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In this paper, we investigate the conditions under which direct laser acceleration (DLA) of electrons in a laser-irradiated plasma can produce distinct photon emission profiles, focusing on the mechanisms responsible for single-lobed versus double-lobed angular distributions of emitted γ-rays. Through a combination of particle-in-cell simulations, test-electron simulations, and theoretical analysis, we demonstrate that the efficiency of DLA is a key determinant of the resulting emission pattern. Our results show that inefficient DLA, characterized by electrons rapidly gaining and losing energy within a single laser cycle, leads to a double-lobed emission profile heavily influenced by laser fields. In contrast, in the efficient DLA regime, where electrons steadily accumulate energy over multiple cycles, the emission is primarily governed by the quasi-static azimuthal magnetic fields generated by the laser in the plasma, resulting in a well-collimated single-lobed emission profile. Additionally, we identify that reducing the electron density in the target enhances the efficiency of DLA, thereby transforming the emission from a double-lobed to a single-lobed profile. These findings provide valuable insights into the optimization of laser-driven γ-ray sources for applications requiring high-intensity, well-collimated beams.

Ultrafast laser systems, those with a pulse duration on the order of picoseconds or less, have enabled advancements in a wide variety of fields. Of particular interest to this work, these laser systems are the key component to many High Energy Density (HED) physics experiments. Despite this, previous studies on the shape of the laser pulse within the HED community have focused primarily on pulse duration due to the relationship between pulse duration and peak intensity, while leaving the femtosecond scale structure of the pulse shape largely unstudied. To broaden the variety of potential pulses available for study, a method of reliably adjusting the pulse shape at the femtosecond scale using sub-nanometer resolution Direct Phase Control has been developed. This paper examines the capabilities of this new method compared to more commonplace dispersion-based pulse shaping methods. It also will detail the capabilities of the core algorithm driving this technique when used in conjunction with the WIZZLER and DAZZLER instruments that are common in high intensity laser labs. Finally, some discussion is given to possible applications on how the Direct Phase Control pulse shaping technique will be implemented in the future.