Simulation Study Of Edge Localized Mode Dynamics And Transport Process On EAST

The high edge particle and energy confinement regime,known as the H-mode,with an edge pressure pedestal in a tokamak is currently considered the most promising scenario to achieve magnetic confinement fusion.Edge-localized modes（ELMs）are often excited in an H-mode plasma due to the appearance of edge pressure pedestal.When gradients in the H-mode transport barrier grow to exceed the magnetohydrodynamic（MHD）stability limit,the ELM grows explosively,and then rapidly transports heat flux and particles to the open field line region and material surfaces,casuing damage to divertor targets.For future application in devices such as the International Thermonuclear Experimental Reactor（ITER）,the heat load problem on the divertor target plate in H-mode of ELM is an urgent problem to be solved.The filamentary structure in the process of ELM outbreak is a typical feature of ELM.Various diagnoses show that the filaments move radially outwards at the boundary,and the phenomena of each device are different.Experimental results of some devices show that the filaments are accelerated in the process of outward movement,while others are decelerated,so there is no clear theoretical explanation.Therefore,the simulation and analysis of the dynamic process of ELM filaments will help to understand the mechanism of ELM generation and filaments formation under H-mode.At the same time,with the outbreak of ELM,the radial outward movement of ELM filaments will be accompanied by the outward transport of particles and energy.Therefore,the simulation study of heat flux and particle flux transport generated in the process of ELM can improve the understanding of the edge transport process of plasma in ELM outburst and the thermal deposition of the first wall and target plate,and find out the control and mitigation method prior to the basic operational regime of ITER.BOUT++is a three-dimensional nonlinear finite-differencing plasma simulation framework written in C++,using real tokamak configuration,targeted particularly on studies of tokamak edge plasmas.Using peeling-ballooning model,the instability growth rate and mode structure obtained by linear simulation are in good agreement with the results of other codes.In addition,BOUT++also contains nonlinear physical effects,which can be used to simulate the nonlinear process of plasma pedestal collapse and study the turbulence and transport problems of different magnetic field configurations in different nuclear fusion devices.In this paper,the six-field two-fluid model in BOUT++is used to simulate the dynamics evolution and transport process of ELM in H-mode discharge on Experimental Advanced Superconducting Tokamak（EAST）.The content of the paper is as following:（1）The six-field two-fluid model in BOUT++is used to simulate the ELM crash event in a typical H-mode discharge on EAST,and the ELM instability was analyzed.The linear growth rate is calculated using an ideal MHD model and a full model containing all nonlinear physical mechanisms.It is found that the linear growth rate is dependent on the electron and ion temperature gradients and not on the particle density gradients.This indicates that the resistive ballooning mode and drift-Alfven wave are two dominant instabilities for the equilibrium,and play important roles in driving ELMs.After the nonlinear effect is added,the ratio of energy loss caused by ELM to energy stored in the plasma pedestal region is obtained,that is,the ELM size is about 15%.The time evolution of the poloidal structures of perturbed electron temperature T_e in peak gradient region of the out middle plane（OMP）is well simulated,indicates the linear growth stage,nonlinear stage and saturation stage of ELM progress.And the dominant toroidal mode number in each stage of the ELM crash process is found.The studies show that in linear stage,the dominant toroidal mode number n=20,during the nonlinear phase,the dominant toroidal mode number is 5,and it changes to 0 when the nonlinear phase goes to saturation.（2）Based on the experimental measurement of the dynamic characteristics of the filamentary structures,filamentary structures of Type-I ELM and Type-III ELM discharges were simulated on the poloidal section of EAST using the BOUT++six-field two-fluid model for the first time.The results show that clear filamentary structures can be seen in the early linear stage,and when the nonlinear stage begins,the filamentary structures leave the separatrix and enter the scrape-off layer（SOL）.With the eruption of the ELM,the turbulent transport increases and the perturbation level gradually increases to the saturation stage.At saturation phase the filaments still exist,but the filamentary structure is not as clear as before.As the filamentary structures spread,their positions change.The calculation results show that the poloidal velocity of the filament structure is greater than the radial velocity of the same discharge.The filament of Type-I ELM moves faster than that of Type-III ELM.As the filament moves radially outward,the speed is also increasing,but Type-I ELM increases faster.The radial velocity range of Type-I ELM filament structure is from 0.1 km·s-1 to 0.95 km·s^-1,and the poloidal velocity range is from 0.73 km·s^-1 to 3.87 km·s^-1.The radial velocity of Type-III ELM filamentary structure ranges from 0.19 km·s^-1 to 0.33 km·s^-1,and the poloidal velocity ranges from 0.7 km·s^-1 to 1.54 km·s^-1.The range of velocity variation is consistent with the diagnostic data obtained by high speed charge-coupled device（CCD）camera on EAST.The width of Type-I ELM filamentary structure is larger than that of Type-III ELM.And in the process of radially outward movement,the width of the filaments gradually decreased,Type-I ELM filament width decreased from 34.19 mm to 24.82 mm,Type-III ELM filament width decreased from 25.31 mm to 16.29 mm,The variation range and trend of width are consistent with experimental diagnosis.The time evolution of the width of the filament shows that the width decreases with the process of ELM.Tracing one single filament,it is found that the width of the filament decreases with time,and the maximum of the perturbed electron temperature decreases at the same time.The results indicate that when the filaments are propagating radially outwards,the energy and particles carried by them is dissipated,which is consistent well with both theroy and experimental conclusions.（3）The transport process of particles and heat flux after ELM explosion on EAST was simulated by BOUT++.The radial particle flux and heat flux transport coefficients and heat flux deposition on the divertor plate were calculated.The influence of sheath boundary condition（SBC）and magnetic flutter term in thermal conductivity on the transport process was discussed.The time evolution of the radial electron heat flux,ion heat flux,and particle density flux at the outer midplane（OMP）are obtained,and the corresponding transport coefficients D_r,χ_ir,andχ_er reach maximum around 0.3～0.5 m²·s^-1 atΨ_N=0.9.The profile evolution of electron and ion heat flux on the inner and outer target plates after ELM burst was given.The heat flux on the outer target plate was much larger than that on the inner target plate,and the peak value of electron heat flux on the target plate was 0.95 MW·m^-2,which was about two orders of magnitude larger than the peak value of ion heat flux 0.012 MW·m^-2,,indicating that electron dominated the heat flux on the target plate during ELM burst.The simulated profiles of ion saturation current density（j_s）at the lower outboard divertor target are compared to those of experiments by Langmuir probes.The profiles near the strike point are similar,and the peak value of j_s from simulation 7.5Acm^-2 is very close to the measurements 8.9Acm^-2.In order to find out the effects of sheath boundary conditions（SBCs）and magnetic flutter term on particle transport and heat flux evolution and distribution during ELM outbreak on EAST,and to understand the physical mechanism of transient heat flux distribution during ELM collapse,the BOUT++code was used to simulate three different cases:with SBCs and flutter,with SBCs but without flutter,and with neither SBCs nor flutter.The results show that SBCs hardly change the linear results of the resistive ballooning mode,and the magnetic flutter term contributes to the edge instability.In the nonlinear phase,SBCs and magnetic flutter will change the dominant toroidal mode,and magnetic flutter will restrain the reverse cascade,so that the dominant modes does not evolve to low n.The magnetic flutter term in the parallel thermal conductivity can change the peak position of the perturbation.However,SBCs have little influence on frequency,but the doppler effect is introduced due to the addition of shear flow,which widens the frequency distribution range.Because the larger flutter causes more energy to flow out of the pedestal region,the magnetic flutter increases the size of ELM from 2.8%to 8.4%relative to the plasma storage in the simulated region,and doubles the radial heat flux and particle flux transport coefficient of OMP to about 1.0m²s^-1.Moreover,as the magnetic flutter term significantly enhances the radial transport,more particles are transported from the pedestal region to SOL,and finally reach the divertor target plate.Therefore,the magnetic flutter term in thermal conductivity can increase the particle flux,which in turn increases the particle flux and heat flux on the divertor target plate,and broadens the parallel heat flux distribution to the target plate.