Characterization of femtosecond filaments in air

Abstract

Laser-induced plasmas (LIPs) offer a promising alternative to conventional ignition methods in high-performance engines, addressing inherent limitations of traditional approaches. Firstly, LIPs can be precisely located within the combustion chamber, enabling ignition in regions with a homogeneously mixed air-fuel ratio. Secondly, LIPs eliminate the need for solid electrodes that act as heat sinks and suffer from erosion in high-pressure environments. Finally, LIPs can achieve combustion with mixtures of lower air-fuel ratios than what is possible via conventional ignition, leading to an increase in fuel efficiency. Consequently, laser-based ignition systems are well-suited for engines with pressures and environments beyond the operating range of conventional spark plugs. In a nominal nanosecond laser ignition method, a high-power laser is focused down to a point until it reaches an intensity level high enough to begin to breakdown the gas molecules in which it is being focused. Whilst the study of nanosecond laser-induced plasma has been investigated thoroughly, this work aims to provide justification for the use of femtosecond lasers as a "foundational" pulse to prepare the combustion area through pre-ionization. Using femtosecond (fs) lasers for ignition purposes is supported by the large body of research which has investigated the formation of plasma filaments through the self-focusing effect that occurs when a laser exceeds a critical power. The length of these filaments has been shown to be on the order of meters under certain conditions and provide exciting promise in the fields of remote sensing, weather control, and waveguides. This work obtains information about the filament through the use of optical emission spectroscopy and finds that the lifetime of femtosecond filaments are on the order of nanoseconds whist studied nanosecond plasmas last in excess of tens of microseconds. The spectral emission from the filament is dominated by the N2(C-B) and N2+(B-X). These transitions are observed in nanosecond plasmas, but only at times of greater than 1 μs after the pulse. Femtosecond filaments produce these species almost immediately and their lifetimes are only a few nanoseconds. Using a radiative emission and absorption code, we were able to determine the gas temperatures of the filament to be 500±100 K. A zero-dimensional plasma kinetics model was developed to simulate experimental conditions derived from optical emission spectroscopy. It was found that N2(C) formation comes initially from direct electron excitation up from the ground state of N2(X) then from the dissociation of N4+. N2(C) is destroyed by quenching via O2 primarily, N2 secondarily, and deexcited down to N2(B) ternately. This work answers two questions. First, how is the plasma generated by a femtosecond laser different from that of a nanosecond laser? Second, what chemical reactions occur in a femtosecond plasma. Experimental testing found physical and thermochemical differences in the femtosecond plasma versus the extensively studied nanosecond plasma, namely the shape, lifetime, emissive properties, and temperature. Theoretical modeling unveiled the reaction pathways responsible for the dominant emissive species found spectrally. Concurrency was found between experimental and theoretical results for the reaction pathway of the dominant emissive species in femtosecond plasmas. This concurrence provides confidence to the reaction pathways presented and opens the door to further research using the knowledge of those reaction pathways to increase the viability of femtosecond laser filaments in combustion applications.