Relativistic ponderomotive effect on the propagation of rippled laser beam and the excitation of electron plasma wave in collisionless plasma

This paper presents an investigation of the propagation of rippled laser beam in a collisionless plasma and its effect on and the excitation of electron plasma wave and particle acceleration, when relativistic and ponderomotive nonlinearities are simultaneously operative. Electron plasma wave (EPW) coupling with rippled laser beam arises on account of the relativistic change in the electron mass and the modification of the background electron density due to ponderomotive nonlinearity. When the electron plasma wave gets coupled to the rippled laser beam, a large fraction of the pump energy gets transferred to EPW and this excited EPW can accelerate the electrons. Analytical expressions for the growth rate of the laser spike in plasma, beam width of the rippled laser beam and excited electron plasma wave have been obtained using paraxial ray approximation. These coupled equations are solved analytically and numerically to study the growth of laser spike in plasma and its effect on the self focusing of rippled laser beam in plasma, amplitude of the excited electron plasma wave and particle acceleration. The result shows that the effect of including ponderomotive nonlinearity significantly affects the growth of laser spike in plasma, excitation of electron plasma wave as well as the number of energetic electrons in particle acceleration process. The results are presented for typical laser plasma parameters.