Plasma dynamics in the tokamak edge

托卡马克边缘和刮擦层中的输运对磁约束聚变反应堆的性能起着至关重要的作用。边缘的湍流可能决定了在高密闭模式放电中形成的基座的高度，而相关的传输则决定了容器壁上的热负荷。由于开放场线、中性粒子、磁几何形状的极端变化和大幅波动等因素的存在，边缘和SOL中的等离子体动力学建模和模拟尤其具有挑战性。作为正在进行的亚瑟王的神剑/海王星项目的一部分,在牛津大学的一组研究人员——与UKAEA合作的同事已经创建了一个小说,基于当下漂移动力学代码研究动力学在边缘和索尔。代码的基本目的是有效地解决漂移的自洽版本,和/或gyro-kinetic方程,而使一个简单的连接到流体码在遥远的溶胶和δf gyrokinetic代码的核心。据我们所知，目前所有的代码要么对常规物理研究来说过于昂贵，要么使用简化的流体处理（可能考虑减少维数），要么使用不一致的动力学处理。我们所追求的动量-动力学方法应该允许在需要的地方进行自一致的动力学处理，而不需要过多的数值费用。目前，代码只能解决并行动力学（有或没有力矩动力学方法）或均匀的螺旋磁场（只有标准漂移动力学）。两个版本的代码都假定电子的简化响应符合玻尔兹曼统计平衡。我们的建议是聘请一位牛津大学的博士生来推进代码开发；继续进行更先进的核查、确认和性能基准；并最终使用这些代码进行基础物理研究。可以探索的代码开发的关键方面包括扩展处理动态电子，增加带电粒子之间的碰撞，扩展到3D（非螺旋）几何，处理将开放和封闭场线区域连接在一起的分离矩阵区域，增加更复杂的壁边界条件以及在单个模拟中从漂移动力学过渡到回旋动力学或流体模型的方案。有很多途径可以探索，哪些是由学生解决的，这将取决于代码的状态，一旦学生达到速度，什么将被视为最新颖和及时的贡献，它提供了最直接的物理结果路径和目标物理与反应堆设计研究的相关性。该项目属于EPSRC“等离子体和激光”和“英国磁聚变研究计划”的研究领域。

Transport in the tokamak edge and scrape-off layer (SOL) plays a crucial role in determining the performance of a magnetic confinement fusion reactor. Turbulence in the edge likely sets the height of the pedestal that forms in high-confinement-mode discharges, and the associated transport sets the heat load to the vessel walls. Plasma dynamics in the edge and SOL are particularly challenging to model and to simulate due to, e.g., the presence of open field lines, neutral particles, extreme changes in magnetic geometry and large-amplitude fluctuations. As part of the ongoing ExCALIBUR/Neptune project, a group of researchers at Oxford - in collaboration with UKAEA colleagues - have created a novel, moment-based drift kinetic code to study the dynamics in the edge and SOL. The fundamental aim of the code is to solve efficiently a self-consistent version of the drift- and/or gyro-kinetic equation, while enabling a straightforward connection to fluid codes in the far SOL and delta-f gyrokinetic codes in the core. To our knowledge, all current codes are either prohibitively expensive for routine physics studies, use a simplified fluid treatment (possibly with a reduction in the number of dimensions considered) or use an inconsistent kinetic treatment. The moment-kinetic approach that we are pursuing should allow for a self-consistent kinetic treatment where required without undue numerical expense. At the moment the code solves only for either parallel dynamics (with or without the moment kinetic approach) or for a homogeneous, helical magnetic field (with only standard drift kinetics). Both versions of the code assume a simplified response for electrons that corresponds to a Boltzmann statistical equilibrium.Our proposal is for a PhD student based at Oxford to carry forward the code development; to continue with more advanced verification, validation and performance benchmarks; and to ultimately use the code to do fundamental physics studies. Key aspects of code development that could be explored include an extension to treat kinetic electrons, addition of collisions amongst charged particles, extension to 3D (non-helical) geometry, treatment of the separatrix region that ties together open and closed field line regions, addition of more sophisticated wall boundary conditions and a scheme for transitioning from drift kinetic to gyrokinetic or fluid models within a single simulation. There are a lot of avenues to explore, and which ones are addressed by the student will depend on the state of the code once the student gets up to speed, what will be seen as the most novel and timely contribution, which provides the most direct path to physics results and the relevance of the targeted physics to reactor design studies.This project falls within the EPSRC "Plasma and Lasers" and "UK Magnetic Fusion Research Programme" research areas.