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

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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.