Evolution of Plasma Channel Driven by Ultra-Short and Ultra-Intense Laser in Near-Critical Density Plasma with a Density Ramp

When an ultra-intense, ultrashort laser pulse interacts with near-critical-density plasma, the ponderomotive force of the laser expels electrons along its propagation path, forming a plasma channel. However, strong coupling effects lead to relatively rapid depletion of the laser energy. Thereafter, the plasma channel continues to extend via a magnetic dipole vortex structure. Owing to the highly nonlinear nature of this process, it is challenging to describe analytically and is influenced by multiple physical parameters. Previous studies have reported this phenomenon and analyzed effects such as plasma channel deflection and ionization. In this work, we go beyond the idealized uniform density profile by incorporating a density ramp at the plasma edge. Specifically, when the density ramp is sufficiently gradual, the laser first excites a cavitation bubble in the plasma, which then rapidly evolves into a strongly coupled plasma channel. Ionization effects are also considered in this scenario. We investigate this complex interaction using two-dimensional particle-in-cell simulations and analyze the evolution of plasma channel and the associated electron bubble. Our findings contribute to a more comprehensive understanding of the interaction between ultra-short and ultra-intense lasers and near-critical density plasma.