Simulation Study Of China Fusion Engineering Test Reactor With Grassy ELM Regime

Edge localized modes(ELMs)are repetitive MHD instabilities at the plasma edge that occur in H-mode operation and could lead to a rapid loss of energy and particles from the plasma edge that potentially poses a crucial wall material and divertor erosion risk and decreases the confinement time of tokamak,which must be avoided to maintain the burning plasma.China Fusion Engineering Test Reactor(CFETR)is the proposed next-generation fusion facility in China,one main goal of which is achieving high performance with steady state buring plasma.To accomplish it operation with low amplitude grassy ELMs is a good alternative for high confinement operation.This thesis focuses on ELMs physic research and exploring CFETR operation with small ELMs or no ELMs.The regime exists within a pedestal top electron collisionality window at high global poloidal beta.Using EPED and BOUT++,a theoretical model that quantitatively explains the physics of the grassy-ELM is presented for the first time.The first step of performing a reliable edge stability analysis is to construct a self-consistent equilibrium.Previously,we only used the fitting method that unphysically decouples the pedestal pressure gradient and current as they are varied,which also didn't consider the relation between pedestal and core region.To account for these two factors,a self-consistent workflow is built for CFETR.With the linear fitting method we connect the core profile generated by TGYRO and pedestal profile form EPED.And then the whole profile is inputted to ONETWO/TGYRO/EFIT loop to be iterated until convergence.This self-consistent core-pedestal coupling equilibrium is crucial for more realistic and accurate results in our study.Neural network method in EPED-NN is also applied to save calculation time of this workflow for the first time,but there will be obvious difference between simulation and realistic when the input parameters are out the range of machine learning.According to the peeling-ballooning theory,we mainly use ELITE and BOUT++to study ELM activities in CFETR.Firstly a benchmark is done with other stability simulation codes such as GATO and NIMROD,which identifys the reliability and validity of the results obtained by ELITE and BOUT++.Then we study the effects of shape parameters such as triangularity and elongationon on ELM activities,and explore the optimization parameter spaces of pedestal top density and effective charge for CFETR.Peeling-ballooning boundary of CFETR initial baseline scenario is also plotted with ELITE,which is consistent with experimental results and illustrates that the modes with low(high)toroidal mode number dominate on peeling(ballooning)boundary.At the same time,we explore nonideal effects of diamagnetic,shear flow and resistiveity on instabilities with different toroidal numbers.Furthermore,we use BOUT++to make a nonlinear study of varies of equilibria,and identify the grassy-ELM operation regime within a pedestal top electron collisionality window(0.3-0.7)at high global poloidal beta(?2),similar to results previously reported in experiments.A peeling-ballooning stability boundary is obtained by scanning the operating density space.The change in density corresponds to a change in that affects the pedestal bootstrap current.High?_pleads to a strong Shafranov shift,which affects the flux surface averaged pressure gradient drive.The two effects combine to create a peeling-dominated window in intermediatebuffered by ballooning-dominated regimes.Only the peeling-dominated regime shows a cyclic behavior during nonlinear simulation of an ELM crash,reminiscent of grassy-ELM dynamics.The quick recovery of the ELM crash is explainable by the rapid rise of a low n kink-peeling instability when pedestal current I_pedexceeds a threshold at high?_p,which limits the excursion beyond marginal stability and is absent in the ballooning-dominated regime.