THE MONOHELIX .1. 5 YEARS OF OPERATION AT THE FBNML AND .2. FINITE-ELEMENT STRESS-ANALYSES

The monohelix is a radially cooled Bitter coil with at least some of its slit annular plates replaced by one or more continuous conducting helices. The "complete" or "perfect" monohelix incorporates the cooling passages in faces of the helices themselves. The "incomplete" or "partial" monohelix employs conventional etched cooling plates slid into the kerf between consecutive turns of the helices. The F.B.N.M.L. 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 teslas 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. We have employed the finite element stress analysis program ABAQUS 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 which we 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.