Theory of transient self-focusing of a CO2 laser pulse in a cold dense plasma

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Abstract
The self‐focusing of laser pulses in a plasma is theoretically investigated for the situation in which the refractive index change is not in equilibrium with the pulse. In particular, the analysis models a CO2 laser–plasma interaction experiment in which the ponderomotive and thermal conduction dominated mechanisms of self‐focusing are dominant. The moment method and the Gaussian shape ansatz are used to describe the laser pulse propagation. Threshold powers are derived for the transient self‐focusing process, which reveals that self‐focusing can occur even for times much less than the characteristic dielectric relaxation time. Because of the behavior of the pulse heating, thermal conduction dominated self‐focusing is inherently a transient phenomenon without a true steady state limit. The governing equations are also numerically solved, revealing the existence of ion acoustic waves created by the action of moving focal spots. These transversely propagating waves may be responsible for breakup or filamentation of the laser pulse.

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