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

Abstract 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 a 0 , 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 28 pC , an energy of approximately 350 MeV , an rms energy spread of about 3%, and a normalized projected emittance of about 80 nm rad . 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 a 0 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.