Harnessing Ionization Effects for Direct Laser Acceleration of Electrons

Direct laser acceleration (DLA) may efficiently accelerate high-energy electron beams when a high-intensity short-pulse laser propagates through an underdense plasma, thereby creating a charge density channel. In this research, we investigate the effect of ionization on the DLA. Typically, helium is used as the target gas due to its relatively low ionization threshold; helium gas is fully ionized by the laser pre-pulse at intensities that are orders of magnitude lower than the pulse peak. By doping the target with heavier gases that fully ionize near the peak intensity, electrons are ionized within a developing channel in which the laser can accelerate them more effectively. Thus, a higher signal and more energetic yield of electrons, and by extension betatron X-rays, may be obtained. This is similar to a method developed to enhance the electron beam charge from laser-wakefield acceleration. Experiments were performed at the OMEGA-EP facility where different gas targets with helium, nitrogen, and argon were tested at varying densities. The experimental results exhibit an elevated electron and X-ray signal that is dependent on the gas density and mixture. Additionally, we use particle-in-cell simulations to further explore this parameter space.