Simulation Study of Ionization and Laser Beam Quality in CO₂-Laser Plasma Interaction

3D numerical simulations of the interaction of a powerful CO₂laser with hydrogen jets demonstrating the role of ionization and laser beam quality on the characteristics of induced wakes are presented. Simulations are performed in support of the plasma wakefield accelerator experiments being conducted at the BNL Accelerator Test Facility (ATF). SPACE, a parallel relativistic Particle-in-Cell code, developed at SBU and BNL, has been used in these studies. A novelty of the code is its set of efficient atomic physics algorithms that compute ionization and recombination rates on the grid and transfer them to particles. The influence of ionization and laser beam quality on the spectrum of pump laser has been studied for a range of gas densities. Simulations accurately explain the variations in Stokes/anti-Stokes signal intensities observed in the experiments for different gas-reservoir pressures.