Influence Of Toroidal Flow On Plasma Equilibrium And Ideal Ballooning Mode Growth Rate In EAST Tokamak

Tokomak is one of the most promising magnetic confinement fusion devices to achieve the controlled thermonuclear fusion.The tokomak equilibrium is one of the most basic and most important issue of the nuclear fusion science.Tokomak plasma equilibria not only affect physical processes in a tokomak such as magnetohydrodynamic(MHD)instabilities,heating and fueling,and transport,but provide a basis for scenario development and plasma control.Only with plasma equilibrium reconstruction,can experimental data analysis and related simulations become possible.Tokamak plasma equilibria can be obtained by solving the Grad-Shafranov equation,combining with diagnostic measurements.In last two decades,it was found that plasma flows exist in tokamaks,and their effects have been investigated extensively.Many studies focused on how the MHD instabilities are affected by plasma flows.However,most published papers included the contributions of flows only by adding new terms from flows in the fluid equations describing the variation of perturbed physical variabies.The modification of the plasma equilibrium induced by flows are ignored.Such inconsistent treatment motivates the thesis work.In this work,the effects of a toroidal flow on a plasma equilibrium are taken into account by numerically solving the modified Grad-Shafranov equation.BOUT++is then used to study the effects of toroidal flow on ballooning instabilities in an EAST plasma while taking the equilibrium modified by the flow into account.Firstly,a numerical code based on the fixed boundary has been developed to solve the Grad-Shafranov equation with a toroidal flow in an EAST plasma in the cylindrical coordinate system.For normal equilibrium configurations(i.e.the equilibrium with nested flux surfaces,NEC),it is found that the flow lead to an outward shift of the magnetic flux surfaces,together with the profiles of pressure,and mass and current densities.The shifts could become significant when the toroidal flow Mach number exceeds 0.2.Such modification of the EAST equilibrium may affect macro MHD instabilities.In some EAST parameter regimes,for instance,L-mode discharges with a low current,the flows could cause the appearance of a novel current reversal equilibrium.Such change in the topology of plasma equilibrium may degrade EAST confinement.For current reversal equilibrium configurations(CRECs),the equilibrium magnetic flux surfaces may change from a configuration with three-magnetic-islands into one with two-magnetic-islands when the toroidal Mach number is sufficiently large.Secondly,the effects of toroidal flow on the ballooning modes in the modified EAST plasma equilibrium with flows are numerically investigated by using BOUT++.It is found that a toroidal flow with shear can significantly modify the safety factor and pressure gradient,which then affects the ballooning instability.The simulation results show that in the presence of rigid flow,the growth rate of ballooning mode will be affected notably if the modified equilibrium is considered.Especially for a shear flow,the growth rates of high-n modes are significantly reduced.For instance,the growth rate of the mode n=60,is reduced by 84%for a strong shear flow.Meanwhile the mode structure is also twisted by the flows.It can be speculated that the turbulence spectrum could be altered in the nonlinear stage considering the stabilization and mitigation effects of the toroidal flows,which may influence EAST transport processes.In conclusion the influence of a toroidal flow on ballooning instabilities should include the modification of EAST equilibria.