Turbulence and transport in the presence of electromagnetic fluctuations and supra-thermal particles in tokamaks.

为世界能源问题和气候变化提供长期解决方案是人类在全球范围内面临的最具科学挑战性的努力之一。核聚变能源是一种特别有吸引力的解决方案，在未来几十年有望成为一种可行的能源，因为它提供无碳、稳定、高能量密度的能源，而且没有放射性废物。然而，在聚变条件下，热等离子体的约束是非常复杂的。磁约束聚变由托卡马克概念所倡导，它在一个环形装置中使用强磁场来产生约束。尽管存在磁场限制，实验和理论已经提供了强有力的证据，证明等离子体中的湍流过程会不断地从热的限制核心中泄漏热量和粒子，这阻碍了聚变过程的有效产生。这促使人们从基本的角度理解等离子体湍流过程导致的热和粒子输运，以及它对现实生活中的托卡马克实验的影响。最近的数值和理论研究表明，在足够高的β值（等离子体与磁压之比）下，电磁场的波动会导致球形托卡马克核心中输运损失的增加，从而导致传统的静电描述“所谓的”离子尺度湍流（外核传统托卡马克的特征）到完全电磁描述和新输运过程之间的范式转变。电磁、中尺度不稳定性（alfv<s:1>本征模）也可以由超热粒子的存在驱动，并且能够稳定静电离子尺度的湍流波动。离子尺度湍流在ST和内部托卡马克核心的稳定导致未开发的约束制度，可能是由电子热损失主导。在传统的核心托卡马克等离子体场景中，这些传统上次于离子热损失。这些机制对未来的核聚变反应堆至关重要，因为未来的核聚变反应堆预计将以电子加热和输运为主导。这项研究的新颖之处在于，通过结合对电子波动的实验测量来研究这些未开发的约束制度，这些电子波动被认为是电子热输运的原因（迄今为止极为罕见），直接数值湍流模拟（陀螺动力学模拟），对实验测量进行定量解释的合成诊断，和分析理论，以产生对实验和数值结果的基本理解。从程序上讲，研究由快粒子驱动的电磁波动和alfv<s:1>本征模是很重要的，因为它们弥补了当前机器操作和未来聚变反应堆之间的差距。即将到来的JET DT运动（英国CCFE）， MAST-U（英国CCFE）和NSTX-U实验（美国普林斯顿）是当今机器与聚变燃烧等离子体实验（如ITER， STEP（英国）和SPARC（美国））之间缺失的一环。科学上，这项研究将导致突破性的发现，如超热粒子和湍流之间新的相互作用机制，或发现未来聚变反应堆预期的增强约束制度。这将直接影响到未来STEP和ITER燃烧等离子体实验的预测和设计，并将使英国在未来几十年内完全实现内部能源独立于磁聚变。

Providing a long-term solution to the world energy problem and climate change is one of the most scientifically challenging endeavours that faces humanity on the global scale. Fusion energy is a particularly attractive solution and is poised to become a viable energy source in the coming decades by providing carbon-free, steady-state, high energy density in the absence of radioactive waste. However, confinement of the hot plasma is remarkably complicated to achieve in fusion conditions. Magnetic confinement fusion is championed by the tokamak concept, which uses strong magnetic fields in a donut-shaped device to produce the confinement. Despite the confining magnetic fields, experiments and theory have provided strong evidence that turbulent processes in the plasma produce a constant leakage of heat and particles out of the hot confining core, which impedes efficient generation of the fusion processes. This motivates understanding the plasma turbulent processes leading to heat and particle transport from a fundamental perspective, as well as its implications for real life tokamak experiments.Recent numerical and theoretical studies have shown that fluctuations in the electromagnetic field can lead to enhanced transport losses in the spherical tokamak core at sufficiently high values of beta (ratio of plasma to magnetic pressure), leading to a paradigm shift between the traditional electrostatic description of the 'so-called' ion-scale turbulence (characteristic of the outer-core conventional tokamak) to a fully electromagnetic description and new transport processes. Electromagnetic, meso-scale instabilities (Alfvén eigenmodes) can also be driven by the presence of supra-thermal particles, and are capable of stabilising electrostatic ion-scale turbulence fluctuations. The stabilisation of ion-scale turbulence in the ST and the inner tokamak core leads to unexplored confinement regimes that are likely dominated by the electron heat losses. These are traditionally subdominant to ion heat losses in conventional core tokamak plasma scenarios. These regimes are critically relevant for future fusion reactors which are expected to have dominant electron heating and transport. The novelty of this research is to study these unexplored confinement regimes by combining experimental measurements of the electron fluctuations that are believed to be responsible for the electron heat transport (extremely scarce to date), direct numerical turbulence simulation (gyrokinetic simulation), synthetic diagnostics for the quantitative interpretation of the experimental measurements, and analytical theory to yield a fundamental understanding of the experimental and numerical findings. Programmatically, the study of electromagnetic fluctuations and Alfvén eigenmodes driven by fast particles is important as they bridge the gap between current machine operation and future fusion reactors. The upcoming JET DT campaign (CCFE, UK), MAST-U (CCFE, UK) and NSTX-U experiments (Princeton, USA) are the missing link between present-day machines and fusion burning plasma experiments such as ITER, STEP (UK) and SPARC (US). Scientifically, this research will lead to ground-breaking discoveries such as new interaction mechanisms between supra-thermal particles and turbulence or the discovery of enhanced confinement regimes expected of future fusion reactors. This will have direct influences affecting the projections and design of the future STEP and ITER burning-plasma experiments, and will enable the UK to gain full in-house energy independence from magnetic fusion in the coming decades.