The effect of density scale length on the plasma scattering and heating in relativistic laser interaction with under dense plasma

In this paper, the effect of plasma density ramp and laser pulse length on the energetic particle generation and pulse scattering were investigated in an under- dense plasma for short and long pulse lengths, of τL</sub> = 60 fs, and τL</sub> = 300 fs, respectively. In our simulations, we used a kinetic particle in cell simulation 1D-3V code. It is found that the laser pulse length and density ramps play an important role on the energetic particles generation and pulse scattering in plasma. So that, the simulations of the laser pulse length impact indicate that, in the case of short pulse interaction with the under-dense plasma, electrons are accelerated to the higher energy level. Furthermore, among the three different ramps: step-like, ramp 1 with a steep slope, and ramp 2 with a gentle slope, in the case of the step-like density ramp, electrons accelerate to high energies, in comparison with two other ramps. A fourier analysis of the total radiation spectrum indicate that in the case of the step-like density profile, the growth rate of the electromagnetic modes have maximum regular picks and minimum growth rate obtained in the case of the smooth ramp. Meanwhile, the time evaluation analysis of the energy distribution function shows that with the time increment of the pulse propagation, in the shorter pulse case, and for the step density scale length, plasma particles can reach a high energy level.