Coupled multiphysics analysis of the TF coil structure in the NSTX upgrade

The National Spherical Torus Experiment (NSTX) is being upgraded to increase plasma current to 2 MA and central toroidal magnetic field to 1 Tesla [1]. The upgrades include a replacement of the centerstack and addition of a second neutral beam. The Inner legs of the TF coil which are at the core of the new larger NSTX upgrade center stack are designed to carry twice the current of the existing TF coils. The inner and outer legs of the TF coil are connected through a set of flexible straps that allow for the thermal growth of the center stack and the out of plane twisting moments in the TF coils. Electromagnetic interaction of the TF, OH and PF coils results in loads on the TF coil structure. The TF flexible straps and structural components of the machine have to withstand these loads. Coupled electromagnetic, thermal and structural analyses were performed. The results of these analyses, which will be presented in detail in this paper, have led to the validation of the design of TF flexible straps and compliance with the requirements of the NSTX structural design criteria.