BOUT++simulation Of Pedestal Turbulence And Divetor Heatflux

Edge plasma turbulence in H-mode regime constrains the pedestal structure and cross-field transport in tokamak devices.Theoretically,turbulence driven by kinetic ballooning mode(KBM)is believed to be the constraint of the pedestal structure.However,no global simulation has been done to verify this assumption.Cross-field transport by the turbulence in L-mode regime has large impacts on the heat flux width at the divertor target,while its role in the H-mode regime is not clear.BOUT++two-fluid and gyro-fluid code package has been widely used to simulate the edge-localized mode(ELM)events and the turbulence transport in edge plasma studies.Therefore,the BOUT++two-fluid and gyro-fluid code package is used to simulate the edge plasma turbulence in ideal and real equilibriums.First,the Gyro-Landau-Fluid(GLF)3＋1 module is used to perform the global simulation of the KBM.To verify our code on the KBM case,the beta scan based on "Cyclone base case parameter set" is performed.We find that the growth rate is almost the same from GYRO and GLF code.Using JET-like global circular equilibria,by including small electron-electron collision to damp electron modes,GYRO generated mode structures and parity suggest that they are kinetic ballooning modes,and the growth rates are comparable to the GLF results.However,a radial scan of the pedestal,using GYRO code,shows different trends for the low-n and high-n modes.The low-n mode scan shows the linear growth rate peaks at peak pressure gradient position as GLF results.However,for high-n modes,the growth rate of the most unstable mode shifts outward to the bottom of pedestal and the real frequency of what was originally the KBMs in ion diamagnetic drift direction steadily approaches and crosses over to the electron diamagnetic drift direction.Second,the quasi-coherent mode(QCM)during inter-ELM period in HL-2A is studied with the six-field two-fluid module.Linear simulations show the peeling-ballooning mode(P-B mode)is stable,while the resistive ballooning mode(RBM)is unstable.From the linear resistivity scan,the growth rate match the theory of resistivity dependence,and thus,the linear result is verified.With an unstable resistive-ballooning mode from linear simulations,the nonlinear simulation has successfully reproduced the QCM inside the separatrix at the outboard mid-plane.The poloidal wave number and frequency from fluctuation analysis are almost the same as the experimental results.The phase shift between the electrical potential and the density fluctuation matches.The transport induced by the QCM dominants the particle transport in the simulation,and the ambient turbulence transport is small.This phenomenon can be found in similar experiments in other devices.Linear and nonlinear global profile scans show that the QCM is triggered by the density gradient,and stabilized by the temperature and temperature gradient.Based on these scans,the observed saturated density and density gradient profiles and the increasing temperature profile are explained.Using the "surface instability" model,based on ballooning mode,the predicted poloidal wave numbers match the similar experimental results in different devices.In the last part,the turbulence simulation of the heat flux width in DIII-D is performed.The linear simulations indicate there are two dominant modes:resistive ballooning mode and drift-Alfven wave(DAW)mode.The locations of these two modes are outer mid-plane and the crown of the device,respectively.And the drive terms are the pressure gradient and the inverse electron pressure scale length.Using a simplified model,theory analysis gives a criterion on the drift-Alfven wave mode.Local calculations show the drift-Alfven wave is unstable,consistent with the simulation result.From the frequency-ke spectra in the nonlinear saturated stage,a quasi-coherent mode,near the separatrix at the outboard mid-plane,is found.The dominant frequency and poloidal wave number match the experimental results from similar discharge.The contour plot of the coherent structure and the statistical analysis of the fluctuations suggest that cross-field transports near the separatrix are blob type.The parallel heat flux widths from the BOUT++ simulations are in good agreements with the experimental measurements.However,the parallel heat fluxes only qualitatively agree with the experimental measurements with higher amplitudes for all three cases.Two of the reasons are:(1)the lack of radiative energy loss and(2)the choice of flux-limiting parameter aj in conductivity model.