Achieving Collimated Gamma-Ray Emission from Direct Laser Acceleration*

In direct laser acceleration (DLA), an ultra-relativistic laser pulse transfers its energy directly to plasma electrons, which then emit gamma rays via synchrotron radiation. Optimized DLA gamma-ray sources using PW-class lasers can potentially convert over 10% of the laser energy into high-energy photons. However, the emission typically forms a broad angular distribution with two lobes, limiting its effectiveness for applications. Despite extensive research on improving conversion efficiency, the underlying mechanism of the angular photon distribution remains unclear. Through particle-in-cell simulations, test-electron simulations, and theoretical analysis, we identify a regime that optimizes the DLA-based gamma-ray source to produce a collimated single-lobed angular distribution. In this regime, the gamma-ray beam direction is well-defined, significantly enhancing its suitability for applications.