Kinetic Study Of Physical Mechanisms Of The E×B Staircase Induced By The Turbulence In Tokamak

The anomalous transport,which is mainly induced by the drift-wave turbulence,is considered to be the key issue to limit the confinement performance of toroidal fusion devices such as Tokamaks.Among all kinds of driving forces of the anomalous transport,the ion temperature mode(ITG)instability is believed to lead to the large ion heat transport along the radial direction,and is widely investigated theoretically,experimentally and numerically.Electromagnetic turbulence is a highly nonlinear and complex system,in the absence of a fundamental and first-principles turbulence theory,statistical methods are often utilized to investigate the characteristics of the turbulence transport.In recent decades,a series of models including SOC(Self-Organized Critical),self-similarity,1/f noise,etc.have been discussed to describe the turbulence system.On the other hand,it is found that the E×B shear flows can significantly suppress the turbulence transport in Tokamak through the effect of decorrelation,and have been widely studied in recent years.Among all types of the E×B shear flow patterns,the E×B staircase,which has been discovered recently in both experiment and numerical simulations,is believed to provide a new way on improving the confinement performance.Since the E×B staircase show quasi-regular shear layers on the radial direction and reveals quasi-stationary property,it is also named as the "micro-barriers" in some works.Turbulence behaviors and EXB shear flows are not isolated,but affected by each other through a complex manner,and play critical roles in transport processes.In recent decades,the development of the massive gyro-kinetic simulation provided a powerful tool to investigate the micro-turbulence in Tokamaks.In the present thesis,through the simulations utilizing the state-of-the-art 5-D gyro-kinetic toroidal numerical code GKNET(Gyro-Kinetic Numerical Experimental Tokamak),we investigate the formation and sustainment mechanisms of the E×B staircase in the ITG turbulence system,as well as the spatial-temporal characteristics of the ITG turbulence.The GKNET code is developed by Kishimoto-Lab in Kyoto University.This is a gyro-kinetic Valsov code,and is a powerful tool to study transport physis in Tokamaks.The content of this thesis are described as follows:In Chapter 1,starting with the discussion of the world energy structure,the introductions of the background and the purpose of this thesis are presented.The brief introductions of transport processes and E×B shear flows are given.In Chapter 2,the gyro-kinetic Valsov equations are deduced from the "one-form",and the conservation properties are proved.The gyro-kinetic simulation code "GKNET" is introduced and the global parameters are given.In Chapter 3,the spatial-temporal characteristics of turbulent transport are investigated through a series of statistical methods including the size probability distribution function(PDF)based on the flux-driven GKNET simulation.Through the 2-D auto-correlation analysis,the transport behavior in the flux-driven Tokamak system is proved to be a classical non-diffusive system;through the analyses of spatial wavenumber spectrum and temporal frequency spectrum,the basic spatial-temporal characteristics of turbulent transport are investigated;through the size PDF of turbulent transport structures,the scaling laws of the distribution function are given,and the effect of turbulent structures on the net transport level are studied.In Chapter 4,the E×B staircase is studied through simulations by GKNET code.Firstly,the properties of the EX B staircase in the flux-driven simulation are described.Through the ?f profile-driven GKNET simulation,the formation mechanism of the EXB staircase and the effect of background mean flow are studied.It is found that radial electric field pattern of the E×B staircase is primarily generated from the excitation of zonal flow via the action of Reynolds stress from the turbulence,and enhanced by the radial mean field variation caused by the temperature fluctuation.Moreover,it is found that the background mean field is critical for the selection between E×B staircase and the avalanche.In Chapter 5,the formation of the intermittent transport burst in the presence of the E×B staircase and the sustainment mechanism of the E×B staircase are investigated based on the ?f profile-driven GKNET simulation.It is found that the phase matching of the meso-scale mode structures can lead to radially extended global mode structures,and is believed to be responsible for the generation of the intermittent transport burst,which also provides the energy to enhance the existing EXB staircase.The extrapolation of the mechanisms discussed above are investigated through the flux-driven GKNET simulation in the larger system size Tokamak,the co-existence of avalanche and intermittent transport burst in sustainment the E×B staircase is discussed,and a new energy path of the free energy in ITG turbulence system are given.Finally in Chapter 6,a brief summary and future work plan are presented.