This paper presents a scheme for strong self-focusing of a laser beam interacting with a cone-guided fast-ignition inertial confinement fusion target using cone pre-plasma filling as an optical medium for reducing the laser beam waist. The objective is to reduce the focal spot size at the interior of the tip of the re-entrant cone to that required for efficient coupling to the dense imploded fuel core. This is challenging to achieve in a large laser system using the standard optical components of a chirped-pulse-amplified (CPA) laser-beam chain where the spot sizes produced are often significantly larger than would be desirable for fast ignition. The approach described also makes use of the presence of pre-plasma in the cone. Such pre-plasma filling is difficult to avoid entirely when illuminating a cone with a high-energy CPA laser system due to the challenges of reducing laser pre-pulse to below the threshold for plasma production. For deriving the differential equation which governs the progress of the laser beam-width with propagation distance, paraxial theory in a WKB approximation has been used. A simulation is performed assuming strong self-focusing in accordance with the laser parameters and plasma density profile chosen.

本文提出了一种方案，利用锥形预等离子体填充物作为减少激光束腰的光学介质，使与锥形制导的快速点火惯性约束聚变靶相互作用的激光束具有较强的自聚焦能力。目的是将凹角锥尖端内部的焦斑尺寸减小到有效耦合到密集的内燃燃料芯所需的焦斑尺寸。在使用chi脉冲放大（CPA）激光束链的标准光学组件的大型激光系统中，要实现这一点具有挑战性，在这种系统中，产生的光斑大小通常明显大于快速点火所需的光斑大小。所描述的方法还利用了锥体中等离子体的存在。由于将激光预脉冲减小到等离子体产生阈值以下的挑战，在用高能CPA激光系统照射圆锥体时，很难完全避免这种等离子体预填充。为了导出控制激光束宽度随传播距离变化的微分方程，已使用WKB近似中的近轴理论。根据所选择的激光参数和等离子体密度曲线，假设进行了强烈的自聚焦，就可以进行仿真。