Magnetic characterization of austenitic stainless steel for nuclear magnetic resonance coils

Magnetic field uniformity is critical to nuclear magnetic resonance (NMR). Ferromagnetic materials can cause field inhomogeneity. Historically magnet constructions have avoided ferrous alloys to minimize field distortions. This strategy becomes problematic when fabricating large high field magnets because austenitic stainless steel is the preferred and necessary material for bore tubes and winding reinforcement in Nb/sub 3/Sn coils. The selection of a structural austenitic stainless steel alloy must include an understanding of the potential for weak ferromagnetism in the context of the design. Magnetic characterization entails a permeability measurement of the base metal, its welds, and those sections under mechanical stress. This paper presents a review of this subject concerning the selection of a limited 316L stainless steel chemistry. We report 4.2 K permeability measurement results from base metals, weld sections, and tensile test samples.