Effects of dielectric joints on the structural behavior of tokamak devices

An inherent requirement in the design of tokamak fusion devices is the electrical isolation of the structural support system. In the case of metallic support systems, this isolation is accomplished by inserting dielectric joints between adjacent structural components. These joints, however, have a significantly lower stiffness than typical structural connections as a result of the lower elastic moduli of the dielectric materials as compared to metals. Consequently, stiffness discontinuties occur within the support structure that affect its response to the imposed loads. This paper investigates the effects of the dielectric joint designs used in the ORMAK Upgrade and Impurities Studies Experiment (ISX) machines on the stresses and deformations of these two systems. The joints examined include designs for resisting primarily compressive forces (ORMAK Upgrade torque rings). Techniques for modeling these joint types as linear springs for use in finite element models are discussed. The results of finite element analyses using the NASTRAN code are discussed and comparisons of the stresses and deformations for each of the two systems with and without dielectric joints are presented. These results are examined to determine the relative influence of the geometry, dielectric material and location of the joint on the overall behavior of the system