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. Th is strategy becomes problematic when fabricating large high field magnets b ecause austenitic stainless steel is the preferred and necessary material f or bore tubes and winding reinforcement in Nb3Sn coils. The selection of a structural austenitic stainless steel alloy must include an understanding o f the potential for weak ferromagnetism in the context of the design, Magne tic 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 stainle ss steel chemistry. We report 4.2 K permeability measurement results from b ase metals, weld sections, and tensile test samples.