Laser wakefield acceleration (LWFA) and particle wakefield acceleration (PWFA) are two prominent approaches in the field of high-gradient plasma-based acceleration. We propose and systematically demonstrate, via comprehensive start-to-end simulations, a novel hybrid LPWFA scheme for driving soft X-ray FELs with high-gain performance and sufficient robustness. Using a 200 TW laser and two simple yet versatile plasma density profiles, we obtain a witness beam with an energy of 609 MeV, a global (average slice) energy spread of 0.42% (0.15%), and a charge of 32.82 pC. The minimum slice energy spread reaches 0.08%. This ultra-low slice-energy-spread beam enables the generation of soft X-ray pulses at 10 nm with energy up to 27.2 uJ (peak power up to 5.7 GW). Importantly, the scheme exhibits strong robustness against practical drive laser fluctuations—including laser energy jitter of -2% to 2% and focal position jitter of -0.4 mm to 0.4 mm—robustly delivering pulse energies exceeding 5 uJ (~2e11 photons). This work represents a significant advance in compact soft XFEL development, ensuring stable operation and paving the way toward a reliable and widely accessible 10 nm radiation source.

LWFA, PWFA, Soft-XFEL, Robustness