Influence Of Asymmetric Magnetic Perturbation And Radial Pressure Force On Double Tearing Modes

In International Thermonuclear Experimental Reactor(ITER project),the reversed magnetic shear configuration is confirmed to be used.In previous study,for the former,reversed magnetic shear is found to induce the double tearing mode(DTM),which can grow fastly by the mode coupling and destroy the plasma confinement significantly.In the last two decades,the control of DTMs has been extensively studied in theory and experiment.Due to the complexity of the problem,the nonlinear studies of DTMs have not been well understood,especially in the nonlinear evolution and the suppression mechanism of the mode.In this thesis,the combining method of computer simulations and theoretical analysis are used to study the evolution of DTMs.In Chapter 1,the background and significance of this thesis,tokamak magnetic field configuration and previous work are introduced.In Chapter 2,the effects of asymmetric magnetic perturbation on the triggering and evolution of DTMs are investigated using nonlinear magnetohydrodynamics simulations in a slab geometry.We find that for reversed magnetic shear plasmas the resistive reconnection process induced by the initial perturbation at one rational surface can drive a new island at the other rational surface with the same mode number.The four typical states of the mode for the time evolution are found,and include:(i)a linear growth stage;(ii)a linear/nonlinear stable stage;(iii)an interactively driving stage;and(iv)a symmetric DTM stage.These differ from previous simulation results.Moreover,nonlinear DTM growth is found to strongly depend on the asymmetric magnetic perturbation,particularly in the early nonlinear phase.The initial perturbation strength scale of island width suggests that the left island enters into a Sweet–Parker growth process when the right island is sufficiently large to effectively drive the other.These results predict that although externally applied magnetic perturbations can suppress the neoclassical tearing mode they can also trigger new instabilities such as asymmetric DTMs.In Chapter 3,The role of radial pressure force in the interlocking dynamics of DTMs is investigated by force balance analysis based on the compressible magnetohydrodynamics model.It is found that the stability of symmetric DTMs is dominated by the radial pressure force rather than the field line bending force.Owing to the compressibility of rotating plasmas,unbalanced radial forces can just result in the rotating islands drift toward each other in the radial direction but do not trigger the explosive growth of the mode in the interlocking process,which is different from that of antisymmetric DTM without flow.