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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. Performance of the method and instrumentation is examined using data taken with a single shot FROG system. Finally, some discussion is given to possible applications on how the Direct Phase Control pulse shaping technique will be implemented in the future. 

Abstract We investigate the mechanisms responsible for single-lobed versus double-lobed angular distributions of emitted γ-rays in laser-irradiated plasmas, focusing on how direct laser acceleration (DLA) shapes the emission profile. Using test-particle calculations, we show that the efficiency of DLA plays a central role. In the inefficient DLA regime, electrons rapidly gain and lose energy within a single laser cycle, resulting in a double-lobed emission profile heavily influenced by laser fields. In contrast, in the efficient DLA regime, electrons steadily accumulate energy over multiple laser cycles, achieving much higher energies and emitting orders of magnitude more energy. This emission is intensely collimated and results in single-lobed profiles dominated by quasi-static azimuthal magnetic fields in the plasma. Particle-in-cell simulations demonstrate that lower-density targets create favorable conditions for some electrons to enter the efficient DLA regime. These electrons can dominate the emission, transforming the overall profile from double-lobed to single-lobed, even though inefficient DLA electrons remain present. These findings provide valuable insights for optimizing laser-driven γ-ray sources for applications requiring high-intensity, well-collimated beams.