Collimated $\gamma$-ray emission enabled by efficient direct laser acceleration

TL;DR

This paper explores how efficient direct laser acceleration (DLA) in laser-irradiated plasmas produces highly collimated, single-lobed gamma-ray emission, advancing the understanding of laser-driven gamma-ray source optimization.

Contribution

It reveals the critical role of DLA efficiency in shaping gamma-ray emission profiles and demonstrates how target density influences DLA regimes and emission collimation.

Findings

Efficient DLA leads to higher energy, collimated gamma-ray emission.

Inefficient DLA results in double-lobed, less collimated profiles.

Lower-density targets favor efficient DLA and single-lobed emission.

Abstract

We investigate the mechanisms responsible for single-lobed versus double-lobed angular distributions of emitted $\gamma$-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 $\gamma$-ray sources for applications requiring high-intensity, well-collimated beams.