Simulation of a chirped femtosecond relativistic laser pulse interaction with underdense plasma by using a hydrodynamic approach

摘要

A chirped laser pulse indicates that the laser frequency changes over the duration of the pulse: a positively (negatively) chirped pulse implies that the laser frequency increases (decreases) with time. In this paper, we use a simplified, fully relativistic hydrodynamic approach to simulate the influence of chirp on the propagation of a femtosecond relativistic laser pulse in underdense plasma. Based on this simplified cold-fluid model, the influence of chirp on the main dynamics of the laser pulse, such as self-steepening, red-shift in the leading edge, variation of the frequency chirp, and the generated wakefields can be studied self-consistently. The simulation results show that a pulse with a positive chirp results in a larger increment in the intensity parameter a(0) when propagating a certain distance into an underdense plasma compared with an un-chirped and a negatively chirped pulse, which is largely because of a much greater forward shift of the peak amplitude and more severe pulse self-steepening effect due to the frequency red-shift at the leading edge when exciting a plasma wave. The ponderomotive force, which relates to the first-order differential of the laser pulse intensity envelope, is expected to be stronger for a positively chirped pulse because of its steeper leading edge and larger intensity parameter a(0). As a result, the wakefield driven by the positively chirped laser pulse is more intense than that driven by an un-chirped and a negatively chirped laser pulse, which is confirmed by our self-consistent hydrodynamic simulation.