The effect of the laser pulse shape on the wakefield generation in field-ionized plasma

In this paper, the effect of the laser pulse shape on the generation and evolution of the wakefield during the interaction of the intense laser pulse with the gas have been studied utilizing the parallel relativistic PIC simulation code. In order to reach this aim, three pulses with length 300 fs and different rise-times 30, 45, and 60 are typically selected. Our results show that, the amplitude of the laser wakefield produced in the gas in comparison with the plasma strongly depends on the laser pulse shape. The simulation results indicate that for the high-slope laser pulse time (here 30 fs), ionization and thus density fluctuations have no significant effect on the wakefield generation because of rapid increase of the laser electric field. While by increasing the laser pulse rise-time to 45 fs, the rapid wave breaking due to the change in the medium refractive index during the gas ionization, prevents from the wakefield amplitude growth, so that the wakefield with larger amplitude is emerged in the plasma. For a slow-sloping pulse (here 60 fs), the ratio of the wakefield generation in the gas to the plasma is altered for the different gas densities and laser intensities. Moreover, it is represented that the longer the laser pulse rise-time, the sooner difference between the wakefield produced in the gas and plasma is observed. In fact, the larger the rise-time, the greater the density fluctuations and, consequently, the larger the initial noise is generated to seed the Raman instability.