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

Abstract

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. K E Y W O R D Schirp, femtosecond relativistic laser, plasma, wakefield INTRODUCTIONUsing the chirped pulse amplification (CPA) technique, ultrashort-pulse laser sources with durations on the femtosecond time scale can produce multi-terawatt and even petawatt power. [1][2][3] These laser pulses can be focused to intensities well above 10 18 W/cm 2 , [4] from which an electron will acquire a quiver velocity approaching the speed of light; that is, the electron motion in the laser fields becomes highly relativistic and non-linear. Thus, these laser pulses are called relativistic femtosecond laser pulses and have great applications in high energy density science, such as energetic electron acceleration, [5] ion acceleration, [6] attosecond pulses generation, [7,8] positron beams for laboratory astrophysics, [9] and so on. These applications require long laser

Space charge effects on radiative ultrashort laser‐plasma interactions: Relativistic fluid model

“…It should be noted that the fluid models as well as the kinetic ones have been widely used for the investigation of the laser-gas interactions. [56][57][58][59][60][61][62][63][64][65] Regarding the latter one, the Vlasov-Maxwell equations are used as the first-principle equations for investigating the plasma processes in which the individual plasma particles and their velocity distribution and also their short range forces are of great importance. Moreover, these processes are non-linear and have a kinetic and collective nature.…”

“…Moreover, these processes are non-linear and have a kinetic and collective nature. [15,57,58] Therefore, the relevant numerical methods are based on the direct solution of the Vlasov equation as well as the particle-in-cell (PIC) method which relies on the solution of the Newton-Maxwell equations of the system instead of solving the Vlasov Equation. [66][67][68][69][70][71][72][73] Due to the fact that simulating the intense laser-air interactions via the particle simulation methods is computationally intense, investigating the process through the plasma fluid equations is a favourable alternative.…”

“…It is noteworthy that these relativistic fluid equations give a good approximation for many phenomena in laser-plasma interactions and their results are valid below the laser intensity of 10 22 W/m 2. [15,54,[56][57][58] Regarding the fact that the ions are considered stationary, the electron-ion, the electron-electron, and the electron-neutral collisions can affect the ultrashort laser-plasma interactions. However due to the ionization degree of more than 0.5 in our case, the collisions between charged particles are the dominant ones.…”