Multiscale turbulent dynamics of tokamak plasmas

等离子体湍流支撑着广泛的现象，包括恒星和星系的形成；太阳风的特性，以及——本节目的重点——托卡马克中等离子体的约束。它是复杂的反馈机制，耦合空间和时间尺度跨越几个数量级。完整的问题是极具挑战性的，因此为了在现实世界的应用中取得进展，我们必须开发出能够捕捉基本物理特性的简化模型。我们提出的计划的目标是通过在基础科学水平上推进我们对这些多尺度相互作用的理解来解决这个问题。这将通过耦合分析理论，先进的计算和实验能力，包括新升级的MAST-U托卡马克来实现。等离子体湍流是复杂的，因为至少有两种相互作用的“流体”——电子和离子——它们是带电的。因此，密度的波动驱动电荷分离，从而驱动静电场的波动，而速度的波动驱动电流，从而驱动磁场的波动。然后，这些场将电子和离子“流体”的相对运动耦合起来。磁化等离子体支持的各种波，以及当波的相速度与粒子速度相匹配时存在的共振，使情况进一步复杂化。要正确处理这些共振，需要了解粒子速度分布；反过来，这需要一个动力学或先进的流体方法-一个艰巨的任务。湍流，通常在托卡马克的毫米厘米尺度上，以一种复杂的方式与全球平衡剖面（例如密度，温度和流动梯度）相互作用，这是在米尺度上的。为了量化托卡马克等离子体湍流和轮廓之间复杂的多尺度反馈机制，从而提供准稳定最终状态的预测能力，我们将解决和整合一些主题。我们将首先了解平均流如何与静电湍流相互作用（即忽略磁场中的波动），需要耦合具有特征尺度的波动，从电子拉莫尔半径（亚毫米）到离子拉莫尔半径（几毫米）甚至更大（厘米），到系统长度尺度的轮廓(m)。我们的新理论和模拟将为MAST-U的实验提供信息，利用已经计划用于该设备的两种诊断仪器（光束发射光谱和多普勒后向散射）。这些仪器所能测量的波长范围很可能存在差距，因此我们预计需要开发和安装一种新的微波成像系统。这将使用从该计划的早期阶段获得的知识来设计，并部署到后期的进一步实验中。对电磁湍流的理解还不太发达，需要新的理论模型。基于从静电湍流中获得的知识，我们将寻求再次理解多尺度的相互作用和反馈，包括流动。然而，现在的情况更加复杂，因为电磁湍流可以驱动大尺度电流，改变限制等离子体的磁场，并耦合成大尺度电磁模式。一个关键的动机是优化托卡马克等离子体的聚变性能，这需要我们了解快速粒子的影响。这些可以通过它们激发的不稳定性直接驱动湍流，或者影响由热粒子驱动的湍流。我们的模拟将评估中性束加热系统产生的快速粒子对MAST-U的影响，以及聚变反应产生的高能α粒子对未来设备（如ITER）的影响，以及计划在JET上使用氘-氚混合聚变燃料的实验。

Plasma turbulence underpins a wide range of phenomena, including the formation of stars and galaxies; the properties of the solar wind, and - the focus of this programme - the confinement of plasmas in tokamaks. It is complicated by feedback mechanisms that couple space and time scales spanning several orders of magnitude. The full problem is extremely challenging, and so to make progress for real world applications we must develop reduced models that capture the essential physics. The goal of our proposed programme is to address this by advancing our understanding of these multi-scale interactions at a fundamental science level. This will be achieved by coupling analytic theory, advanced computation and experimental capabilities, including the newly upgraded MAST-U tokamak.Plasma turbulence is complicated by the fact that there are at least two types of interacting "fluids" - electrons and ions - and these are charged. Fluctuations in density therefore drive charge separation and hence fluctuations in the electrostatic field, while fluctuations in velocity drive currents and hence fluctuations in the magnetic field. These fields then couple the relative motions of the electron and ion "fluids". The situation is further complicated by the rich variety of waves that a magnetised plasma supports, and the resonances that exist when the phase velocity of a wave matches the particle velocity. To properly treat these resonances requires knowledge of the particle velocity distribution; this, in turn, requires either a kinetic or an advanced fluid approach - a daunting task. Turbulence, typically at the millimetre-centimetre scale in tokamaks, interacts in a complex way with the global equilibrium profiles (density, temperature and flow gradients, for example), which are on the metre-scale. To quantify the complex, multi-scale feedback mechanisms between tokamak plasma turbulence and profiles, and so provide a predictive capability for the quasi-steady final states, we will address and integrate a number of topics. We will first learn how mean flows interact with electrostatic turbulence (ie neglecting fluctuations in the magnetic field), requiring coupling between fluctuations with characteristic scales ranging from the electron Larmor radius (sub-mm) through to the ion Larmor radius (few mm) and beyond (cm), to the system length scale of the profiles (m). Our new theory and simulations will inform experiments on MAST-U, exploiting two diagnostic instruments already planned for the device (beam emission spectroscopy and doppler back-scattering). It is likely there will be gaps in the wavelength range that these instruments can measure, so we anticipate a need to develop and install a new microwave imaging system. This will be designed using knowledge gained from the early phase of the programme, and deployed for further experiments towards the end.Understanding of electromagnetic turbulence is less developed and new theoretical models will be required. Building on the knowledge gained from the electrostatic turbulence, we will seek to again understand the multi-scale interactions and feedbacks, including flows. However, now the situation is more complicated as electromagnetic turbulence can drive large scale currents, modifying the magnetic field which confines the plasma, and coupling into large scale electromagnetic modes.A key motivation is to optimise tokamak plasmas for fusion performance, and this requires us to understand the impact of fast particles. These can drive turbulence directly through the instabilities they excite, or influence the turbulence driven by the thermal particles. Our simulations will assess the impact of the fast particles created by the neutral beam heating systems on MAST-U, and also the impact of energetic alpha particles from fusion reactions on future devices like ITER, as well as experiments planned on JET with the deuterium-tritium mix fusion fuel.