Intense laser interactions in the atmosphere

Abstract

This dissertation consists of three research projects which deal with laser-matter interactions and potential applications of the processes.In the first part of this dissertation, I present, analyze, and simulate a mechanism for remotely generating RF radiation in air by using a laser pulse train (LPT). The LPT ionizes the air through photo detachment and collisional ionization, generating a plasma filament. The ponderomotive forces associated with the LPT drive radial and longitudinal plasma currents. These plasma currents generate radiation at the LPT repetition rate, which can be tuned in the 0.5 – 6.0 GHz range. Of particular interest is the RF generation in the 1.4-1.7 GHz regime, which is the operating frequency range of all GPS signals. The analysis and simulation results of the RF radiation are shown to be in agreement with experimental data. The experiments were performed in collaboration with the theory and computational work at the University of Maryland. The second part of this dissertation analyzes and simulates long range propagation and stability of laser pulses traveling vertically through the turbulent atmosphere. By tailoring the initial laser pulse parameters, the pulse can be focused transversely and longitudinally at remote locations (>50 km) with intensity enhancements of > 103. In addition, nonlinear self-focusing of the laser pulse and dispersion lead to the growth of a hybrid modulation-filamentation instability, which causes both transverse and longitudinal pulse breakup. This hybrid instability is analyzed and numerically simulated for pulses propagating in the atmosphere. The potential application of this research is on the long-range generation of RF signals in a turbulent atmosphere. The third part of this dissertation presents a mechanism for generating tunable low frequency radiation in the ionosphere (F layer) and propagating the low frequency signal back to the ground. This process has the potential to be used as a new type of over-the-horizon (OTH) radar which would be compact, mobile, and far less costly than conventional OTH systems. Conventional OTH radar systems utilize dipole antenna arrays that span multiple kilometers, and can cost well over a billion dollars. Most major countries employ conventional OTH radar systems. The proposed new mechanism relies on a ground-based, modulated RF source. By tuning the modulation frequency to the local plasma frequency in the ionosphere (F layer), a current can be resonantly excited by the ponderomotive force of the modulated signal. This current grows in time and generates a low frequency signal at the local plasma frequency, which is in the MHz range. The low frequency signal propagates back to the ground, far from the source, where it can be detected. We find that using available RF sources, i.e., gyrotrons, detectable low frequency signals can be generated and detected on the ground. The minimum detectable intensity is ~10-18 W/m2, though the theoretical limit may be lower. This mechanism has the potential to detect OTH objects using a compact, mobile, and far less expensive system than presently exists.