L-H transition studies on the ST40 tokamak

托卡马克等离子体从低到高(L-H)禁闭区的转变是聚变史上最显著的发现之一，指的是当输入功率增加到一个临界值以上时，禁闭的突然改善。伴随着L-H跃迁的是在受限等离子体边缘形成的基座，这是一个相对较窄的边缘等离子体区域，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，从而改善了与L模式相比的聚变性能。自20世纪80年代首次发现以来，这种现象已在不同的聚变装置中可靠地再现；L-H跃迁、倒向H-L跃迁和基座的研究一直是磁约束聚变中最重要的研究课题之一。对于ITER来说，如何在可用加热功率下进入和离开H模的问题仍然是实际机器运行规划的关键部分。最近，新的托卡马克概念被提出，这些概念进一步偏离了传统的ITER、JET等设计，这需要了解在新体制下获得H模的途径。因此，关于L-H转变的话题正在迅速演变，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、核心和偏滤器等离子体诊断套件，并利用先进的分析技术。该项目将通过分析托卡马克能源有限公司新的ST40超导球形托卡马克的实验结果，为正在进行的全球H模式接入研究做出独特而重要的贡献。为了促进这一点，将开发一个新的解释模型。ST40等离子体将占据低限制和高限制操作的新领域，这种球形托卡马克具有高磁场、低纵横比和增强加热功率的特殊组合。传统分析方法的有效性，使用进入和离开H模式的瞬时转变的平均值或方差，由于大的、与时间相关的波动而具有局限性。因此，本项目的重点将是研究L-H/H-L跃迁的时间演化、可用的边缘基座以及一种新的概率分布函数统计方法的发展。该项目的实验方面将是在一系列可用的参数扫描(如密度、磁场、Ip、扭矩或附加加热方案)上绘制L-H(和H-L)跃迁的操作空间。这些结果有望对正在进行的关于MAST、MAST-U和NSTX的跨球面托卡马克H模式访问研究做出重大贡献。在理论和计算方面，该项目将使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较轻的PDF统计模型。代码将需要并行化以加快计算速度，并进行升级以包括更广泛的噪声源。

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.