured nb sub 3 sn surface layers; and surface refinement by removal of coarse structured nb sub 3 sn surface layers. the results of these investigations are reported in detail. it was found that ac losses increased directly and sensitively with sn loading, whereas formation of the coarse surface layer responded limitedly and sluggishly to this parameter. no quantitative correlation was identified between losses and the observed thickness of the coarse structure. although the effect of coating composition on losses was apparent in some samples, compositional influences tended to be obscured by the predominantly regulatory role of the sn loading parameter. nb sub 3 sn formation under ''sn limited'' reaction conditions appeared as the only useful guideline as inferred from the data for achievement of low loss nb sub 3 sn product. in contrast, the investigation focused upon surface refinement methods which led to a viable and attractive method for achieving low losses in nb sub 3 sn tapes, produced by the liquid diffusion process. this method, which was developed, involved a supplementary treatment of nb sub 3 sn tape performed after completion of the diffusion heat treatment step in accordance with standard procedure. the process demonstrated reliability and uniformly successful results in both the investigative