L-H transition studies on the ST40 tokamak

在托卡马克中，等离子体从低约束到高约束的跃迁是核聚变史上最显著的发现之一，它是指当输入功率增加到一个临界值以上时，约束的突然改善。L-H跃迁伴随着在受限等离子体边缘形成一个基座，这是一个相对狭窄的等离子体边缘区域，其压力分布梯度显著增强，输运减少。这反过来又提高了核心等离子体的温度，从而提高了与l模式相比的聚变性能。自20世纪80年代首次发现以来，这种现象已经在不同的聚变装置中可靠地重现；对L-H跃迁、后H-L跃迁及其基架的研究，一直是磁约束聚变领域最重要的研究课题之一。对于ITER来说，如何在可用的加热功率下进入和退出h模式仍然是实际机器运行规划的关键部分。最近，人们提出了新的托卡马克概念，这些概念偏离了ITER、JET等传统设计，这需要理解在新体制下进入h模的途径。因此，L-H跃迁的主题正在迅速发展，越来越多的实验研究计划在现有的机器上进行。这些研究包括升级的边缘、岩心和分流器等离子诊断套件，并利用先进的分析技术。该项目将通过对托卡马克能源有限公司新型ST40超导球形托卡马克的实验结果分析，为正在进行的全球h模存取研究做出独特而重要的贡献。为了促进这一点，将开发一种新的解释模型。ST40等离子体将占据低和高约束操作的新领域，这种球形托卡马克的高磁场，低宽高比和增强的加热功率的特殊组合。传统的分析方法（使用进入和退出h模式的瞬时转换的平均值或方差）的有效性由于大的、随时间变化的波动而受到限制。因此，本项目的特别重点将放在L-H/H-L转换的时间演变的调查，可用的边缘基座和一种新的概率分布函数（PDF）统计方法的发展。该项目的实验方面将通过一系列可用的参数扫描，如密度、磁场、Ip、扭矩或额外加热方案，绘制出L-H（和H-L）转换的操作空间。这些结果预计将对正在进行的MAST、MAST- u和NSTX上的跨球面托卡马克h模存取研究作出重大贡献。在理论和计算方面，该项目将采用现有的模拟代码来求解3D Fokker-Planck方程，并使用它来开发和调整更轻的PDF统计模型。代码需要并行化以加快计算速度，并升级以包含更大范围的噪声源。

The plasma transition from the low to high (L-H) confinement regime in tokamaks, one of the most remarkable discoveries in fusion history, refers to the sudden improvement of confinement when input power is increased above a critical value. The L-H transition is accompanied by the formation of a pedestal at confined plasma edges, a relatively narrow edge plasma region with significantly enhanced pressure profile gradients and reduced transport. This in turn boosts the temperature of the core plasma thus improving the fusion performance compared to L-mode. This phenomenon has been reliably reproduced in different fusion devices since its first discovery in 1980s; the study of the L-H transition, the backwards H-L transition and the pedestal, has remained one of the most important research topics in Magnetically Confined Fusion ever since.For ITER, questions on how to enter and exit the H-mode with the available heating power remain a crucial part of the practical machine operation planning. Recently, new tokamak concepts wandering further away from the conventional design of ITER, JET, etc., have been proposed, which require understanding of the access to H-mode in new regimes. The topic of L-H transitions is thus rapidly evolving, with a growing number of experimental studies planned on existing machines. These studies involve upgraded suites of edge, core and divertor plasma diagnostics and utilise advanced analysis techniques.This project will make a unique and important contribution to ongoing world-wide H-mode access studies through the analysis of experimental results on the new ST40 super-conducting spherical tokamak at Tokamak Energy Ltd. To facilitate this, a novel interpretative model will be developed. ST40 plasmas will occupy new domains of low and high confinement operation with this spherical tokamak's exceptional combination of high magnetic field, low aspect ratio and augmented heating power. The validity of traditional analysis methodology, using mean values or variance across the instantaneous transitions into and out of H-mode, has limitations due to large, time-dependent fluctuations. Thus, the particular emphasis of this project will be on the investigation of the time-evolution of the L-H/H-L transitions, the edge pedestal whereavailable and the development of a novel probability distribution function (PDF) statistical method.The experimental aspect of the project will be to map out the operational space for L-H (and H-L) transitions over a series of available parameter scans, such as density, magnetic field, Ip, torque or additional heating scheme. These results are expected to make a significant contribution to ongoing cross-spherical tokamak H-mode access studies on MAST, MAST-U and NSTX. On the theoretical and computational side, the project will employ an existing simulation code to solve the 3D Fokker-Planck equation and use it to develop and tune the lighter PDF statistical model. The code will need to be parallelised to speed up computations and upgraded to include a wider range of noise sources.