Pulse-duration dependence of electron acceleration and betatron radiation in tightly focused laser wakefield...

Betatron X-ray sources generated via laser wakefield acceleration (LWFA) hold strong potential for diverse applications. Recent progress in tightly focused, high-intensity LWFA has enabled high-charge electron bunches and enhanced betatron radiation. Systematic experimental scans of the laser pulse duration τ_FWHM, a key control parameter for both acceleration and radiation, remain challenging, motivating particle-in-cell simulations. Accordingly, we simulate tightly focused laser pulses propagating in underdense plasma and show that τ_FWHM governs transitions between distinct interaction regimes. An optimal duration range of 20-40 fs is identified, where direct laser acceleration contributes substantially, and the conversion efficiencies from laser energy to electron energy and to X-ray energy are simultaneously maximized, reaching 70% and 0.4%, respectively, at ∼1 PW.