Optimization of the Stress Distribution of Helical Coils With a Cable-in-Conduit Configuration

Virial-limit coils for application to magnetic confinement nuclear fusion devices are hybrid helical coils that level electromagnetic stresses. In past research, a possibility that a large bending stress is caused in helical coils made of a cable-in-conduit configuration (CICC) was pointed out. In this paper, we analytically evaluated the stresses of helical coils made of CICC by modeling them as springs and calculated a detailed stress distribution by finite-element method (FEM) analysis. FEM results show that the bending and torsional stresses, which are ten times or more as large as the membrane stress, are caused. These stresses are caused by the moment of electromagnetic forces, which can be explained by modeling helical coils as coil springs. We propose a helical coil that lessens the moment and confirms the effect of the reduced moment, and the coil under this optimal condition of moment reduction is termed a moment-free coil.