Time-Resolved Characterization of the Formation of a Collisionless Shock

Abstract

We report on the temporally and spatially resolved detection of the precursory stages that lead to the formation of an unmagnetized, supercritical collision-less shock in a laser-driven laboratory experiment. The measured evolution of the electrostatic potential associated with the shock unveils the transition from a current free double layer into a symmetric shock structure, stabilized by ion reflection at the shock front. Supported by a matching Particle-In-Cell simulation and theoretical considerations, we suggest that this process is analogeous to ion reflection at supercritical collisionless shocks in supernova remnants.Collision-less shocks represent particularly intriguing phenomena in plasma physics, due to their implications in a broad range of physical scenarios, extending from laboratory-based laser-plasma experiments to astrophysics. In the latter case, particular attention has been devoted to shock waves generated during the propagation of a supernova remnant (SNR) blast shell into the interstellar medium (ISM), since they are thought to be the dominant source of galactic high energy cosmic rays [1][2][3][4][5]. In this case, non-collisionality is guaranteed by the low particle collision frequency in the ISM [6]; the dynamics of SNR shocks is expected to be dominated by electromagnetic fields, thus setting stringent limits on their speed and stability. However, despite the considerable number of recent observations of such structures, the intrinsic difficulty in directly probing the plasma conditions around the SNRs has left the debate upon their generation mechanism and dynamics still open in the scientific community. A possible solution to this impasse might be provided by studying small-scale reproductions of these phenomena in laser-based laboratory experiments. Within this framework, promising results have indeed already been obtained [7][8][9], especially concerning the stationary stage of these structures. Nonetheless, a full understanding of the dynamics of a collisionless shock would also require a detailed characterization of the transient phase in which it is formed, a regime that has hitherto eluded experimental detection.Employing a time-resolved proton imaging technique [10], we present here the first experimental observation of the precursory stages that lead to the generation of a supercritical electrostatic collision-less shock at the boundary of a blast shell of laser-ablated plasma expanding into a dilute ambient medium. By following the temporal evolution of the propagation speed and of the profile of the associated electrostatic potential, it has been possible to distinguish the intermediate steps that let an initially freely expanding structure, similar to a current free double layer (CFDL) [11,12], evolve into a forward shock propagating at a supercritical speed (observed Mach number of the order of 4). A matching Particle-In-Cell (PIC) simulation allowed us to reproduce the transformation of the plasma contact boundary by collision-less electrostatic processes into a forward ...

Experimental Observation of Thin-shell Instability in a Collisionless Plasma

“…A series of zoomed RCF images of this structure (obtained within the same laser shot) are shown in panels (c)-(e), which correspond, respectively, to t 0 , + t 0 2 ps, and + t 0 6 ps. Such a modulation can be interpreted as a thin plasma shell, bounded by double layers or electrostatic shocks (Hershkowitz 1981;Ahmed et al 2013). Furthermore, a pronounced periodic spatial oscillation along the thin shell can also be seen in Figure 1(c), similar to the perturbations observed on the surface of a blast wave (Edens et al 2005) in hydrodynamically expanding plasma.…”

Particle-in-cell simulation study of a lower-hybrid shock

“…This overlap layer is the first stage of the shock formation. 17 Time t 2 ¼ 7.4: The ion density hump has spread out in space, forming a plateau between 4.1 mm and 4.25 mm in Fig. 3.…”

“…Self-consistent and steady-state solutions of an ion acoustic wave that steepened into an electrostatic shock exist, provided that the speed of the upstream plasma measured in the shock frame does not exceed a few times the ion acoustic speed. 8 Electrostatic shocks are now routinely produced in laser-plasma experiments [11][12][13][14][15][16][17][18] and they attract considerable interest because they allow us to study in-situ some of the plasma processes that develop in remote astrophysical environments. For example, the shocks that ensheath the blast shells of supernova remnants.…”

Collisionless electrostatic shock generation using high-energy laser systems

“…Basic physics of collisionless ES shock generation has been studied numerically [89][90][91][92][93] and experimentally using high-energy laser systems [94][95][96][97][98][99]. In the experiments using high-energy laser systems, there are two schemes for the generation of collisionless ES shock.…”