Numerical method and experimental validation of the magneto-thermal-mechanical coupling problem with application to tokamak structures

Abstract

Tokamak is a kind of device that uses strong magnetic fields to confine the hot nuclear fuel in plasma state and achieve the controlled thermonuclear fusion reaction. It is well-known that increasing the plasma elongation is beneficial for improving the performance of tokamak. However, high elongation is always along with a strong intrinsic vertical instability. The control system to neutralize the vertical instability of the plasma may fail. As a result, the plasma will move vertically and finally strike on the plasma-facing components accompanied by rapid thermal and current disruption. During this process, large eddy currents will be induced in the plasma surrounding conductive structures and high heat flux will be injected into the first wall of the vacuum vessel (VV). The eddy currents and the temperature gradient then produce strong Lorentz forces and thermal stress, which may cause excessive deformation or even failure of the device. Additionally, there are complicated magneto-thermal-mechanical coupling (MTMC) phenomena in the dynamics of the tokamak structures, which may have non-negligible influences on the distribution of the electromagnetic and mechanical loads and the dynamic response of the structure. To ensure the safety of the tokamak device, it is necessary to make quantitative evaluations of the coupling effects on the transient response of the VV and in-vessel structures during plasma disruptions. In this paper, efficient numerical methods were proposed for solving the structural dynamic problem with MTMC effects. The nonlinear interaction between the magneto-mechanical terms was linearized with a block-Gauss-Seidel iterative algorithm, and an adaptive matrix update algorithm was proposed to cope with the temperature effect on material properties. In addition, an MTMC experiment system with both electromagnetic load and thermal load was set up to demonstrate the validity and efficiency of the proposed numerical methods. Finally, the dynamic response of the vacuum vessel of HL-2M tokamak during a typical plasma disruption was simulated using our updated numerical code, and influences of the MTMC effects were quantitatively evaluated.