Multiscale turbulent dynamics of tokamak plasmas

等离子体湍流是一系列现象的基础，包括恒星和星系的形成;太阳风的特性，以及本方案的重点-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。完整的问题是非常具有挑战性的，因此为了在真实的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来实现，包括最新升级的MAST-U托卡马克。等离子体湍流由于至少有两种相互作用的“流体”--电子和离子--而这些流体都是带电的，因此变得复杂。因此，密度的波动驱动电荷分离，从而导致静电场的波动，而速度的波动驱动电流，从而导致磁场的波动。然后这些场耦合电子和离子“流体”的相对运动。磁化等离子体支持的波种类繁多，当波的相速度与粒子速度相匹配时会产生共振，这使得情况更加复杂。为了正确处理这些共振，需要粒子速度分布的知识，这反过来又需要一个动力学或先进的流体方法-一个艰巨的任务。湍流在托卡马克中通常是毫米-厘米级的，它以复杂的方式与米级的全球平衡分布（例如密度、温度和流量梯度）相互作用。为了量化托卡马克等离子体湍流和轮廓之间的复杂的多尺度反馈机制，从而提供准稳态终态的预测能力，我们将解决和整合一些主题。我们将首先学习如何平均流与静电湍流相互作用（即忽略磁场中的波动），需要波动与特征尺度之间的耦合，范围从电子拉莫尔半径（亚毫米）到离子拉莫尔半径（几毫米）和超过（厘米），到轮廓的系统长度尺度（米）。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）。这些仪器可以测量的波长范围可能会有差距，因此我们预计需要开发和安装新的微波成像系统。这将使用从该计划的早期阶段获得的知识进行设计，并部署到最后的进一步实验中。对电磁湍流的了解较少，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，这需要我们了解快粒子的影响。这些可以直接通过它们激发的不稳定性来驱动湍流，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粒子对MAST-U的影响，以及聚变反应产生的高能α粒子对ITER等未来设备的影响，以及计划在JET上进行的氘氚混合聚变燃料实验。

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.