Nonlinear Simulations And Analysis On The Instabilities Of Pedestal Plasmas In Tokamaks

The tokamak pedeatal plasma plays a key role in global confinement.On the one hand,the pressure at the pedestal top(or pedestal height)strongly impacts global confinement in tokamak.On the other hand,the steep gradient of pressure profile produces strong instability,which will largely increase the heat and particles transport to the first wall and divertor.It will cause potential damage to the vacuum vessel and divertor targets.Therefore,the simulations and analysis of the pedestal plasmas are important to realize the high confinement operation of tokamak.In this thesis,the extensible BOUT++framwork with multi-field two-fulid module is used to study the pedestal instability and turbulence of EAST and D?-D tokamaks.The instability in pedestal includes ballooning mode,peeling mode,drift-Alfven wave(DAW)and so on.The ion diamagnetic effect,shear Alfve wave and shear flow can suppress the instability.In this work,the effects of ion diamagnectic and shear flow on ideal ballooning mode are analyzed by linear simulations.The dispersion relation is used to qualitatively analyze the physical mechanism of these effects,and the analysis restults of balloon mode and diamagnetic effect are consistent with the simulation.However,the dispersiton relation is local,which is not suitable to study the global effect of shear flow.In order to accurately analyze the contribution of these effects,as well as the effects of shear flow on ballooning mode,the integral dispersion relation is proposed in this work.It uses the numerical integral of the mode structure to solve the dispersion relation.Then the growth rates of different physical terms are obtained by this method.In addition,the numerical integration of kinetic energy in the whole space is used to analyze the contribution of different physical terms to free energy.The results of above linear analysis show the instability of ballooning mode is driven by curvature,and it provides free energy.The diamagnetic effect and shear flow suppress curvature driven and absorb free energy.Based on the above analysis methods,the linear instability of the EAST edge coherent mode(ECM/CM)is studied,and the methods are extended to the analysis of nonlinear turbulence driving mechanism.The linear analysis shows that the instabilities of the EAST CM are driven by the peeling-ballooning mode and drift Alfven wave(DAW).However,the turbulence in nonlinear phase is driven by the Reynolds stress and Maxwell stress,which redistribute free energy to the turbulence.The contribution of Reynolds stress is about seven times as large as Maxwell stress,which indicates that the EAST CM is a predominantly electrostatic mode.In order to study the process of EAST CM formation and evolution,the energy transfer between three-wave coupling is used to analysis.The results show that the energy is transferred from the middle-n modes to low-n modes during the formation of EAST CM,and the modes coupling effect is enhanced in this process.During the evolution of EAST CM,Ni fluctuation tends to generate the 'single-mode' coupling and Te tends to be 'multiple-mode'.The 'single-mode' coupling is easy to cause the profile collapse,which indicates that the density profile provides more free energy to drive turbulence than the electron temperature.The pedestal turbulence is not only transported outwards to the divertor target,but also spreads inwards.It may affect the core plasmas.Based on the EAST/D?-D joint experiments,the effects of density profiles on the turbulence between ITB foot and ETB are studied using the equilibrium with large radius(normalized p>0.5)ITB in D?-D.The nonlinear simulations show that the particle and heat fluxes without density ITB are about five and three times larger than that with ITB.The pedestal turbulence without density ITB can spread inwards deeper,which can reach the inside of ITB foot.The physical analysis indicates that the E×B shear flow is not the dominant factor for the turbulence transport,while the density profile is found to be important.In addition,the kinetic energy is used to analyze the reason of turbulence enhancement without density ITB.The results show that the Reynolds stress without density ITB redistributes larger energy to the turbulence.It generates larger perturbation kinetic energy and increases the inward propagation of the pedestal turbulence.The linear analysis method proposed in this work enriches the research of pedestal,and it reveals the physical mechanism about the effect of ideal balloon mode,diamagnetic effect and shear flow on instability.The energy transfer between three-wave interaction shows the process of EAST CM formation and evolution.In addition,the kinetic and magnetic energy analysis reveal the driving mechanism of EAST CM turbulence,as well as the electrostatic and electromagnetic characteristics.At last,the simulation and analysis of ITB show that the density ITB structure plays an important role in suppressing the turbulence.