Overmodulation of an upper hybrid electromagnetic beam in a plasma

Using the fluid equations for the nonlinear response of electrons, we have studied the self distortion of an amplitude-modulated upper hybrid laser beam and a Gaussian laser pulse in a collisionless magnetoplasma. The beam possesses a Gaussian distribution of intensity in the transverse plane and gives rise to a ponderomotive force on the electrons. The parallel component of ponderomotive force causes a redistribution of electron density, which gets modulated at the modulation frequency of the beam, the profile of modified electron density results in the selffocusing of the beam. The high- and low-amplitude portions of the beam are focused to different extents, and a severe distortion in the amplitude envelope of the beam results. For a typical 10.6-μ, CO2 laser of 10⁸ W the change in the index of modulation becomes ∼100% after propagating a distance ∼0.4 cm. Similarly, the intensity envelope of a Gaussian laser pulse also gets severely distorted as it propagates in the plasma.