Ion and Electron Acoustic Bursts during Anti-Parallel Reconnection Driven by Lasers

Magnetic reconnection is ubiquitous in space and astrophysical plasmas, rapidly converting magnetic field energy into thermal and kinetic energy of plasma particles. Among numerous candidate kinetic mechanisms, ion-acoustic instabilities driven by the relative drift between ions and electrons, or electric current, have long been hypothesized to play a critical role in dissipating magnetic energy in collisionless plasmas. But their effectiveness and even existence during reconnection remain elusive 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 in high-Z plasmas at low beta in a novel platform using high-power lasers. The ion-acoustic bursts are followed by electron acoustic bursts with electron heating and bulk acceleration. These observations are successfully reproduced by 1D and 2D Particle-in-Cell simulations in which ion-acoustic instabilities, driven by electron jet currents in the reconnection exhaust, grow rapidly to form electrostatic double layers. These double layers induce an electron two-stream instability that generates electron acoustic bursts, during which electrons are heated and accelerated in accordance with the experimental measurements. Our results demonstrate the importance of ion and electron acoustic dynamics during magnetic 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.