Optimizing laser focal spot size using self-focusing in a cone-guided fast-ignition ICF target
Department of Physics, Lovely Professional University, 144411, Phagwara, Punjab, India
2 Extreme Light Infrastructure-Nuclear Physics (ELI-NP), “Horia Hulubei” National Institute of Physics and Nuclear Engineering, 30 Reactorului Street, 0077125, Magurele, Jud. Iifov, Romania
3 York Plasma Institute, Department of Physics, University of York, YO10 5DD, Heslington, York, UK
Accepted: 23 April 2021
Published online: 4 May 2021
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.
© The Author(s), under exclusive licence to Società Italiana di Fisica and Springer-Verlag GmbH Germany, part of Springer Nature 2021