Asymmetric Magnetic-Modulated Collisionless Shocks Retrieved from Laser-Driven Directional Protons

Collisionless shocks are crucial mechanisms for energetic particle generation in the universe. Recent advances in ultraintense laser technology have enabled laboratory investigations of shock-driven ion acceleration, yet the observed directional asymmetries of ion beams remain poorly understood. Here, using particle-in-cell simulations supported by a theoretical model, we reveal that the intrinsic asymmetry of proton beams generated in near-critical-density plasmas is determined by the carrier-envelope phase ( ϕ CEP ) of the laser. This asymmetry originates from the ϕ CEP -dependent formation of magnetic islands, which deflect refluxing electrons and induce imbalanced transverse currents and charge separation. The resulting asymmetric electrostatic shocks accelerate proton beams preferentially toward the side with larger magnetic islands, with the deflection direction reversing upon a π -shift in the ϕ CEP . Our results provide a mechanistic understanding of directional ion beam generation and suggest a new framework for interpreting the origin of high-energy cosmic rays, while offering a potential method for ϕ CEP retrieval in ultraintense laser–matter interactions.