MHD Stability of Fusion Plasma with Ferromagnetic Wall Effect

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Two important issues in developing the fusion DEMO reactor are the realization of high beta plasma and the development of low activation materials. As for the low activation materials, the ferritic steel is considered to be the most promising candidate as the structural material in DEMO reactor. Because of its strong ferromagnetism, however, the ferritic steel is also considered to have a bad influence to plasma ignition, control, MHD stability and plasma confinement etc.. Among others, MHD stability is related directly to the realization of high beta plasma, and the ferritic steel has possibility to deteriorate MHD stability and decrease the critical beta value. It is, therefore, one of the most important issues in development of fusion reactor to investigate the ferromagnetic effect on the MHD stability. The purpose of this study is to investigate the effect of ferromagnetic effect on critical beta by MHD stability analyses for low aspect ratio and circular cross section equilibrium … More using linear MHD stability code, which uses the full set of resistive MHD equations. The obtained results in these studies are in the following :The reduction of critical beta by the effect of ferromagnetism is shown to be 8 % and 20 % against the ferromagnetic wall of relative permeability of 2 and 3, respectively, for tokamaks of aspect rate 3. The reduction value is 10 % against the ferromagnetic wall of relative permeability of 2 for high aspect ratio tokamaks, and the toroidal effect is considered to be small for ferromagnetic effect on the critical beta. It is shown that the attraction of the perturbed poloidal magnetic field due to ferromagnetism in the wall destabilizes the external kink mode, and reduces the critical beta values.The existence of the stability window in the minor radius of resistive wall opened by both effects of the toroidal plasma rotation and the plasma dissipation is shown for tokamaks of aspect ratio 3, while they have not been observed for high aspect ratio tokamaks. The width of stability window is shown to be widened in small values of relative permeability less than 3, while it is shown that the width of stability window is reduced in large values of relative permeability greater than 3. The stabilization effect comes from the coupling the unstable mode to the stable mode, whose reduction rate is decreased by the ferromagnetism. Less