  Magnetic reconnection converts magnetic energy into thermal and kinetic
energy in plasma. Among numerous candidate mechanisms, ion acoustic
instabilities driven by the relative drift between ions and electrons, or
equivalently electric current, have been suggested to play a critical role in
dissipating magnetic energy in collisionless plasmas. However, their existence
and effectiveness during reconnection have not been well understood due to ion
Landau damping and difficulties in resolving the Debye length scale in the
laboratory. Here we report a sudden onset of ion acoustic bursts measured by
collective Thomson scattering in the exhaust of anti-parallel magnetically
driven reconnection using high-power lasers. The ion acoustic bursts are
followed by electron acoustic bursts with electron heating and bulk
acceleration. We reproduce these observations with 1D and 2D particle-in-cell
simulations in which electron outflow jet drives ion-acoustic instabilities,
forming double layers. These layers induce electron two-stream instabilities
that generate electron acoustic bursts and energize electrons. Our results
demonstrate the importance of ion and electron acoustic dynamics during
reconnection when ion Landau damping is ineffective, a condition applicable to
a range of astrophysical plasmas including near-Earth space, stellar flares,
and black hole accretion engines.
