Plane strain optimization of conductor and structure grading in the inner leg of a Tokamak toroidal field coil

Abstract

I present the results of the analytic plane strain optimization of the structure and conductor grading in the inner leg of a Tokamak toroidal field coil. The coil is assumed to be made of regions of soft conductor inside a stiff grid of conduit. Calculus of variations is used to determine the optimal profile of this structure. The optimal solution is found to be two-layered. An outer layer is bucked only, not wedged, and the structure fraction is graded so that all structure is at a uniform stress. An inner layer is an advanced bucking cylinder, similar to a Florida Bitter plate, whose radial stiffness is tuned so that its azimuthal stress is uniform. Such an advanced bucking cylinder would require advanced manufacturing to fabricate. These results should be seen an upper limits rather than achievable performance. Concepts that arise from this optimization, such as selectively softening layers of the bucking cylinder by drilling or cutting, may be of use to real designs in a less-optimized form.