Formation of a Density Bump in a Collisionless Electrostatic Shock Wave During Expansion of a Hot Dense Plasma into a Co
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Formation of a Density Bump in a Collisionless Electrostatic Shock Wave During Expansion of a Hot Dense Plasma into a Cold Rarefied One A. A. Nechaeva, *, M. A. Garaseva, A. N. Stepanova, and V. V. Kocharovskya a Institute
of Applied Physics, Russian Academy of Sciences, ul. Ul’yanova 46, Nizhny Novgorod, 603950 Russia *e-mail: [email protected] Received March 25, 2019; revised February 10, 2020; accepted March 16, 2020
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. Keywords: collisionless plasma, decay of strong disruption, non-maxwellian particle distribution, electrostatic shock wave, magnetic field generation DOI: 10.1134/S1063780X2008005X
INTRODUCTION It is well known [1–7] that electrostatic shock waves sustained by a self-consistent separation of charges of nonequilibrium fractions can exist in both laboratory and space inhomogeneous collisionless plasmas containing hot electrons and cold ions. Since ion acoustic oscillations (solitons, in particular) [7– 10] can propagate in such plasma and are easily excited by, e.g., explosions, they are usually assumed to be responsible for the nonlinear dynamic processes of changes in concentration and fine structure of the shock front. For example, it was suggested [1, 11–13] using stationary structures discovered in the approximation of the laminar ion motion for description of shock waves at low ion temperature, including taking into account partial reflection of ions from the shock front [13]. At the same time, double- and multiflow ion motion leading to various instabilities, those of the beam type in the first place, along with additional generation of various waves, ion-acoustic and Langmuir in particular, can appear in the shock waves when particle collisions play a minor role, and ion sound velocities substantially exceed thermal velocities of ions [8, 14–18]. As a result, wave turbulization of the shock front and instability
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