SELF-GUIDING AND STABILITY OF INTENSE OPTICAL BEAMS IN GASES UNDERGOING IONIZATION

The propagation of intense optical beams in gases undergoing ionizatio n is analyzed. Two types of optical beam modes are considered: a funda mental Gaussian and a higher-order radially polarized beam. The propag ation dynamics include the effects of diffraction, nonlinear self-focu sing, and ionization. For sufficiently intense optical beams the neutr al gas undergoes ionization, generating a plasma which tends to defocu s the beam. An envelope equation governing the spot size for both type s of beams is derived, analyzed, and solved numerically. Self-guided s olutions, which result from a balancing of diffraction, plasma defocus ing, and nonlinear self-focusing, are analyzed for both types of beams . These equilibrium solutions are found to be unstable due to an ioniz ation-modulation instability for which asymptotic growth rates are obt ained. A self-guided inverse Cherenkov accelerator based on the higher -order radially polarized mode is proposed and analyzed. In addition, the depletion of the optical field due to collision and ionization los ses is analyzed and the attenuation length is derived.