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 ...

We report on the experimental observation of the instability of a plasma shell, which formed during the expansion of a laser-ablated plasma into a rarefied ambient medium. By means of a proton radiography technique, the evolution of the instability is temporally and spatially resolved on a timescale much shorter than the hydrodynamic one. The density of the thin shell exceeds that of the surrounding plasma, which lets electrons diffuse outward. An ambipolar electric field grows on both sides of the thin shell that is antiparallel to the density gradient. Ripples in the thin shell result in a spatially varying balance between the thermal pressure force mediated by this field and the ram pressure force that is exerted on it by the inflowing plasma. This mismatch amplifies the ripples by the same mechanism that drives the hydrodynamic nonlinear thin-shell instability (NTSI). Our results thus constitute the first experimental verification that the NTSI can develop in colliding flows.

The ultrafast charge dynamics following the interaction of an ultra-intense laser pulse with a foil target leads to the launch of an ultra-short, intense electromagnetic (EM) pulse along a wire connected to the target. Due to the strong electric field (of the order of GV m −1 ) associated to such laser-driven EM pulses, these can be exploited in a travelling-wave helical geometry for controlling and optimizing the parameters of laser accelerated proton beams. The propagation of the EM pulse along a helical path was studied by employing a proton probing technique. The pulse-carrying coil was probed along two orthogonal directions, transverse and parallel to the coil axis. The temporal profile of the pulse obtained from the transverse probing of the coil is in agreement with the previous measurements obtained in a planar geometry. The data obtained from the longitudinal probing of the coil shows a clear evidence of an energy dependent reduction of the proton beam divergence, which underpins the mechanism behind selective guiding of laser-driven ions by the helical coil targets.