Role of field ionization in laser pulse evolution during interaction of long laser pulse with gaseous hydrogen atoms

In this paper, the laser pulse evolution arising from the field ionization during the interaction of a long laser pulse with gaseous hydrogen atoms is investigated using the kinetic 1D‐3 V Particle‐In‐Cell (PIC) Smilei simulation code. After performing various simulations, it is shown that the field ionization of hydrogen atoms has a non‐negligible effect on the evolution of the laser pulse compared to the pre‐ionized plasma case. The results of our simulations show that the amount of these evolutions is strongly dependent on the parameters of the laser and initial ionization assumed. In this regard, two main mechanisms are responsible for the changes in the generated radiations and then the evolution of the laser pulse. When the average degree of ionization is weak, the backscattered Raman radiations can provide the necessary conditions for the chaotic behavior to occur and the laser pulse to evolve. When the laser and plasma pulse parameters (such as the laser pulse amplitude, hydrogen atoms density, and the rise time of pulse) are selected so that a strong space charge field is formed, the wave breaking (which happened faster due to density changes during the field ionization) is the main factor for evolutions in the laser pulse.