Filamentation due to the growth of a Weibel-type instability was observed in the interaction of a pair of counter-streaming, ablatively-driven plasma flows, in a supersonic, collisionless regime relevant to astrophysical collisionless shocks. The flows were created by irradiating a pair of opposing plastic (CH) foils with 1.8 kJ, 2-ns laser pulses on the omega ep laser system. Ultrafast laserdriven proton radiography was used to image the Weibel-generated electromagnetic fields. The experimental observations are in good agreement with the analytical theory of the Weibel instability and with particle-in-cell simulations.Astrophysical shock waves play diverse roles, including energizing cosmic rays in the blast waves of astrophysical explosions [1], and generating primordial magnetic fields during the formation of galaxies and clusters [2]. These shocks are typically collisionless, and require collective electromagnetic fields [3], as Coulomb collisions alone are too weak to sustain shocks in high-temperature astrophysical plasmas. The class of Weibel-type instabilities [4][5][6] (including the classical Weibel and currentfilamentation instabilities) is one such collective mechanism that has been proposed to generate a turbulent magnetic field in the shock front and thereby mediate shock formation in cosmological shocks [7] and blast wave shocks in gamma ray bursts [8][9][10] and supernova remnants [11]. These instabilities generate magnetic field de novo by tapping into non-equilibrium features in the electron and ion distributions functions. The classical form of the Weibel instability is driven by temperature anisotropy [4], but counterstreaming ion beams, as occurs in the present context, provides an equivalent drive mechanism [6]. A related current filamentation instability of relativistic electron beams [12] has also previously been observed in experiments driven by ultraintense lasers [13].We report experimental identification an ion-driven Weibel-type instability generated in the interaction of two counterstreaming laser-produced plasma plumes. A pair of opposing CH targets was irradiated by kJ-class laser pulses on the OMEGA EP laser laser system, driving a pair of ablative flows toward the collision region at the midplane between the two foils. Due to the long mean-free-path between ions in opposing streams, the streams interpenetrate, establishing supersonic counterstreaming conditions in the ion populations, while the electrons form a single thermalized cloud. Meanwhile, the plasma density is also sufficient so that the the ion skin depth d i = (m i /µ 0 ne 2 ) 1/2 , is much smaller than the system size L. These conditions allow the growth of an ion-driven Weibel instability, for which d i is the characteristic wavelength [14][15][16]. The Weibel-generated electromagnetic fields were observed with an ultrafast pro- ton radiography technique [17], and identified through good agreement with analytic theory [6] and particle-incell simulations, discussed below. Figure 1 shows a schematic of the experiments...

[1] A previous analysis of electron distributions measured in situ by the Wind spacecraft has revealed that electrons were trapped in the electromagnetic geometry of the reconnection event encountered in the deep magnetotail. In this paper we develop a detailed theory that can account for the main anisotropic features of the electron distributions associated with trapping in reconnection. The analysis shows that electron trapping in electric fields is generic in reconnection, as it is required in order to maintain the condition of quasineutrality. In addition to the spacecraft data, evidence of trapping in numerical simulations is also presented. Trapping is effective in eliminating free-streaming electrons along magnetic fields and thereby reduces parallel electron currents. Its importance for fast reconnection is discussed and emphasized by observations in a laboratory plasma.

Direct in situ observation of magnetic reconnection in the Earth's magnetotail as well as kinetic numerical studies have recently shown that the electron pressure in a collisionless reconnection region is strongly anisotropic. This anisotropy is mainly caused by the trapping of electrons in parallel electric fields. We present new equations of state for the parallel and perpendicular pressures for magnetized electrons. This model-derived here and tested against a kinetic simulation-allows a fluid description in a collisionless regime where parallel electric fields and the dynamics of both passing and trapped electrons are essential.