Laser and Plasma Parameter Optimization for Direct Laser Acceleration of Electron Beams*

Direct laser acceleration (DLA) is a mechanism for generating superponderomotive electrons with energy up to hundreds of MeV through the interaction of high-intensity picosecond laser pulses with underdense plasma. The relativistic, broadband DLA electrons are capable of producing bright secondary x-ray radiation and high-energy protons/neutrons. As a complex and dynamic process, the DLA electron acceleration can be affected by a number of nonlinear processes. To investigate the dependency of laser-particle energy exchange on different factors, we performed a wide range of parameter scans on the OMEGA EP facility using supersonic gas nozzles. An optimized combination of laser focusing geometries, laser power and plasma density [H Tang et al.  New J. Phys. 26, 053010 (2024)] and an ideal gradient of the plasma density ramps were demonstrated in the experiment. Meanwhile, 2D particle-in-cell OSIRIS simulations replicating the interactions provides insight into the laser propagation, plasma channel evolution, as well as the corresponding electron dynamics. Our studies identify an optimal experimental condition for DLA electron generation and offer a path towards future applications like betatron x-ray beams for radiography.