  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.
