Self-organized electromagnetic field structures in laser-produced counter-streaming plasmas

Abstract

Self-organization 1,2 occurs in plasmas when energy progressively transfers from smaller to larger scales in an inverse cascade 3 . Global structures that emerge from turbulent plasmas can be found in the laboratory 4 and in astrophysical settings; for example, the cosmic magnetic field 5,6 , collisionless shocks in supernova remnants 7 and the internal structures of newly formed stars known as Herbig-Haro objects 8 . Here we show that large, stable electromagnetic field structures can also arise within counter-streaming supersonic plasmas in the laboratory. These surprising structures, formed by a yet unexplained mechanism, are predominantly oriented transverse to the primary flow direction, extend for much larger distances than the intrinsic plasma spatial scales and persist for much longer than the plasma kinetic timescales. Our results challenge existing models of counter-streaming plasmas and can be used to better understand large-scale and long-time plasma self-organization.Our experiments were performed at the OMEGA EP laser facility, where two kilojoule-class lasers irradiated two polyethylene (CH 2 ) plastic discs that faced each other at a distance of 8 mm, creating a system of high-velocity laser-ablated counter-streaming plasma flows. The experimental details are described in Fig. 1 and in the Methods. At early times, up to at least 8 ns, intra-jet ion collisions are known to be strong (owing to relatively low-particle thermal velocities) but inter-jet ion collisions are rare (owing to relatively high flow velocities), permitting the evolution of both hydrodynamic and collisionless plasma instabilities 9,10 (Table 1). We visualized the electric and magnetic field structures in the counter-streaming plasmas with short-pulse laser-generated proton beam imaging 11,12 , taken from two orthogonal views to evaluate the possible azimuthal symmetry of the field structures. After roughly 3 ns, caustics (large-intensity variations 13 ) in the proton images indicate the formation of strong field zones within the plasma, probably due to sharp structures with strong gradients, as reported elsewhere 14 . By 4 ns, the features have changed markedly into two large swaths of straight transverse caustics that extend for up to 5 mm. This extent is large compared with the fundamental scale lengths of the plasma (Table 1) such as the Debye length (50,000 times larger) and the ion inertial length (nearly 100 times larger), indicating a high degree of self-organization. This organization

Filamentation Instability of Counterstreaming Laser-Driven Plasmas

“…The beam foci had a minor diameter of 900 µm, and used distributed phase-plate (DPP) beam smoothing, for ontarget laser intensities near 5 × 10 13 W/cm 2 . The laser setup was similar to the recent interacting plume experiments of Kugland et al [19], which observed large-scale "self-organized" plasma structures, except for smaller separation of the targets and the use of broader laser foci and DPP phase plate beam smoothing, which may limit the density and decrease the magnitude of self-generated magnetic fields.The electromagnetic fields formed in the interaction region were probed using an ultrafast diagnostic proton beam [17], generated with a third, high-intensity laser …”

“…The first image, at t = 3.8 ns relative to the start of the driver pulse, shows a prominent and sharp"X"-like structure at the midplane, with the protons deflected into pairs of thin lines, reminiscent of the caustic proton structures observed in experiments in a similar laser-energy regime with larger initial target separation [19,20]. However, for the present discussion, we focus on the filamentary instability visible above the "X" structure.…”

Experimental evidence for collisional shock formation via two obliquely merging supersonic plasma jets

“…Related studies include counter-streaming laser-produced plasmas supporting hohlraum design for indirect-drive inertial confinement fusion [17][18][19] and for studying astrophysically relevant shocks, [20][21][22][23][24] colliding plasmas using wire-array Z pinches, 25,26 and applications such as pulsed laser deposition 27 and laser-induced breakdown spectroscopy. 28 Primary issues of interest in these studies include the identification of shock formation, the formation of a stagnation layer [29][30][31] between colliding plasmas, and the possible role of two-fluid and kinetic effects on plasma interpenetration.…”

Structure of a Magnetic Flux Annihilation Layer Formed by the Collision of Supersonic, Magnetized Plasma Flows

“…The structure of the reconnection layer in these conditions is unknown, but is expected to adjust to accommodate the rate of magnetic flux delivered into the layer, where, for example, a pileup of the magnetic flux could contribute to controlling the reconnection rate [5,6]. A number of recent laser-driven, high energy density physics (HEDP) experiments [7][8][9][10] have investigated magnetic reconnection in the strongly driven regime, as well as the formation of astrophysically relevant collisionless shocks [11] and self-organized field structures [12]. Large-scale field structures produced by collisions between laser-driven plasma flows have, for example, been interpreted [11] as being due to the accumulation of advected toroidal magnetic fields generated via the Biermann battery mechanism at the laser spots [13].…”