【作者】 张敏；

【导师】 马志为；

【作者基本信息】 浙江大学 ， 等离子体物理， 2022， 硕士

【摘要】 在托卡马克装置H模放电过程中,等离子体边界常伴随着周期性的边缘局域模(ELM)。通常认为ELM不稳定性是由边界的压强梯度和局域电流共同驱动,并根据二者驱动的强弱划分为以压强梯度驱动为主导的气球模、电流驱动为主导的剥离模和二者共同驱动的剥离-气球模。本文利用大型环位形托卡马克磁流体模拟程序(CLT)和新开发的自由边界偏滤器位形的环向流平衡代码(CLT-EQ),线性模拟研究了边缘局域模不稳定性,主要通过改变边界局域电流来研究ELM的线性增长率和模结构变化规律,以及磁剪切对ELM模式转换的影响。此外还讨论了等离子体的环向剪切流对剥离-气球模的线性影响。本文利用平衡代码CLT-EQ构造了具有H模放电特性的静平衡位形。线性模拟结果表明在高比压台基区,边缘局域的三种不稳定性分别为:气球模,剥离-气球模和剥离模。随着边界电流逐渐增加,在台基区不稳定从边界压强驱动的气球模不稳定性,变成剥离-气球模不稳定性,到边界电流驱动的剥离模不稳定性。我们模拟发现磁剪切主导着这些不稳定模式的转变。随着边界电流密度的逐渐增大,边界的q剖面和局部安全因子v=rBφ/RBθ随之减小。我们引入了整体磁剪切sglobal=(r/q)(dq/dr)和局域磁剪切slocal=(r/v)((?)v/(?)r)。在弱场侧的局域磁剪切slocal=0时,不稳定性模式从气球模主导向剥离-气球模主导发生转变;在sglobal=0附近时,不稳定性模式从剥离-气球模主导向剥离模主导发生转变。此外,我们分析了不同环向模数的增长率,结果表明,在“气球模主导”阶段,最不稳定模式由高n环向模主导;在“剥离-气球模主导”阶段,最不稳定模由中等环向模主导。最后,利用CLT-EQ构造了具有H模放电特性的环向流平衡位形,结合CLT代码线性模拟了环向剪切流对ELM的稳定效应。模拟结果表明环向剪切流对剥离-气球模的高n有具有较强的稳定作用,而对低n有轻微的解稳效应。更多还原

【Abstract】 During a discharge of the H-mode in tokamaks,edge localized mode(ELM)periodically occurs at the plasma boundary.In general,it is believed that the ELM instability is driven by a pressure gradient at the plasma pedestal.According to its driving properties,the ELM can be resulted from the ballooning mode driven by the pressure gradient,the peeling mode driven by the plasma current,or the peelingballooning mode.In this paper,the linear properties of the edge localized mode instability is studied by using three-dimensional toroidal magnetohydrodynamic code(CLT)and the newly developed free-boundary divertor plasma equilibrium code(CLTEQ).The characteristics of the ELM are explored by adjusting the local current in the plasma pedestal.It is found that that the magnetic shear is the most crucial factor to determine which unstable mode drives the ELM.In addition,the role of the toroidal plasma shear flow on the dynamics of the ELM is also studied.In the thesis,we first construct a H-mode configuration using CLT-EQ.The simulation results confirm that the ELM is associated with three different types of the instabilities:ballooning mode,peeling-ballooning mode,and peeling mode,which are respectively driven by either the plasma current density,or a plasma pressure gradient,or both at the plasma pedestal:.As the local current density at the boundary increases from small to large value,the unstable mode is from the ballooning mode driven by the pressure gradient,to the peeling-ballooning mode by both the pressure gradient and the plasma current,and finally to the peeling mode driven by the plasma current.It is found from the simulation results that that magnetic shear plays a dominate role on the change of these unstable modes.As the edge current density gradually increases,the q profile and the local pitch angle of the magnetic field v=rBφ/RBθ decrease and flatten.We define the global magnetic shear to be Sglobal=(dq/dr)and the local magnetic shear to be Slocal(r/v)((?)v/(?)r).Our simulation results suggest that the dominant unstable mode changes from the ballooning mode to the peeling-ballooning mode when the local magnetic shear on the low field side(LFS)decreases to zero.It is further found that Sglobal=0 is another critical point to determine the transition between the peelingballooning and the peeling mode.Finally,the H-mode equilibrium configuration with the toroidal plasma flow equilibrium is constructed by CLT-EQ.It is indicated from the CLT simulation that the toroidal shear flow has a strong stabilization effect on high-n modes,but has a weak destabilizing effect on low-n modes of the Peeling-Ballooning mode.更多还原