Abstract
A self-ducted Gaussian laser beam, propagating through a plasma, is unstable to a perturbation in its spot size, r0. A reduction (or enhancement) in r0 leads to an enhancement (or reduction) in axial intensity, translating r0 modulation into intensity variation with axial distance and time, causing ponderomotive force on the electrons that drive a Langmuir wave. The density troughs/crests of the Langmuir wave, moving with the group velocity of the laser, cause self-focusing/defocusing of the laser, enhancing the original perturbation and leading to a combined instability of self-focusing, and Raman scattering of the laser beam. The thermal effects allow the Langmuir wave to be treated as a radially localized eigenmode with finite group velocity in the axial direction. The spatial growth rate of the instability decreases with electron temperature while it increases with plasma density.
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