Observation of magnetic field generation via the Weibel instability in interpenetrating plasma flows

Abstract

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).

Collisionless electrostatic shock generation using high-energy laser systems

“…Successful generation of collisionless MHD [45,46] and ES shocks, and experimental approach to study collisionless EM shock [60][61][62]64,65] in laboratory indicate that laboratory experiment can be an alternative approach to study space and astrophysical plasma physics. Furthermore, recent achievement of mono-energetic proton acceleration by collisionless ES shock produced by ultrahigh-intensity laser system [84][85][86] shows a possibility to apply the collisionless shock to several applications including medical treatment.…”

“…We have conducted the Weibel-instability mediated collisionless EM shock experiments with Omega and Omega EP laser systems (Rochester U., U.S.A), and measured plasma parameters such as electron and ion temperatures, electron density, and flow velocity of counter-streaming plasmas [60][61][62] with collective Thomson scattering (CTS) [63], and filamentary structure produced by the Weibel instability [62,64,65] with proton radiography [66,67]. Now the NIF experiment is going on.…”

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

“…Rather, proton-radiography or Thomson scattering, via the injection of external particles, is the prescribed diagnostic tool. 27,28 In principle, the sub-Larmor-scale ions should emit Weibler radiation, but this will be orders of magnitude less intense (because of their higher mass) than the radiation produced by electrons via alternative radiation mechanisms. In addition, plasma dispersion would certainly screen out any ion Weibler radiation, since the characteristic emission frequencies will be well below the electron plasma cutoff frequency.…”

“…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. [28][29][30] Electrons moving in small-scale magnetic turbulence emit radiation that is distinct from both synchrotron and cyclotron radiation.…”

“…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.…”