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 sufficiently 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 Weibel-generated 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 efficiently 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.
Self-organization occurs in plasmas when energy progressively transfers from smaller to larger scales in an inverse cascade. Global structures that emerge from turbulent plasmas can be found in the ...laboratory and in astrophysical settings; for example, the cosmic magnetic field, collisionless shocks in supernova remnants and the internal structures of newly formed stars known as Herbig-Haro objects. 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. PUBLICATION ABSTRACT
Rapid electron and ion heating is observed in collisionless counterstreaming plasma flows and explained via a novel heating mechanism that couples the electron and ion temperatures. Recent ...experiments measure plasma conditions 4 mm from the surface of single foil (single plasma stream) and double foils (two counterstreaming plasmas) targets using Thomson scattering. Significant increases in electron and ion temperatures (from <100 eV to >1 keV) compared to the single foil geometry are observed. While electrons are heated by friction on opposite going ions, ion-ion collisions cannot explain the observed ion heating. Also, density and flow velocity measurements show negligible slow down and rule out stagnation. The nonlinear saturation of an acoustic two-stream electrostatic instability is predicted to couple the ion temperature to the electron temperature through the dynamic evolution of the instability threshold. Particle-in-cell simulations including both collisional and collisionless effects are compared to the experimental measurements and show rapid electron and ion heating consistent with the experimental measurements.
Laser-produced proton beams have been used to achieve ultrafast volumetric heating of carbon samples at solid density. The isochoric melting of carbon was probed by a scattering of x rays from a ...secondary laser-produced plasma. From the scattering signal, we have deduced the fraction of the material that was melted by the inhomogeneous heating. The results are compared to different theoretical approaches for the equation of state which suggests modifications from standard models.
In this paper, scaling relations pertinent to collisionless-shock experiments are derived. Two oppositely directed, non-relativistic interpenetrating plasma streams are considered. A full set of ...collisionless Vlasov-Maxwell equations is used to describe the plasma streams. Two scaling parameters are identified for those streams in which the initial temperature is small compared with the final temperature. If the scaling parameters are held constant between the two systems, the knowledge of the behavior of one of the systems allows for a direct prediction of behavior of the other, irrespective of differences in, for example, the plasma density. This similarity covers both hydrogen and non-hydrogen ion species. Reduced versions suitable for assessing electrostatically mediated and magnetically mediated (Weibel-like) models of the shocks are described. In these limiting cases, the number of scaling parameters reduces to one, making the similarity quite broad. The presence of these scaling relations can serve as a basis for making comparisons between models and selecting the most plausible models. A brief discussion is presented of the constraints associated with the role of intra-jet collisions, whose frequency may be non-negligible even if the collisions between the particles of two jets are very rare.