Transient and time periodic self focusing of a laser beam in a plasma

A Gaussian laser beam with step function temporal profile, propagating through a plasma exerts radial ponderomotive force on the electrons and creates a plasma channel. The channel evolves on a time scale similar to r(0)/c(s), where r(0) is the beam spot size and c(s) is the sound speed, accompanied by time-dependent self focusing of the laser. In the case of an amplitude modulated laser, strong self distortion occurs when modulation frequency Omega < r(0)/c(s). For Omega>r(0)/c(s), self focusing occurs at a higher laser power and the beam width parameter has a small time-dependent ripple. (C) 1999 American Institute of Physics. [S1070-664X(99)00205-0].