The monohelix: (1) five years of operation at the FBNML and (2) finite element stress analyses

The FBNML (Francis Bitter National Magnet Laboratory) has built and operated seven partial monohelices employing helices of Be-Cu, Cu-alumina, or Cr-Cu. Fabrication was by conventional lathe, numerically controlled lathe, or wire electric discharge machining. Each monohelix is the innermost coil of a hybrid magnet system which can generate 27 T in a 53-mm bore or 31 T in a 33-mm bore (35 T with holmium poles). Compared to their Bitter coil predecessors, these monohelices have demonstrated comparable performance and superior life. One helix gave 270 hours of service; another is still active after 230 hours. The finite element stress analysis program ABAQUS was used to calculate the stresses and strains in radially cooled Bitter coils and monohelices. At a maximum conductor strain of 0.8% (the approximate empirical limit for acceptable coil life), a perfect monohelix is structurally a third stronger than the Bitter coil analyzed. The strength of the partial monohelix design appears to depend on the relative thicknesses of the helix and cooling plates, and on the friction between them