Collisionless shocks can be produced as a result of strong magnetic fields in a plasma flow, and therefore are common in many astrophysical systems. The Weibel instability is one candidate mechanism for the generation of su ciently strong fields to create a collisionless shock. Despite their crucial role in astrophysical systems, observation of the magnetic fields produced by Weibel instabilities in experiments has been challenging. Using a proton probe to directly image electromagnetic fields, we present evidence of Weibelgenerated magnetic fields that grow in opposing, initially unmagnetized plasma flows from laser-driven laboratory experiments. Three-dimensional particle-in-cell simulations reveal that the instability e ciently extracts energy from the plasma flows, and that the self-generated magnetic energy reaches a few percent of the total energy in the system. This result demonstrates an experimental platform suitable for the investigation of a wide range of astrophysical phenomena, including collisionless shock formation in supernova remnants, large-scale magnetic field amplification, and the radiation signature from gamma-ray bursts.The magnetic fields required for collisionless shock formation in astrophysical systems may either be initially present, for example in supernova remnants or young galaxies 1 , or they may be selfgenerated in systems such as gamma-ray bursts (GRBs; ref. 2). In the case of GRB outflows, the intense magnetic fields are greater than those which can be seeded by the GRB progenitor or produced by misaligned density and temperature gradients (the Biermannbattery effect) 3,4 . It has long been known that instabilities can generate strong magnetic fields, even in the absence of seed fields. Weibel considered the development of an electromagnetic instability driven by the electron velocity anisotropy in a background of resting ions 5 . The signature of the instability is a pattern of current filaments stretched along the axis of symmetry of the electron motion. The process is quite general, and subsequent work has shown that such instabilities can be excited in both non-relativistic and relativistic shocks. This general nature makes the Weibel instability common in astrophysical systems [6][7][8] . The instability provides a mechanism by which the electromagnetic turbulence associated with the formation of collisionless shocks is fed by the flow anisotropy of the protons (and ions) stochastically reflecting off of the shock 9-11 , and leading ultimately to strong particle acceleration in GRB's (ref. 12).

Laser wakefield acceleration of electrons holds great promise for producing ultracompact stages of GeV scale, high-quality electron beams for applications such as x-ray free electron lasers and high-energy colliders. Ultrahigh intensity laser pulses can be self-guided by relativistic plasma waves (the wake) over tens of vacuum diffraction lengths, to give >1  GeV energy in centimeter-scale low density plasmas using ionization-induced injection to inject charge into the wake even at low densities. By restricting electron injection to a distinct short region, the injector stage, energetic electron beams (of the order of 100 MeV) with a relatively large energy spread are generated. Some of these electrons are then further accelerated by a second, longer accelerator stage, which increases their energy to ∼0.5  GeV while reducing the relative energy spread to <5% FWHM.

Magnetic fields are ubiquitous in the Universe. The energy density of these fields is typically comparable to the energy density of the fluid motions of the plasma in which they are embedded, making magnetic fields essential players in the dynamics of the luminous matter. The standard theoretical model for the origin of these strong magnetic fields is through the amplification of tiny seed fields via turbulent dynamo to the level consistent with current observations. However, experimental demonstration of the turbulent dynamo mechanism has remained elusive, since it requires plasma conditions that are extremely hard to re-create in terrestrial laboratories. Here we demonstrate, using laser-produced colliding plasma flows, that turbulence is indeed capable of rapidly amplifying seed fields to near equipartition with the turbulent fluid motions. These results support the notion that turbulent dynamo is a viable mechanism responsible for the observed present-day magnetization.

A series of Omega experiments have produced and characterized high velocity counter-streaming plasma flows relevant for the creation of collisionless shocks. Single and double CH2 foils have been irradiated with a laser intensity of ∼10 16 W/cm 2 . The laser ablated plasma was characterized 4 mm from the foil surface using Thomson scattering. A peak plasma flow velocity of 2,000 km/s, an electron temperature of ∼110 eV, an ion temperature of ∼30 eV, and a density of ∼10 18 cm −3 were measured in the single foil configuration. Significant increases in electron and ion temperatures were seen in the double foil geometry. The measured single foil plasma conditions were used to calculate the ion skin depth, c/ωpi ∼0.16 mm, the interaction length, int, of ∼8 mm, and the Coulomb mean free path, λ mf p ∼27 mm. With c/ωpi int < λ mf p we are in a regime where collisionless shock formation is possible.

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