Optimizing laser focal spot size using self-focusing in a cone-guided fast-ignition ICF target

Abstract

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.