Plasma kinetics, pre-heat, and the emergence of strong shocks in laser fusion: the hydro-kinetic regime

激光惯性约束聚变（ICF）的目标是创造并点燃一颗微小的恒星。热核聚变释放的能量可以被利用，为人类提供安全、可持续、可靠、无碳的无限量电力。如果实现，激光核聚变不仅将为全球变暖提供解决方案，还将使英国成为净能源出口国，并在英国目前处于世界领先地位的领域（如激光和靶材制造）创造一个超高科技技术出口的新市场。数十亿美元的国家点火设施（NIF）是目前唯一的激光，原则上，有足够的能量来实现点火（“恒星”燃烧），尽管迄今为止NIF尚未实现点火。基线“间接驱动”NIF设计使用一系列激光束在金属圆柱体（hohlraum）中产生x射线，这些x射线依次烧蚀球形ICF目标，驱动会聚内爆。这导致目标被压缩，产生类似于太阳中心的密度和温度条件，从而点燃“恒星”。虽然间接驱动方法对ICF有一些优势，但它效率极低，而且目前尚不清楚是否有可能利用NIF上可用的激光能量实现间接驱动点火。备选ICF方案包括“直接驱动”和“冲击点火”。在这里，激光直接照亮目标，提高效率的一个因素~5，这意味着它应该有可能实现点火与NIF的能量。冲击点火是最近发明的直接驱动的一种变体。在这里，内爆速度可以低于点火所需的最小速度，相反，点火是由内爆结束时发射的强激波发起的。与其他ICF方案相比，激波点火具有许多潜在的优点；点火所需的激光能量完全在NIF上的可能范围内，因为内爆速度可以降低，对有害流体不稳定性（瑞利-泰勒）的敏感性也降低了。重要的是，能量增益（聚变能输出/电能输入）应该足以用于发电。激光-等离子体相互作用的不稳定性（LPI），如受激拉曼散射、双等离子体衰变和受激布里渊散射，在所有的ICF方案中都存在。这些lpi改变了激光吸收的时空特征，并能产生大量的高能（或热）电子。确定lpi和相关热电子的特性对ICF至关重要，因为它们决定了聚变燃料是否会在燃料被压缩之前被加热（预热）——可能会阻止点火——或者热电子的能量是否可以被利用，增强激波点火方案中的激波产生，潜在地导致聚变能量的获得，足以用于当今激光器的能量应用。ICF物理学的这个关键领域是本提案的重点。在Omega激光设备上的新实验将测量点火尺度直接驱动和激波点火参数空间中的LPI和热电子特性。一个关键的成果将是将实验数据封装在创新的新型激光等离子体相互作用和热电子模拟模型中，这将与英国的辐射流体动力学代码框架：Odin保持一致。这些将显著提高我们的预测模拟能力，提供基准的、高保真的模拟工具，这些工具将公开提供给英国学术激光等离子体物理社区。这项工作直接参与并领导了大型设施上的ICF实验，为英国社区提供了一条明确的途径，通过该途径，英国社区可以获得技能、专业知识和工具，为本世纪20年代世界上最大的激光器开发下一代ICF设计并进行实验。

The goal of Laser Inertial Confinement Fusion (ICF) is to create and ignite a minute star. The energy liberated through thermonuclear fusion can be harnessed, providing mankind with an essentially limitless source of safe, sustainable, secure, carbon-free, electricity. If realised, laser-fusion would not only provide a solution to global warming, but enable the UK to become a net energy exporter, and also create a new market in ultra-high-tech technology exports in areas where the UK is currently world-leading, such as laser and targetry manufacture.The multi-billion dollar National Ignition Facility (NIF) is currently the only laser which, in principal, has sufficient energy to achieve ignition (where the 'star' burns), although to-date NIF has not achieved ignition. The base-line 'indirect-drive' NIF design uses an array of laser beams to create x-rays in a metallic cylinder (hohlraum), these x-rays in turn ablate the spherical ICF target, driving a convergent implosion. This causes the target to be compressed, creating density and temperature conditions similar to those within the centre of the Sun, thereby igniting the 'star'. While there are some advantages to the indirect-drive approach to ICF, it is extremely inefficient, and it is currently unclear whether it will be possible to achieve indirect drive ignition with the laser energy available on NIF. Alternative ICF schemes exist including 'direct drive' and 'shock ignition'. Here, the lasers directly illuminate the target improving efficiency by a factor of ~5, meaning it should be possible to achieve ignition with NIF's energy. Shock ignition is a recently invented variant of direct drive. Here the implosion velocity can be lower than the minimum required for ignition, instead ignition is initiated by a strong shock launched towards the end of the implosion. Shock ignition has many potential advantages over other ICF schemes; the laser energy requirements for ignition are well within those possible on NIF, as the implosion velocity can be lower, the susceptibility to deleterious fluid instabilities (Rayleigh-Taylor) is also reduced. Importantly, the energy gain (fusion energy out/electrical energy in) should be sufficient for power generation.Laser-plasma interaction instabilities (LPI) such as Stimulated Raman Scatter, Two Plasmon Decay and Stimulated Brillouin Scatter occur in all ICF schemes. These LPIs alter the temporospatial characteristics of laser absorption and can create significant populations of energetic (or hot) electrons. Determining the characteristics of the LPIs and the associated hot electrons is of critical importance for ICF as they dictate whether the fusion fuel will be heated prior to the fuel being compressed (pre-heat) - potentially precluding ignition - or whether the hot electrons' energy can be harnessed, enhancing shock generation in the shock ignition scheme, potentially leading to fusion energy gains sufficient for energy applications on today's lasers. This crucial area of ICF physics is the focus of this proposal. New experiments on the Omega laser facility will measure the LPI and hot electron characteristics in the parameter spaces of ignition-scale direct drive and shock ignition. A key outcome will be the encapsulation of the experimental data in innovative new laser-plasma interaction and hot electron simulation models, which will run in-line with the UK's radiation-hydrodynamics code framework: Odin. These will significantly improve our predictive simulation capabilities, providing benchmarked, high-fidelity simulation tools which will be made openly available to the UK academic laser-plasma physics community. This work, with direct involvement and leadership of ICF experiments on large scale facilities, provides a clear route by which the UK community can attain the skills, expertise, and tools to develop next-generation ICF designs for, and execute experiments on, the world's largest largest lasers into the 2020s.