Physics of high-charge laser-plasma accelerators for few-MeV applications

Laser-plasma accelerators represent a promising technology for future compact accelerating systems, enabling the acceleration of tens of pC to above 1GeV over just a few centimeters. Nonetheless, these devices currently lack the stability, beam quality, and average current of conventional systems. While many efforts have focused on improving acceleration stability and quality, little progress has been made in increasing the beam's average current, which is essential for future laser-plasma-based applications, such as three-dimensional X-ray tomography for cargo inspection. In this paper, we investigate a laser-plasma acceleration regime aimed at increasing the beam average current with energies up to few MeVs, efficiently enhancing the beam charge. We present experimental results on configurations that allow reaching charges of 5-30 nC and a maximum conversion efficiency of around 14%. Through comprehensive particle-in-cell simulations, we interpret the experimental results and present a detailed study on electron dynamics. From our analysis, we show that most electrons are not trapped in a plasma wave; rather, they experience ponderomotive acceleration. Thus, we prove the laser pulse as the main driver of the particles' energy gain process.