The Source of Gamma-Ray Angular Distribution in Relativistically Transparent Laser-Irradiated Plasmas

State-of-the-art laser facilities have made significant advancements in focusing lasers to ultra-relativistic intensities, enabling laser-plasma experiments capable of generating highly energetic particles. One highly researched mechanism is direct laser acceleration (DLA), where the ultra-high power of the laser is directly coupled into plasma electrons. These ultra-relativistic electrons, self-consistently within the plasma and laser fields, exhibit efficient emission of gamma rays through synchrotron radiation. However, the underlying mechanism governing the angular distribution of these emitted photons by DLA electrons remains incompletely understood. Particularly noteworthy are numerical simulations that observe the angular distribution of photon energy peaking either in a single lobe along the laser direction or in two lobes slightly deviating from it. This study employs test-electron simulations and theoretical analysis to gain deeper insights into the behavior of the angular distribution of photon energy observed in particle-in-cell kinetic simulations. Our findings indicate that a single lobe distribution arises when DLA electrons 'efficiently' gain energy from the laser across multiple laser periods.