Oscillatory zonal flows and their interaction with turbulence in magnetically confined plasmas

核聚变约束等离子体是非平衡驱动系统，其中微不稳定性从温度和密度梯度中提取能量，驱动湍流波动。湍流输运对整体等离子体约束特性有强烈的影响，因此是未来任何反应堆设计的核心。在一定条件下，湍流波动可以组织成大规模的纬向流，如下图所示。这些流动可以通过剪切来调节湍流，并且被认为是等离子体向高约束模式（h模式）过渡的关键因素，这是目前ITER反应堆的主要运行场景。纯纬向流没有频率，在实验室中很难观察到。托卡马克的环形曲率引起了纬向流的附加压缩模态，即测地线声模态（GAM）。该模态以实际频率振荡，可以深入了解湍流和纬向流之间的非线性相互作用和反馈。GAM频率取决于离子和电子温度、等离子体的旋转和形状。这一建议解决了湍流、GAMs和非振荡纬向流之间的相互作用。本项目的主要兴趣领域包括GAM的径向结构，了解电子沿磁场线的传输如何影响GAM的耗散，以及快速等离子体旋转对这种模式的影响。实际方面将探索利用观察到的GAM特征来推断电子和离子温度的比例的可能性，这在目前的诊断中是不可用的。与CCFE MAST团队的密切合作对项目至关重要。该项目结合了令人兴奋的核聚变物理与实用技能，如高级数据分析和使用高性能计算资源进行数值模拟。用于描述纬向流的数学技术非常丰富和通用，在木星的大气、洋流和托卡马克等离子体中都有相同的方程。

Nuclear FusionMagnetically confined plasmas are out of equilibrium driven systems in which micro-instabilities extract energy from temperature and density gradients, driving turbulent fluctuations. Turbulent transport strongly impacts on global plasma confinement properties and thus is in the core of any future reactor design. Under certain conditions turbulent fluctuations can organise into large-scale zonal flows, as shown in the figure below. These flows can regulate turbulence by shearing and are believed to be a key factor in plasma transition to high-confinement mode (H-mode), currently the main operating scenario for the ITER reactor.Pure zonal flows have no frequency and are difficult to observe in the laboratory. The toroidal curvature of a tokamak induces additional compressional mode of zonal flows known as the Geodesic Acoustic Mode (GAM). This mode oscillates with real frequency and can give an insight into the non-linear interactions and feedback between the turbulence and zonal flows. The GAM frequency is dependent on the ion and electron temperatures, rotation and shaping of the plasma.This proposal addresses interaction between turbulence, GAMs and non-oscillatory zonal flows. Main areas of interest for this project include radial structure of GAM, understanding how electron transport along the magnetic field lines influences dissipation of GAMs and what is the impact of fast plasma rotation on this mode. Practical aspects will explore a possibility of using observed GAM features to infer the ratio of electron and ion temperature, which are not available on MAST with current diagnostics. Close collaboration with CCFE MAST team is essential for the project.The project combines exciting physics of nuclear fusion with practical skills such as advanced data analysis and the use of high performance computational resources for numerical simulations. The mathematical techniques used to describe zonal flows are very rich and generic, with the same equations governing flows in the Jupiters atmosphere, ocean currents and in tokamak plasmas.