Ultralow-emittance electron beams from laser wakefield accelerator based on sharp density transition injection

We numerically investigate a scheme for generating ultralow-emittance electron beams using hydrodynamic optical-field-ionization (HOFI)-induced shock injection in laser wakefield acceleration (LWFA). A steep density down-ramp formed by the HOFI process enables electron injection at low laser amplitude a0, reducing transverse forces and favoring longitudinal injection to minimize the beam emittance. Particle-in-cell simulations demonstrate the production of high-quality electron beams with a charge of , an energy of approximately , an rms energy spread of about 3%, and a normalized projected emittance of about . Unlike mechanically driven shocks commonly used in LWFA, the HOFI-induced shock exhibits superior stability, enabling precise control over the electron injection process. Moreover, because injection occurs where a0 is relatively low and slowly varying, the scheme shows enhanced tolerance to laser energy jitter. This approach provides a promising pathway for generating high-quality electron beams suited for downstream applications such as GeV-class plasma accelerators and free-electron lasers.