Simulation Of Quasi-Coherent Mode Based On BOUT++ In HL-2A

High-confinement steady-state operation mode is essential for achieving magnetic confinement nuclear fusion and confinement of pedestal and instability studies of magnetohydrodynamics in H-mode regime have been key physical issues.Recently,many electromagnetic fluctuations whose frequency spectrum width are between coherent and broadband called QCMs(quasi-coherent modes)are observed at the edge of various tokamak devices during the ELMy H-mode discharge experiment.QCMs are closely related to the pedestal and they appear with the forming of the pedestal while disappear with the onset of the ELM.Current theoretical analysis suggests that the turbulence fluctuation caused by QCMs can enhance the plasma transport and discharge excess plasma particles and energy in the pedestal via the edge of the devices maintaining the pedestal to be stable.Therefore,the research of QCMs under the high-confinement discharge of tokamak devices is a crucial topic at present.Currently,QCMs have also been discovered in the HL-2A device during the discharge under the ELMy H-mode.The frequency of the QCMs is about 50 kHz,the poloidal wave number is 0.6 cm-1 and they propagate into the core of the plasma.In order to analyze the physical characteristics of QCMs observed in HL-2A device,some simulations of the QCMs of HL-2A device are implemented in this paper using the real HL 2A tokamak magnetic field configuration and experimental data and the newly developed BOUT++ program with electromagnetic six-field two-fluid model.In this paper,the experimentally measured plasma electron density and ion temperature profile data subjected by two-step smoothing are used as the initial profile of the numerical simulation.Then,linear and nonlinear simulations of the QCM during the ELMy H-mode discharge are conducted in the real HL-2A tokamak magnetic field configuration,and the position and some physical properties of the QCM are obtained.In linear simulation,after calculating of mode growthrate under different control conditions and different toroidal mode numbers,the properties of the QCM is proved to be the resistive ballooning mode driven by the resistivity and the plasma pressure gradient.Then,in order to analyze the QCM in more details,toroidal compressibility,energy exchange,thermal conductivities,thermal force,resistivity and hyper-resistivity,hall MHD effects,anomalous viscosity and other physical effects are introduced in a nonlinear simulation to obtain the frequency and the poloidal wave number of the QCM,which are consistent with the experimental results.Furthermore,the qualitative numerical simulation of the interaction between the QCMs and the background turbulence is also conducted using BOUT++,and the effects on mode structure and amplitude cased by background turbulence are analyzed.The comparison between the simulation and the bi-spectrum analysis shows that there is a nonlinear coupling between the QCMs and the background turbulence.At last,the effects of the density and temperature profile changing in the radial direction on the growthrate and mode structure of the QCMs are analyzed qualitatively,and the relationship between plasma density,temperature and QCMs is acquired.The results in this paper enhance the understanding of QCMs in HL-2 A devices and also provide some theoretical simulation basis for researching the confinement of pedestal and the interaction between QCMs and background turbulence.