Observation of sub-relativistic collisionless shock generation and breakout dynamics.

Relativistic collisionless shocks, which are ubiquitous in the cosmos, play a significant role in various astrophysical phenomena such as gamma-ray bursts, PeVatrons, and supernova shock breakouts. Here we present a demonstration using a compact femtosecond laser system to generate sub-relativistic collisionless shocks (0.03c) under astrophysically relevant conditions. We attribute the shock formation to a rapidly growing Weibel instability in a precisely tuning low-density preplasma environment, which resembles the interstellar media near an astrophysical central engine. Owing to this Weibel instability, a 5000 Tesla magnetic field is developed within 2.7 ps, leading to the collisionless shock formation and subsequent breakout at the preplasma boundaries. This platform enables direct investigation of astrophysics related to relativistic collisionless shocks. The achieved parameters bridge the gap between astrophysical observations and controlled laboratory experiments, offering unprecedented opportunities to validate cosmic shock models.