Low intensity laser pulse train propagation in air: part I: analysis and simulations

Abstract

In this presentation we outline the results of the analysis and numerical simulations of the physical phenomena associated with the propagation of laser pulse trains (LPT) in air. In the performed studies the intensity of the micropulses in the LPT is far below that of tunneling ionization. The ionization process relies on the background level of radioactivity, which plays an important role in initiating a collisional ionization process. The focused LPT ionizes the air forming a plasma filament. The ponderomotive forces associated with the LPT drive the plasma oscillations predominantly in the radial direction. As the plasma density builds up on axis, the latter portion of the LPT is defocused, resulting in scattering of the incoming laser radiation and shortening of the laser's interaction length. In our model, a low intensity LPT photo-ionizes background negative ions (produced by ambient ionizing radiation) and provides the seed electrons necessary to initiate collisional ionization. The driven radial electron currents in turn generate directed rf radiation. The frequency of the rf radiation is given by 1/T p where T p is the separation time of micro-pulses in LPT.