Linear Simulation Study Of Edge Localized Mode Instability

During a discharge of the H-mode in tokamaks,edge localized mode(ELM)periodically occurs at the plasma boundary.In general,it is believed that the ELM instability is driven by a pressure gradient at the plasma pedestal.According to its driving properties,the ELM can be resulted from the ballooning mode driven by the pressure gradient,the peeling mode driven by the plasma current,or the peelingballooning mode.In this paper,the linear properties of the edge localized mode instability is studied by using three-dimensional toroidal magnetohydrodynamic code(CLT)and the newly developed free-boundary divertor plasma equilibrium code(CLTEQ).The characteristics of the ELM are explored by adjusting the local current in the plasma pedestal.It is found that that the magnetic shear is the most crucial factor to determine which unstable mode drives the ELM.In addition,the role of the toroidal plasma shear flow on the dynamics of the ELM is also studied.In the thesis,we first construct a H-mode configuration using CLT-EQ.The simulation results confirm that the ELM is associated with three different types of the instabilities:ballooning mode,peeling-ballooning mode,and peeling mode,which are respectively driven by either the plasma current density,or a plasma pressure gradient,or both at the plasma pedestal:.As the local current density at the boundary increases from small to large value,the unstable mode is from the ballooning mode driven by the pressure gradient,to the peeling-ballooning mode by both the pressure gradient and the plasma current,and finally to the peeling mode driven by the plasma current.It is found from the simulation results that that magnetic shear plays a dominate role on the change of these unstable modes.As the edge current density gradually increases,the q profile and the local pitch angle of the magnetic field v=rBφ/RBθ decrease and flatten.We define the global magnetic shear to be Sglobal=(dq/dr)and the local magnetic shear to be Slocal(r/v)((?)v/(?)r).Our simulation results suggest that the dominant unstable mode changes from the ballooning mode to the peeling-ballooning mode when the local magnetic shear on the low field side(LFS)decreases to zero.It is further found that Sglobal=0 is another critical point to determine the transition between the peelingballooning and the peeling mode.Finally,the H-mode equilibrium configuration with the toroidal plasma flow equilibrium is constructed by CLT-EQ.It is indicated from the CLT simulation that the toroidal shear flow has a strong stabilization effect on high-n modes,but has a weak destabilizing effect on low-n modes of the Peeling-Ballooning mode.