HL-2A装置高能量离子驱动的比压阿尔芬本征模的扫频行为

Beta-induced Alfvén eigenmodes with frequency chirping driven by energetic ions in the HL-2A Tokamak

利用混合磁流体-动理学模拟程序MEGA对中国环流器二号装置观测到的具有频率啁啾行为的比压阿尔芬本征模进行分析.区别于动理论方法Berk-Breizman模型,MEGA程序采用真实的实验参数,如平衡位形、电子密度、电子温度和离子温度等,再现了具有向上和向下扫频特性的比压阿尔芬本征模.实验观测到下扫频行为出现时背景等离子体的参数和比压值相对更高.据此设置MEGA程序的输入参数:在下扫频行为模拟中,高能量离子的螺矩角初始分布宽度和芯部比压值,以及扩散系数均更高.模拟结果显示快离子相空间的分布影响了扫频行为.当上扫频行为占主导时,平行于磁场的离子发挥主要作用;而下扫频行为占主导时,垂直于磁场的离子密度显著上升.实验与模拟均表明:下扫频行为占主导的比压阿尔芬本征模激发对比压值和高能量离子的密度要求更高,这与之前的模拟分析得到的结论一致.

The beta-induced Alfvén eigenmodes(BAEs)with frequency chirping,observed in the HL-2A Tokamak,are analysed by a MHD-kinetic hybrid code MEGA.Realistic parameters are applied to the code,such as equilibrium,electron density and temperature,ion temperature,which is different from the kinetic Berk-Breizman theory.The BAEs are observed by Mirnov probes and soft X-ray arrays.Toroidal and porloidal mode number are confirmed to be n/m=2/3 by using the phase shift method with toroidal filtered Mirnov signal arrays.And the soft X-ray arrays’signal shows that BAEs are located at the core of the plasma and they have a relatively broad mode structure.The BAEs with up-and down-chirping are reproduced with MEGA code.The simulation results of mode structure accord well with experimental observations.Compared with upchirping BAEs,the down-chirping BAEs are excited with higher plasma parameters and beta value,thus the energetic ion distribution in pitch angle has a broader width,and the beta value of energetic ions in the core of plasma and diffusion value are higher in the down-chirping simulation.The simulation results show that the phase space distribution of energetic ions affects the wave chirping direction.The energetic ions parallel to the magnetic field drive the up-chirping behavior.When the down-chirping behavior dominates,the density of energetic ions perpendicular to the magnetic field increases significantly.It shows that the down-chirping BAEs require higher beta and energetic ion density,which is consistent with the previous simulation result.