Filament contact within situ Nb3Sn superconducting wire

Experimental evidence is presented which indicates that the Nb₃Sn filaments withinin situ prepared Nb₃Sn-Cu superconducting wire are welded together at point contacts along a thin annular region which borders the surface of the original tin supply. For wires with tin added from an external plated layer the welded annular region lies near the outside diameter just below the original tin plate, whereas for wires with the tin added from an internal core the annular region lies along the original core hole. This welded region is expected to increase the a.c. loss characteristics of these wires. A possible cause of the welded annular region and methods for its elimination are discussed.