外源驱动型托卡马克电磁湍流及全域输运的模拟研究

Turbulent transport in tokamak plasmas is not only characterized by local diffusion but also often exhibits various events of intermittency, bifurcation and nonlocality. For instance, core plasma can timely respond to the external perturbation at edge through non-diffusion, showing strong core-edge transport coupling. Conventional local transport theory and simulation can describe the scaling law of the gradient-driven turbulent transport to some extent, but possess great limitation to represent the dynamics of flux (heat, particle and momentum) driven turbulence and global transport. This project plans to perform global simulation studies on the multi-mode multi-scale electromagnetic turbulence in tokamak with heating and fueling to deeply understand the nonlinear self-organization of high temperature and density plasma and global property of turbulent transport. It is also to explore innovative methods and control means to achieve high confinement tokamak plasmas. Specifically, based on the first principle global gyrokinetic and Landau-fluid simulations with flux (heat, particle and momentum sources), we will investigate the nonlinear interaction of multi-mode multi-scale electromagnetic turbulence; the dynamics of large-scale structure, e.g. shear flows; the global property of turbulent transport; the statistic characteristics of large transport events, e.g. avalanche-type transport, as well as the description methodologies. Furthermore, the validation and comparison between simulation predictions and experimental observations will also be performed.

托卡马克等离子体湍性输运不仅具有扩散型的局域特征，而且常常展现各种类型的间歇性、分叉、非局域特性。例如，边缘和芯部输运高度耦合，对外部扰动表现出非扩散型输运响应。传统的局域输运理论和模拟能较好地描述梯度驱动型湍性输运的定标率，但应用于研究外源(加热、加料)驱动型湍流及其全域输运时具有很大的局限性。本项目计划开展在托卡马克加热加料条件下的多模多时空尺度电磁湍流数值模拟研究，深入理解高参数托卡马克等离子体湍流的非线性自组织过程和全域输运特性，探索实现高约束等离子体的新途径和控制手段。具体地讲，基于第一原理包含外源（粒子、热和动量注入）的回旋动理学和朗道—流体数值模拟研究多模多时空尺度电磁湍流非线性相互作用、大尺度结构（如剪切流）的产生机理及作用、湍性电磁输运的全域特性和大输运事件（如雪崩现象）的统计特征及描述方法，开展数值模拟预测和物理实验的比较和验证研究。

托卡马克等离子体湍性输运制约装置的约束性能，是多模多时空尺度等离子体扰动非线性相互作用与外源驱动平衡剖面演化耦合的结果，不仅具有扩散型的局域特征，而且常常展现各种类型的非局域特性。传统的局域输运理论和模拟应用于研究外源(加热、加料)驱动型湍流及其全域输运时具有很大的局限性。本项目在自主研发外源驱动型扩展流体程序的基础上，利用引进的全域回旋动理学程序GKNET研究了外源驱动多模多尺度托卡马克等离子体湍流的非线性自组织过程和全域输运特性，获得了一批创新性重要成果：1、基于GKNET模拟，发现了E×B剪切流阶梯结构起源于离子温度梯度（ITG）模非线性饱和阶段，其径向电场结构主要来源于湍流所激发的偶对称结构带状流。2、提出了湍流结构的概率分布密度（PDF）分析统计学方法，给出了湍流输运尺寸分布的定标关系。3、自主开发了研究托卡马克外源驱动型多模多尺度湍流和全域输运特性的扩展流体程序ExFC。4、基于ExFC模拟，再现了在轴加热ITG湍流输运主导的离子温度剖面不变性；定性地解释了电子加热L模实验普遍观测到的粒子泵出现象；发现了离子加热驱动ITG湍流能够渗透进入磁岛区域增强离子热输运；发现了HL-2A装置上m/n=2/1磁岛对湍流的调制实验存在一个激发准相干模（QCM）的磁岛宽度阈值，且发生ITG模转换成捕获电子模（TEM）湍流，与实验观测基本一致。5、模拟研究了有限比压电磁ITG和动理学气球模（KBM）湍流的非线性饱和机制以及相关输运特性，发现强带状流是电磁ITG模的主要饱和机制，湍流饱和振幅较低。而KBM湍流经历弱带状流抑制和低模数扰动结构非线性模耦合两级饱和过程，导致相对较高的湍流饱和振幅和输运水平。这些成果应用于探索高约束等离子体运行模式，为ITER和未来的CFETR的先进运行模式提供理论基础。