Matthew R. Edwards

Femtosecond laser electronic excitation tagging for quantitative velocity imaging in air

Time-accurate velocity measurements in unseeded air are made by tagging nitrogen with a femtosecond-duration laser pulse and monitoring the displacement of the molecules with a time-delayed, fast-gated camera. Centimeter-long lines are written through the focal region of a ∼1 mJ, 810 nm laser and are produced by nonlinear excitation and dissociation of nitrogen. Negligible heating is associated with this interaction. The emission arises from recombining nitrogen atoms and lasts for tens of microseconds in natural air. It falls into the 560 to 660 nm spectral region and consists of multiple spectral lines associated with first positive nitrogen transitions. The feasibility of this concept is demonstrated with lines written across a free jet, yielding instantaneous and averaged velocity profiles. The use of high-intensity femtosecond pulses for flow tagging allows the accurate determination of velocity profiles with a single laser system and camera.

The efficiency of Raman amplification in the wavebreaking regime

We compare previous analytic predictions, Vlasov-Maxwell simulations, and particle-in-cell results with a new set of comprehensive one and two dimensional particle-in-cell simulations in an effort to clarify apparent discrepancies between the predictions of different models for the efficiency of Raman amplification in the wavebreaking regime. We find reasonable agreement between our particle-in-cell simulations and previous results from Vlasov-Maxwell simulations and analytic work, suggesting a monotonic decrease in conversion efficiency for increased pump intensities past the wavebreaking threshold.

Simultaneous Temperature and Velocity Measurement in Unseeded Air Flows with FLEET

ows. In this paper, we demonstrate that femtosecond laser electronic excitation tagging (FLEET), a recently developed molecular tagging velocimetry method can also be used to simultaneously measure temperature. A 150 femtosecond pulse populates excited molecular nitrogen states, driving signicant emission in the nitrogen Second Positive and First Negative systems. Spectroscopy of emission from the excited states allows temperature to be determined from both the distribution of rotational energy and the relative strengths of the Second Positive and First Negative systems. Temperature proles are measured to an uncertainty of 20 K and 1 mm spatial resolution between 300 and 600 K in a heated turbulent jet. FLEET temperature and velocity measurements involve separate systems and can be conducted simultaneously without interference.

Enhanced attosecond bursts of relativistic high-order harmonics driven by two-color fields

We study the generation of attosecond x-ray and ultraviolet pulses from relativistically driven overdense plasma targets with two-color incident light. Particle-in-cell simulations show that significant improvement in pulse intensity and isolation is achievable with appropriate laser and plasma parameters. Conversion of 5% of incident laser energy to its second harmonic can enhance the intensity of generated attosecond pulses by an order of magnitude. This approach allows the generation of higher attosecond pulse intensities with existing experimental laser technology and offers a powerful tool for the analysis of the dynamics of relativistic laser-plasma interaction.

Short-pulse amplification by strongly coupled stimulated Brillouin scattering

We examine the feasibility of strongly coupled stimulated Brillouin scattering as a mechanism for the plasma-based amplification of sub-picosecond pulses. In particular, we use fluid theory and particle-in-cell simulations to compare the relative advantages of Raman and Brillouin amplification over a broad range of achievable parameters.

Distinguishing Raman from strongly coupled Brillouin amplification for short pulses

Plasma-based amplification by strongly coupled Brillouin scattering has recently been suggested for the compression of a short seed laser to ultrahigh intensities in sub-quarter-critical-density plasmas. However, by employing detailed spectral analysis of particle-in-cell simulations in the same parameter regime, we demonstrate that, in fact, Raman backscattering amplification is responsible for the growth and compression of the high-intensity, leading spike, where most of the energy compression occurs, while the ion mode only affects the low-intensity tail of the amplified pulse. The critical role of the initial seed shape is identified. A number of subtleties in the numerical simulations are also pointed out.

Strongly Enhanced Stimulated Brillouin Backscattering in an Electron-Positron Plasma.

Stimulated Brillouin backscattering of light is shown to be drastically enhanced in electron-positron plasmas, in contrast to the suppression of stimulated Raman scattering. A generalized theory of three-wave coupling between electromagnetic and plasma waves in two-species plasmas with arbitrary mass ratios, confirmed with a comprehensive set of particle-in-cell simulations, reveals violations of commonly held assumptions about the behavior of electron-positron plasmas. Specifically, in the electron-positron limit three-wave parametric interaction between light and the plasma acoustic wave can occur, and the acoustic wave phase velocity differs from its usually assumed value.

Limitations on High-Spatial Resolution Measurements of Turbulence Using Femtosecond Laser Tagging

The study of the smallest scales of turbulence requires high-resolution spatially-resolved measurements of velocity. Femtosecond Laser Electronic Excitation Tagging (FLEET) provides high-spatial-resolution, minimally-invasive velocity measurements in unseeded air flows, offering both a new method for characterizing turbulence and a potential tool for studying the interaction of turbulent flow with localized energy addition. To clarify the resolution limits achievable with FLEET, we quantify the density and temperature perturbation caused by the laser-gas interaction and examine the statistics of small-scale turbulence measured with FLEET for effects of the perturbation. We combine experimental measurements with a simple numerical model of the interaction of turbulence with a density perturbation. Our results suggest that the perturbation caused by FLEET does not cause errors in velocity measurement for length scales longer than 100 microns.

New diagnostic methods for laser plasma- and microwave-enhanced combustion

The study of pulsed laser- and microwave-induced plasma interactions with atmospheric and higher pressure combusting gases requires rapid diagnostic methods that are capable of determining the mechanisms by which these interactions are taking place. New rapid diagnostics are presented here extending the capabilities of Rayleigh and Thomson scattering and resonance-enhanced multi-photon ionization (REMPI) detection and introducing femtosecond laser-induced velocity and temperature profile imaging. Spectrally filtered Rayleigh scattering provides a method for the planar imaging of temperature fields for constant pressure interactions and line imaging of velocity, temperature and density profiles. Depolarization of Rayleigh scattering provides a measure of the dissociation fraction, and multi-wavelength line imaging enables the separation of Thomson scattering from Rayleigh scattering. Radar REMPI takes advantage of high-frequency microwave scattering from the region of laser-selected species ionization to extend REMPI to atmospheric pressures and implement it as a stand-off detection method for atomic and molecular species in combusting environments. Femtosecond laser electronic excitation tagging (FLEET) generates highly excited molecular species and dissociation through the focal zone of the laser. The prompt fluorescence from excited molecular species yields temperature profiles, and the delayed fluorescence from recombining atomic fragments yields velocity profiles.

Density Scaling and Calibration of FLEET Temperature Measurements

Bulk gas temperature is a critical parameter in characterizing the dynamics, species concentration, quantum state and chemical reactivity of high speed flow. A myriad of techniques have been developed recently to non-intrusively and remotely measure temperature in different conditions and environments. Often these experiments require external flow seeding, are only applicable in a relatively limited range of temperature and rely on a complicated apparatus for thermometry alone. Femtosecond Laser Electronic Excitation Tagging (FLEET) avoids many of the common issues encountered with laserbased techniques and has been shown to be an effective tool for simultaneous measurements of flow velocity and temperature 1,2 . A spectrum of the second positive and first negative emission of molecular nitrogen is captured from a region of interest and the intensity of rotational features can be correlated to the translational gas temperature within the natural lifetime of the fluorescence. Either the experimental spectrum is fit directly to a simulation or certain portions of the spectrum are analyzed to build a database of area ratios from which temperature can be extracted. It is observed that the intensity, shape and appearance of certain prominent rovibrational features change with pressure. The present study investigates the nature of FLEET fluorescence at various temperatures and pressures and establishes a direct scaling relationship to properly account for changes in gas density to the area ratio methodology.