The Nonlinear Interactions Between Tearing Mode And Ion Temperature Gradient Mode In Magnetic Confinement Plasmas

Macro-scale magnetohydrodynamic(MHD)activities are very dangerous for tokamak plasmas because they can act to globally deform the magnetic field and substantially degrade the plasma confinement.For instance,the tearing mode(TM),a kind of resistive MHD instability,can break toroidally nested magnetic flux surfaces into helical magnetic islands,which can significantly limit the achievable plasma beta in tokamak discharges and finally give rise to major plasma disruptions.Micro-scale instability,on the other hand,is also one of the crucially important issues in magnetically confined plasmas.Such an instability is usually driven by the plasma inhomogeneity and is responsible for the excitation of micro-turbulence as well as the associated anomalous transports.It has been recognized that the micro-scale turbulence induced by ion temperature gradient(ITG)modes dominantly generates anomalous ion transport in tokamaks.Usually,the MHD activities or micro-turbulences are investigated independently due to the spatiotemporal scales between them.However,in fact,both macro-scale and micro-scale instabilities are inevitably excited during the long pulse,quasi-steady state operation of advanced tokamak discharges so that multiple temporal-and spatial-scale interactions may occur and induce various types of novel complicated physical processes.In this thesis,nonlinear multi-scale interactions between the tearing mode(TM),ion temperature gradient(ITG)turbulence and shear flow are studied numerically by using a reduced Landau five-field model in slab geometry.The main conclusions in this thesis are summarized as follow:In Chapter 1,an introduction of the background,the fundamental characteristics of tearing mode and ITG instabilities and the work progress of multi-scale interactions between them are presented briefly.In Chapter 2,the derivations of Landau five-field model and the physical assumptions are presented briefly.Numerical methods used are also introduced.In addition,the basic characteristics of the instabilities covered with this model are also shown.In Chapter 3,the characteristics of saturated steady states in nonlinear tearing mode simulations are numerically investigated by using the Landau five-field model in slab geometry.It is found that a structure bifurcation of flow parity inside islands,then induce a new state with a large vortex flow.The mechanism is identified as a secondary instability in the configuration with wide magnetic islands,in which the eigen mode parity is contrary to the conventional tearing mode.The critical values of island width for the bifurcation are demonstrated in simulations,showing a comparable level with experimental observation on LHD devices.Moreover,the rotation directions of vortex around the O-point of island after the bifurcation also show good agreement between simulations and experiments.In Chapter 4,multi-scale interactions between the tearing mode(TM)and ion temperature gradient(ITG)turbulence are studied numerically using the self-consistent Landau five-field model in slab geometry.It is found that the multi-scale system goes through five distinct phases and is then saturated in a dynamic quasi-steady state.During the nonlinear evolution,the macro-scale TM and the micro-scale ITG turbulence can mutually destabilize each other.On the one hand,the fluctuation level of the turbulence is greatly raised when the magnetic island grows beyond a threshold.On the other hand,the island growth is significantly enhanced through increasing the ITG as the island width increases above a critical value or the island propagating velocity is reduced below a critical value.In addition,the generation of zonal flows and the associated turbulent transport in the multi-scale interaction process are analysed in detail.In Chapter 5,effect of shear flow on the multi-scale nonlinear interaction in plasmas is numerically investigated by using the Landau five-field model in slab geometry.Dual roles of shear flow in the process are discovered,significantly suppressing micro-scale fluctuations and dramatically promoting macro-scale fluctuations.Furthermore,its similar dual roles in turbulent transport are also demonstrated.It is found that the typical shearing effect is responsible for the effect of shear flow on small scale fluctuations.The novel underlying mechanism for the promotion of large scale fluctuations is nonlinear and identified as the formation of a large vortex flow induced by shear flow interacted with island.The later mechanism is verified through an analytical model and theoretical prediction is consistent with numerical simulations.Finally,in Chapter 6,a brief summary and future work are presented.