Longitudinal magnetic field effect on enhancing electron bunch quality in laser wakefield acceleration

Abstract

Improving the quality of electron bunches is a crucial challenge for advancing laser wakefield accelerators. This study investigates the properties of high-charge electron bunches produced by two distinct laser pulse profiles— a 6th-order Flattened Gaussian and a Gaussian pulse, both with the same intensity, propagating through a magnetized plasma featuring a density transition. Using particle-in-cell (PIC) simulations, we thoroughly examined the impact of a strong longitudinal external magnetic field (up to 150 T) on the quality of electron bunches, particularly focusing on their injection in a bubble regime via a high-density transition. Our results demonstrate that applying such a magnetic field parallel to the laser propagation direction provides an effective control mechanism for highly charged electron beams. The presence of this longitudinal magnetic field increases the energy of the electron bunches while simultaneously decreasing their energy spread during high-charge injection. While the transverse normalized emittance was observed to increase with the applied magnetic field, the overall beam quality was markedly improved due to the superior energy characteristics. A comparative analysis further reveals that the Flattened Gaussian pulse consistently achieved higher peak energies and greater bunch charges than the Gaussian pulse under the same conditions. These findings underscore that a strong external magnetic field can facilitate the generation of electron bunches with both high charge and superior quality, holding significant promise for advancing applications in fields such as radiography, radiobiology, bright gamma-ray generation, and betatron X-ray production.