Chemistry and Dynamics of Pre-Ionization Controlled Laser Plasmas

Executive Summary This research project has explored the physics of pre-ionization controlled laser induced plasmas in air. The pre-ionization plasma approach is based on the synergistic effect of combining multiple laser pulses where the first pulse provides periodization (not full breakdown) thereby promoting controlled absorption and plasma formation in the presence of the second pulse. We have generally used ultraviolet (UV) pulses for periodization, owing to stronger multiphoton ionization (MPI), and near-infrared (NIR) pulses for energy addition. The expanded window of conditions (size, temperature, active species etc.) achievable with the pre-ionization plasmas makes them potentially attractive for multiple Air Force applications including providing new sources for ignition of propulsion devices, for atmospheric waveguides, and for novel diagnostics. We have followed a combined experimental and modeling approach to investigate the basic laser-induced plasma formation mechanisms, including detailed characterization of plasma properties and species, as well as studies of ignition and combustion of air-fuel mixtures with the pre-ionization laser plasmas. Experimental measurements have included Rayleigh and Thomson scattering to determine gas temperature and electron density, optical emission spectroscopy and chemiluminescence, Schlieren imaging, as well as combustion studies in a fixed volume chamber (heat release from pressure rise). These measurements have been used to support the development and validation of a two-dimensional axisymmetric numeric model of the laser plasma system including a novel optical-solver (for beam-steering) based on the plasma chemistry.