This paper discusses the results of experimental studies of low intensity laser pulse train (LI-LPT) propagation in air. The train of ultra-short laser pulses of adjustable repetition rate became possible with the intra-cavity longitudinal mode selector that improves the efficiency of the mode locking mechanism. This technique enabled the generation of a LPT with an envelope duration TPL ≈ 40 ns (FWHM). The envelope is filled with a train of micro-pulses that form a temporal comb with an individual micro-pulse duration τL ≥ 150 ps. The micro-pulse separation time, TP, can be tuned from ~10 ns to 0.45 ns. Depending on the pump level, the total energy ELPT of the LPT is in the range of 150 mJ and 1.2 J. When focused in air, the LPT with peak micro-pulse intensity ranging from 5×1014 W/cm2 to 1016 W/cm2 generates a plasma. The laser induced plasma leads to laser light scattering, broadband luminescence, and generation of rf radiation. We report the first results of the experimental studies of the interaction of the LI-LPT with air. Theoretical analysis and simulations of the filamentation and rf radiation have been carried out. The results of the experiment are in agreement with the theoretical and simulation results.
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 micro-pulses 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/Tp where Tp is the separation time of micro-pulses in LPT.
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.
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