Optimization of OH coil spiral topology

Abstract

The central solenoid of the Compact Ignition Tokamak (CIT) must produce magnetic fields as high as 20 Tesla during a pulse. Axisymmetric analyses of the solenoid indicate the stresses experienced by the innermost turns at this field are approximately 400 MPa. As the real solenoid is constructed from thick-turn, asymmetric pancakes, and as departure from axisymmetry produces stresses above the axisymmetric levels, it is important to design the individual pancakes so that peak stresses are minimized. In this paper, an optimum transition region topology between circular turns is derived analytically to minimize peak stresses. The method employed is a variational technique which yields a first-order differential equation describing the topology as a function of transition angle and radial offset. A best transition angle is then determined on the basis of current requirements and stress levels. Results from the analytical treatment are compared to results obtained from finite element analyses.