Study On Characteristics Of The L-H Transition Based On A Reduced Model In Tokamak Plasmas

Energy problems is very important for the sustainable development of the economy.Traditional fossil energy reserves are limited,so the development of alternative energy is one of the important tasks of current scientific research.Compared with traditional fossil energy,nuclear energy has many advantages.Thus,it is an important way to develop the nuclear technology,especially for the magnetic confinement fusion,to solve the energy problems,In Tokamak(one kind of magnetic confinement device),the strong magnetic field can make charged particles move around the magnetic field line,which provides favorable condition for the fusion reaction.Even though,the deuterium-tritium plasma can be effectively restrained in the toroidal "magnetic container" conditions,the collision(collective interaction)between charged particles causes the transport of particles across the magnetic line.The neoclassical transport coefficient taking into account toroidal effect of tokamak is significantly enhanced,but it is still not sufficient to explain the experiment observation value.In the edge region,the pressure gradient is weak when the turbulence amplitude is strong.The anomalous transport caused by turbulence results in the operation of the device in low confinement mode(low confinement mode,L-mode).Through the control of Micro-turbulence instability,the pressure gradient of the boundary can be steep,forming a boundary pedestal region.Then the improved confinement(high confinement mode,H-mode)of the plasma can be obtained.This thesis mainly focuses on the phenomenon of confinement mode transition(L-H transition)in Tokamak devices.The typical characteristics of the L-H transition are numerically analyzed in detail.The structure of the paper is as follows:In chapter Ⅰ,the basic features of low confinement mode and high confinement mode are introduced.In chapter Ⅱ,the "Predator-Prey" ecological model and its application in "turbulence-shear flow" system are introduced.In chapter Ⅲ,four-field model including "turbulence-zonal flow-geodesic acoustic mode" is used to analyze the influence of turbulence growth rate on the energy evolution of various components of the system.The results show that Landau damping of geodesic acoustic mode can cause the periodic oscillation,even if there is no collision damping of zonal flow.The oscillation frequency of turbulence,zonal flow and geodesic acoustic mode increases with the increase of turbulence amplitude.The zonal flow gradually accumulates energy during each oscillation period and eventually suppress turbulence.Where the coupling parameters of "turbulence-ion sound wave" are less than zero and the zonal flow is damped,even if the coupling parameters of "turbulence-zonal flow" are greater than the coupling parameters of "turbulence-geodesic acoustic mode",the geodesic acoustic mode will also take precedence over the zonal flow growth.In chapter Ⅳ,we add the evolution of the mean pressure gradient to the previous model including "turbulence-zonal flow-geodesic acoustic mode-mean flow".The numerical analysis of the dynamic characteristics between L-H transition and L-H-L reverse transition is carried out.The results show that the power threshold of triggering H-mode and the dynamic characteristics of the transition phase of L-H transition depend on the coupling parameters of "turbulence-zonal flow" and "turbulence-geodesic acoustic mode".In the transition phase of L-H transition,two kinds of limit cycle oscillations which are similar to the diagnostic results of the HL-2A tokamak are observed clearly.Then the hysteresis of the L-H-L process can be observed when the power increases and then decreases.Finally,a brief summary end the thesis.