Aluminum stabilized multifilamentary NbTi conductor

It has long been recognized that high purity aluminum offers several potential advantages in the stabilization of superconductors. Specifically, aluminum is commercially available with a residual resistance ratio (RRR) in excess of 2500 compared to a maximum of approximately 200 for copper. The magnetoresistance of aluminum also saturates rather quickly such that the specific resistivity of RRR 2500 aluminum is approximately 1/20 that of RRR 200 copper at 12 T. Aluminum also offers well recognized weight and radiation transparency advantages. Previous attempts to fabricate commercially viable aluminum stabilized conductors have, however, been unsuccessful because of the great disparity in the metal forming properties of NbTi and aluminum. This disparity of flow stresses has led to the use of higher resistivity aluminum alloys as conductor matrices. These conductors have met with marginal success. The present approach involves applying aluminum stabilizer to a completely fabricated and optimized NbTi/copper composite. The challenge is, of course, achieving complete metallurgical bonding and uniform cross section. Through the use of an orthogonal extrusion process whereby aluminum is solid state bonded to a cleaned copper conductor we have successfully clad a wide range of superconductors with high purity aluminum. Moreover, with proper control of the cladding process an aluminum resistivity ratio over 2500 has been preserved in the finished conductor with no degradation of the NbTi critical current properties.