Filamentation Instability of Counterstreaming Laser-Driven Plasmas

Abstract

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...

Electromagnetic Field Generation in the Downstream of Electrostatic Shocks Due to Electron Trapping

“…Recent laser-driven shock experiments showed the appearance of an electromagnetic field structure [22][23][24], which was attributed to the ion-filamentation instability [25] that evolves on time scales of ten thousands of the inverse electron plasma frequency, ω −1 pe . As a main outcome of this Letter, we show that these structures can already be seeded and produced on tens of ω −1 pe and remain in a quasisteady state over thousands of ω −1 pe .…”

Time Evolution of Kelvin–helmholtz Vortices Associated With Collisionless Shocks in Laser-Produced Plasmas

“…Recent rapid growth of laser technologies allows us to model space and astrophysical phenomena in laboratories (Drake 1999;Remington et al 1999Remington et al , 2006Takabe et al 1999). For instance, collisionless shocks have been experimentally investigated in laser-produced counterstreaming plasmas (Morita et al 2010Kuramitsu et al 2011Kuramitsu et al , 2012Kugland et al 2012;Ross et al 2012;Fox et al 2013;Yuan et al 2013;Huntington et al 2015;Park et al 2015).W efirst reported the relatively laminar density jump with optical interferometry in collisionless counterstreaming plasmas in the absence of an external magnetic field using the Shenguang II laser facility (Morita et al 2010). In order to distinguish shock from contact surface we measured the emission jump and its time evolutions with self-emission optical pyrometry (SOP) with the Gekko XII (GXII) laser facility (Kuramitsu et al 2011).…”

“…With the OMEGA and OMEGA EP laser facilities, symmetric counterstreaming plasmas can be produced (Kugland et al 2012;Ross et al 2012). In the symmetric conditions, KHI is less effective, however, filamentation instability or Weibel instability can take place where the magnetic filaments grow in the shock transition region (Fox et al 2013;Yuan et al 2013;Huntington et al 2015;Park et al 2015). Currently verifications of collisionless shock formation due to Weibel-type instability are ongoing with the world largest laser facility, the National Ignition Facility.…”

Radiation from particles moving in small-scale magnetic fields created in solid-density laser-plasma laboratory experiments

“…[24][25][26] In contrast to the aforementioned solid-density plasmas, these plasmas flow freely in-between laser ablated metal plates. 23,[27][28][29][30] This is achieved via weaker laser intensities and longer pulse durations ($10 14 W/cm 2 and $1 ns, for a recent Omega laser experiment)-although higher intensities are believed to be required for the creation of a shock. 27,28 Recently, the formation of filamentary structures indicative of ion-driven Weibel-like magnetic fields has been observed in a scaled laboratory experiment at the Omega Laser Facility.…”

“…27,28 Recently, the formation of filamentary structures indicative of ion-driven Weibel-like magnetic fields has been observed in a scaled laboratory experiment at the Omega Laser Facility. [28][29][30] Electrons moving in small-scale magnetic turbulence emit radiation that is distinct from both synchrotron and cyclotron radiation. In the context of plasma astrophysics, this radiation is known as "jitter" radiation.…”