Generation of RF radiation by laser pulse trains in air

Abstract

We analyze and numerically simulate a mechanism for generating directed rf radiation when a laser pulse train propagates and forms a plasma filament in air. The role of background radiation levels plays an important role in the ionization process. Photo-ionization of the background negative ions provides the seed electrons necessary to initiate collisional ionization with the air molecules. In this ionization mechanism, the peak intensity of the laser pulse train is far below the tunneling ionization level. The collisionally-ionized electrons are driven radially outward by the ponderomotive force associated with the laser pulses. The resulting oscillating radial currents generate rf radiation mainly in the direction of the laser pulses. The rf frequency is directly related to the laser pulse separation time in the pulse train. The ionization and rf generation mechanism is analyzed using a non-relativistic fluid model which incorporates, among other things, the effects of background radiation, photo-ionization, collisional ionization, ponderomotive and space charge effects, and attachment/recombination processes. The electron density build-up and rf radiation level and directionality are obtained. The results of our analysis and simulations are in good agreement with experiments employing laser pulse trains ionizing air and generating rf radiation.