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 () of the laser. This asymmetry originates from the -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 -shift in the . 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 retrieval in ultraintense laser–matter interactions.