Thermal stress and relieving method of the ITER lower vertical stabilization coil

•Stress issues on the coil are solved by analyzing the local model based on symmetry features.•Improving filet only is more effective than decreasing notch depth additionally.•Matching the rail with proper notch depth will have unexpected effect of mitigating stress concentration.•Various optimization choices are applicable to solve the overlarge stress issues at clamp interface.

Structural analysis of the ITER lower vertical stabilization (VS) coil has concluded thermal stress concentration is a serious issue obstructing the coil design. To optimize the lower VS coil, its structural performance has been studied in detail to find methods of solving the stress concentration issues. Apparently, increasing filet radius and decreasing the notch depth are the means commonly used to deal with stress concentrations in the singular regions. However, the real effects are not so simple due to very limited optimization space of the coil geometry. Larger filet radius will significant mitigate the stress concentration but still not enough if the material keeps original. On the other hand, stress concentration at the interface between the conductor jacket and clamp also needs to be mitigated. Changing the clamp size or distance is an obvious and effective way to solve this issue. However, introducing sliding contact at the interface is more effective and simpler. It requires the jacket and clamp are not tightly welded together but connected by bolts or other analogous means with which local sliding is permitted. Additionally, the effect of gap between conductor, spine and clamp is also researched. All in all, the stress concentrations of ITER lower VS coil can be mitigated to meet the acceptance criteria based on the specific optimizations and combining them together.