Formation of a Density Bump in a Collisionless Electrostatic Shock Wave During Expansion of a Hot Dense Plasma into a Cold Rarefied One

Abstract

Formation and evolution of a density bump in an electrostatic shock wave during decay of a discontinuity in a plasma characterized by the presence of hot electrons and a large drop in plasma density across the discontinuity are investigated. Numerical particle-in-cell simulation in a wide range of plasma parameters revealed that the appearance of the density bump as a result of the action of the electric field of high-energy electrons in the region of the travelling shock front changes the character of generated ion–acoustic waves and is accompanied by complex nonlaminar kinetics of different fractions of accelerated and thermal ions, including those reflected from the front. Investigation of particle trajectories in real and phase spaces unveiled that ions on both sides of the discontinuity, namely, ions of the rarefied plasma captured by the wave and accelerated ions of the dense plasma catching it up, participate in formation and sustaining of the density bump in the shock wave. A qualitative analysis of contributions of both ion components to the density bump is carried out, and specific features of the latter for typical parameters of laser plasma are found.