ITER CS inner transition evaluation, local stresses, non-axisymmetry and turn loss

A review of the ITER central solenoid (CS) design was initiated due to the expectation that the US will rejoin ITER. The Snowmass reviews of the burning plasma experiments, and the revised design of the ITER CS as a free-standing segmented pancake wound coil system pointed to new design issues that needed to be addressed. The scale-down of the ITER has less of an effect on structural qualification than the conceptual changes. This present design introduces a different set of mechanical design concerns than the large monolithic layer wound coil proposed in the first EDA. Pancake winding employs transitions and joints to build the stack of coil segments that make-up the coil system. The transitions have to be bent to accommodate the elevation change of the stacked pancakes. Inner turn hoop tensions are offset by the elevation change, and bending loads result. The transitions are accompanied by void regions that cause a turn loss, and a non-axisymmetric stiffness of the global coil structure. Local details of the lead break-outs and helium penetrations, cause localized stresses. The CS is now free-standing coil loaded in hoop tension. As a result, fatigue is now a concern, and localized stresses, and global stress multipliers need to be held to a minimum. The US role in ITER has not yet been determined, and the analyses and discussions in this paper are intended to present some of the concerns, and discuss their potential resolution, but not represent a final design solution.