Study On The Interaction Between Beta-induced Alfven Eigenmode And Tearing Mode In Tokamak Plasmas

The interaction between magnetohydrodynamic(MHD)instabilities and energetic particles(EPs)and the interaction of different instabilities are crucial physical issues in tokamaks.In this dissertation,we focus on two types of instabilities:beta-induced Alfven eigenmode(BAE)and tearing mode(TM).The former can be driven by EPs and in turn leads to the redistribution/losses of EPs,and the latter can break the closed magnetic flux surface and form the magnetic island(MI),degrade plasma confinement and even trigger disruptions.Moreover,the TM can also interact with the EPs,leading to EPs transport and redistribution.Besides,since the mode structures of BAE and TM are both localized around the rational surfaces q=m/n,they can be expected to interact with each other strongly.Therefore,the interaction between BAE,TM and EPs is highly complex and remains a topic of investigation.In recent years,this interaction is often observed in many tokamak experiments,including HL-2A,JET,EAST,J-TEXT,etc.To better understand the underlying physics of this complex interaction,based on the experiment in HL-2A,we have systematically investigated the interaction referring to BAE,TM and EPs by using the kinetic-MHD hybrid code M3D-K.The main contents of this dissertation are as follows:Firstly in chapter 4,we studied the EP-driven BAE with a reversed safety factor q profile.The results show that the parameters of EPs directly determine the drive strength of BAE by EPs.The effect of EP’s Larmor radius ph on the BAE is associated with the effect of finite drift orbit width and the power transfer from each single particle to wave,and the growth rate of BAE is peaked when the Larmor radius of EP is close to the perpendicular wavelength of BAE(k丄ρh～O(1)).The effect of injection velocity v0 on BAE is related to the fraction of resonant particles.When the minimum value of safety factor qmin≤1.5,the excited mode is m/n=3/2 BAE;when qmin>1.5,the excited mode becomes m/n=3/2 reversed-shear Alfven eigenmode(RSAE).The mode structure of BAE locates near the q=1.5 rational surface,while the RSAE’s mode structure is localized around q=qmin surface.In addition,it is also found that the nonlinear dynamics of BAE depend on the EP drive.For the strongly driven case,the radial gradient df/dPφ near the q=2 rational surface becomes steeper owing to the interaction of 3/2 BAE and EPs,which in turn drives a 4/2 energetic particle mode(EPM)in the nonlinear stage.Then,the resonances overlapping of two neighboring instabilities triggered a significant redistribution and transport of EPs,and the BAE exhibits a twice-repeated and mostly downward frequency chirping.However,for the weakly driven case,the saturation amplitude of the BAE and the EP’s redistribution level,are all lower than the results in the strongly driven case,and there is no 4/2 EPM being driven in the nonlinear stage.Then in chapter 5,we investigated the interaction referring to BAE,TM and EPs.The results show that the TM can decrease the excitation threshold of BAE.Due to the combined effects of flattened q profile at q=2 surface induced by TM,and steepened EP radial gradient near MI separatrix caused by the MI-induced EP redistribution,the linearly-stable BAE can be excited in the nonlinear stage.Then,the synergistic effect of BAE and TM can produce an enhanced redistribution of EPs.It is also found that the BAE can have a delay effect on the MI saturation.Furthermore,a high-frequency axisymmetric m/n=0/0 "breathing" mode(BM)is generated by the mode coupling of BAE and TM,agreeing well with the experimental observation in HL-2A.The perturbed current of BM(ΔCBM)is localized around 2/1 MI separatrix,and its amplitude exhibits a high-frequency periodic oscillation.This periodic current modification from the then produces the synchronized periodic oscillation of 2/1 MI width in a frequency of BM.The 3/1 MI width remains almost invariant without BM around q=3 surface in the nonlinear stage.In summary,we have systematically investigated the interactions between BAE,TM and EPs in tokamak plasma based on the experiments.The results of this dissertation,based on the verification of experimental observation and related theories,construct a systematic physical picture of the interaction referring to BAE,TM and EPs,which allows us to better understand the underlying interaction physics,and develop relevant predictions to control the relevant instabilities and prevent EPs’ redistribution/losses.