Collisionless shocks, a consequence of supersonic flows sweeping across an interstellar or intergalactic medium, are ubiquitous in the universe. They have been speculated to cause many fundamental astrophysical phenomena by heating and compressing background plasmas; seeding and amplifying magnetic fields; and accelerating cosmic rays. Despite the importance of shocks and their underlying physics, the shock structure and fundamental behaviors remain controversial, as there have been few ways to study them experimentally.
We report on the laboratory experiments on the OMEGA laser at the Laboratory for Laser Energetics at the University of Rochester and their theoretical interpretation involving the generation and observation of Weibel-mediated, electromagnetic collisionless shocks in astrophysical regimes, manifested by critical physical parameters. The spatially resolved filaments at the front of the magnetosonic shocks indicate the compelling evidence of Weibel currents driven by two interpenetrating ion streams in front of a magnetic piston. We reveal a novel physical mechanism explaining how the Weibel-driven filaments penetrate into the magnetized region of interaction and amplified downstream in the pre-compressed magnetic turbulence, resulting in entropy production and shock mediation. We also demonstrate particle acceleration via the first-order Fermi mechanism, a unique and critical feature of astrophysically relevant, electromagnetic collisionless shocks that previously has not been achieved in laboratory experiments.
The experiments directly mimic the scenario of collisionless shocks in nonrelativistic astrophysical contexts such as supernova remnants, and provide a roadmap into the shock physics in relativistic regimes such as afterglow of cosmological γ-ray bursts. This experimental scheme can be reproduced on a larger scale on the ELI high power laser facilities.