SUNION—an algorithm for one-dimensional laser-plasma interaction

Abstract

SUNION solves the combined problem of ion expansion and of resonance absorption of p-polarized electromagnetic radiation. Well-posed initial and boundary conditions are derived within the ponderomotive approximation. Efficiency and accuracy of the code are checked by reproducing known results found in literature for various simplifications. A new type of numerical instability is exposed within the Lagrangian description of the expanding ions, pointing out a fundamental difficulty in treating plasma expansion into a vacuum. It originates from charge separation and is correlated with a singular behavior of the plasma flow. The solution method is considerably simplified by taking into account the first time-derivative in the complex second-order Schrödinger-type wave equation, and, by solving the latter by a Crank-Nicholson scheme, without reference to shooting methods. The absorption coefficient reaching values up to 60 per cent is found to be rather insensitive to the global density structure, and reflects more or less the local scale length at the critical density. Profile steepening caused by radiation pressure is accomplished within the first twenty ion plasma periods.