Quantifying, modelling and interpreting edge plasma turbulence in tokamak and stellarator fusion experiments

磁约束等离子体边缘区域的湍流是聚变科学的中心课题。它的特性有助于确定温度和密度的径向分布，并对其作出响应，这些分布控制着等离子体的整体约束特性。与边缘处的任何磁流体动力不稳定性一起，边缘湍流也决定了等离子体和能量通过最后一个闭合磁通量表面的向外通量，从而向偏滤器或第一壁的方向。因此，边缘等离子体湍流在受限等离子体的物理和相关的准工程约束(如等离子体排气和第一壁载荷)中都起着至关重要的作用。该方案系统地研究了不同等离子体状态和不同约束概念下聚变边缘等离子体湍流的统计特征，即托卡马克和星标器。这项多设备研究与在英国库勒姆UKAEA的Mega Amp球形托卡马克(MAST)上利用共振磁微扰机制的实验计划保持一致，该托卡马克将于2008年交付使用。这一机制在桅杆的边缘区域产生了类似于星体中发现的随机磁场。统计技术将使我们能够在有或没有遍历磁场的情况下，在恒星加速器和托卡马克中对边缘等离子体的统计特性进行新的直接比较，这是建立在我们与日本国家核聚变科学研究所(NIFS)大型螺旋装置(LHD)团队现有合作的基础上。这项研究解决了基本问题，如边缘等离子体湍流表现出的普遍性，用稳健的单一优值系数量化复杂的湍流波动，以及解决在不同约束系统中模拟径向粒子通量分布的实际问题。有三个紧密耦合的研究项目将解决这些广泛的目标。项目1将研究在两种不同的限制系统，即桅杆球形托卡马克和LHD恒星角化器中，在遍历磁场存在的情况下输运的统计特征。我们将讨论电磁涨落在边缘等离子体湍流演化中的基本作用。该项目将建立在我们与UKAEA Culham和日本NIFS的持续合作的基础上。第二个项目涉及桅杆边缘等离子体湍流的系统定量表征。利用复杂系统科学技术，对不同工作状态下的边缘等离子体湍流进行了研究。这一规模庞大的项目将需要对许多数据集进行分析，以建立迄今所缺乏的适当相互关联的全球图景。该项目将验证这样的想法，即大粒子通量事件的分布可以用不依赖于约束方法或等离子体参数的通用函数形式来描述。径向通量概率密度函数的模拟是核聚变研究中的热点问题之一。径向通量可以被与边缘湍流相互作用的极向纬向流显著地改变。我们将使用约化模型来量化具有纬向流的约化湍流模型中粒子通量的减少。第三个项目致力于将等离子体湍流模拟的输出与来自聚变实验的等离子体湍流测量结果进行定量比较。这是聚变科学中尚未被探索的一个方面；申请者将使用2D Hasegawa-Wakatani模型及其扩展(将包括磁场梯度、温度、纬向流和磁场随机化)、多流体代码和A.Peeters(CFSA华威大学)的回转动力学代码。简化后的模型将扩展到考虑磁效应的三维漂移-阿尔芬湍流模拟。

Turbulence in the edge region of magnetically confined plasmas is a central topic in fusion science. Its character helps to determine, and also responds to, the radial profiles of temperature and density which govern the overall confinement properties of the plasma. Together with any magnetohydrodynamic instabilities at the edge, the edge turbulence also determines the outward flux of plasma and energy across the last closed magnetic flux surface, and hence to the divertor or to the first wall. Edge plasma turbulence thus plays an essential role in both the physics of the confined plasma and the associated quasi-engineering constraints such as plasma exhaust and first wall loading. This proposal outlines a systematic study of statistical features of fusion edge plasma turbulence for different plasma regimes and different confinement concepts, namely tokamaks and stellarators. This multi-device study is aligned with the experimental plan to exploit resonant magnetic perturbation mechanisms on the Mega Amp Spherical Tokamak (MAST) at UKAEA, Culham due for commision in 2008. This mechanism generates stochastic magnetic field at the edge region of MAST which is similar to that found in a stellarator. Statistical techniques will allow us to make new direct comparisons between statistical properties of edge plasma in stellarator and a tokamak with, and without, the ergodised magnetic field, building on our existing collaboration with the large Helical Device (LHD) team at National Institute for Fusion Science (NIFS), Japan. This research addresses fundamental issues, such as the extent of universality exhibited by the edge plasma turbulence, quantifying complex turbulent fluctuations with a robust single figures-of-merit and addressing the practical problem of modelling the distribution of radial particle flux in different confinement systems.There are three closely coupled research projects which will address these broad goals. Project 1 will investigate statistical features of transport in the presence of ergodic magnetic field in two different confinement systems, namely MAST spherical tokamak and LHD stellarator. We will address the fundamental role of electromagnetic fluctuations in the evolution of edge plasma turbulence. The project will build on our ongoing collaboration with UKAEA Culham and NIFS, Japan.The second project concerns systematic quantitative characterisation of edge plasma turbulence in MAST. Edge plasma turbulence will be studied under different operating regimes, using the techniques of complex systems science. This high-volume project will require analysis of many datasets in order to build a properly interlinked global picture, which is hitherto lacking. The project will validate the idea that the distribution of large particle flux events can be described with the universal functional form that does not depend on confinement method or plasma parameters. The modelling of the probability density function for the radial fluxes is one of the topical questions in fusion studies. Radial fluxes can be dramatically modified by poloidal zonal flows that interact with edge turbulence. We will use reduced model to quantify the reduction in particle fluxes in reduced turbulence model with zonal flows.The third project is dedicated to the quantitative comparison between the outputs of plasma turbulence simulations and the measurements of plasma turbulence from fusion experiments.This is an underexplored aspect of fusion science; The applicant will use the 2D Hasegawa-Wakatani model and its extension (magnetic field gradient, temperature, zonal flows and magnetic field randomisation will be included), multi-fluid codes and the gyrokinetic code of A. Peeters (CFSA University of Warwick). The reduced model will be extended to 3D drift-Alfven turbulence simulation which will incorporate magnetic effects.