Christopher M. Limbach

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.

Threshold characteristics of ultraviolet and near infrared nanosecond laser induced plasmas

The present contribution compares the energy absorption, optical emission, temperature, and fluid dynamics of ultraviolet (UV) λ = 266 nm and near infrared (NIR) λ = 1064 nm nanosecond laser induced plasmas in ambient air. For UV pulses at the conditions studied, energy absorption by the plasmas increases relatively gradually with laser pulse energy starting at delivered energy of E ∼ 8 mJ. Corresponding measurements of plasma luminosity show that the absorption of UV radiation does not necessarily result in visible plasma emission. For the NIR induced plasmas, the energy absorption profile is far more abrupt and begins at ∼55 mJ. In contrast with UV, the absorption of NIR radiation is always accompanied by intense optical emission. The temperatures of both types of plasma have been measured with Rayleigh scattering thermometry (at times after the Thomson signal sufficiently diminishes). The UV plasmas can attain a wider range of temperatures, including lower temperatures, depending on the pulse energy (e...

Cavity-enhanced rotational Raman scattering in gases using a 20 mW near-infrared fiber laser.

A novel cavity-enhanced laser diagnostic has been developed to perform point measurements of spontaneous rotational Raman scattering. A narrow linewidth fiber laser source (1064 nm) is frequency locked to a high-finesse cavity containing the sample gas. Intracavity powers of 22 W are generated from 3.7 mW of incident laser power, corresponding to a buildup factor of 5900. A triple monochromator and a photomultiplier tube in counting mode are used to disperse and measure the scattering spectra. The system is demonstrated with rotational Raman spectra of nitrogen, oxygen, and carbon dioxide at atmospheric pressure. The approach will allow temporally and spatially resolved Raman measurements for combustion diagnostics and, by extending to higher power, Thomson scattering for diagnostics of low-density plasmas.

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.

Characterization of Dissociation and Gas Heating in Femtosecond Laser Plasma with Planar Rayleigh Scattering and Rayleigh Scattering Polarimetry

Planar Rayleigh scattering and Rayleigh scattering polarimetry are used to characterize the dissociation kinetics, fluid dynamics and energy deposition in the afterglow of a femtosecond laser plasma, relevant for the FLEET (Femtosecond Laser Electronic Excitation Tagging) diagnostic technique. Fast heating is observed in the first 100ns, generating acoustic waves and a low density region with a corresponding temperature rise of at least ∆T = 330 ± 10 K. The energy deposition and temperature rise spatial profiles are observed to scale linearly with the emission intensity. Comparing this emission to the three-body recombination rate of nitrogen atoms shows that recombination processes can adequately explain the time history of the fluorescence after 1μs.

Control of Early Flame Kernel Growth by Multi-Wavelength Laser Pulses for Enhanced Ignition

The present contribution examines the impact of plasma dynamics and plasma-driven fluid dynamics on the flame growth of laser ignited mixtures and shows that a new dual-pulse scheme can be used to control the kernel formation process in ways that extend the lean ignition limit. We perform a comparative study between (conventional) single-pulse laser ignition (λ = 1064 nm) and a novel dual-pulse method based on combining an ultraviolet (UV) pre-ionization pulse (λ = 266 nm) with an overlapped near-infrared (NIR) energy addition pulse (λ = 1064 nm). We employ OH* chemiluminescence to visualize the evolution of the early flame kernel. For single-pulse laser ignition at lean conditions, the flame kernel separates through third lobe detachment, corresponding to high strain rates that extinguish the flame. In this work, we investigate the capabilities of the dual-pulse to control the plasma-driven fluid dynamics by adjusting the axial offset of the two focal points. In particular, we find there exists a beam waist offset whereby the resulting vorticity suppresses formation of the third lobe, consequently reducing flame stretch. With this approach, we demonstrate that the dual-pulse method enables reduced flame speeds (at early times), an extended lean limit, increased combustion efficiency, and decreased laser energy requirements.

Adjoint Optimization of Volumetric Sources in Steady, Supersonic Flow: Energy Addition

Adjoint optimization methods have proven successful for the control of turbulence and boundary layers and in the design of airfoils and aircraft. In this paper, the adjoint equations are extended to the problem of controlling steady, inviscid, supersonic flow with volumetric source terms in the Euler equations, that is, mass, momentum, and energy addition. The adjoint solutions are shown to indicate both the optimal location for the source placement and cost gradient. In the particular case of drag reduction by energy deposition, the gradient is proportional to energy efficiency and becomes a key factor in optimization. The general form of the problem makes these results applicable to all forms of volumetric control with the goals of drag reduction, lift enhancement, and the generation of pitching moments.

Trajectory control of small rotating projectiles by laser discharges

The experimental and theoretical analysis of the trajectory control of small rotating projectiles by laser discharges has been performed. The laser spark was generated by 1.06 μm Nd-YAG laser (300 mJ, 5 ns pulse duration). It was shown that the spark generates a strong shock wave, a hot gas spot, and a slow air jet. The gas heating is considered as a primary mechanism for the pressure redistribution along the projectile surface and the trajectory change.

Properties of ultraviolet and near-infrared laser induced air plasmas and their application for spark ignition

Summary form only given. The present contribution describes a novel means of forming laser induced plasmas in air based on a multiple pulse preionization scheme. The method makes use of a nanosecond duration ultraviolet (UV) 266 nm pulse to generate a weakly ionized plasma kernel. This is followed ~15 ns later by an overlapped Near-Infrared (NIR) 1064 nm pulse for controlled energy addition into the gas. The energy added by the second pulse is due to the inverse bremsstrahlung process in which the free electrons generated during the UV pre-ionization pulse absorb the NIR laser energy and, through collisions with gas molecules, increase the temperature of the gas. The approach allows tailoring of the plasma temperature and potentially size by varying the pulse conditions. One application is to generate weakly ionized, low temperature plasmas in the range of ~2000-3000 K which can then serve as ignition sources for combustion. We report plasma energy absorption and resulting luminosity for the two pulses (wavelengths) acting individually. We show that the UV pulse does not exhibit a threshold behavior, but rather the fraction of energy absorbed inside the plasma changes continuously with pulse energy. This is in contrast with the NIR pulse where there is a discontinuous jump to plasma onset as the pulse energy is increased. Both beams exhibit a stochastic nature of the plasma formation process, i.e. for fixed pulse energy, the fractional energy absorbed inside the plasma varies from shot-to-shot. Rayleigh scattering thermometry is used to measure the plasma temperature for both the UV and NIR pulse as well as for the two pulses combined. Initial gas heating is possible due to the UV pulse (only). A temperature as high as T=1500 K was measured for a UV pulse energy of 20 mJ at 1μs after the end of the pulse. For the NIR, much higher temperatures (>10,000 K) are observed at early times due to the full breakdown of the plasma. Energy absorption and thermometry results for the dual pulse configuration are also presented, along with demonstration of ignition of a propane air mixture by the dual pulse plasma for a total combined pulse energy of ~30 mJ which is lower than the pulse energy needed for either wavelength acting on its own.

Development of cavity enhanced Raman and Thomson scattering diagnostics

Summary form only given. Improved measurement capabilities of electron density (ne) and temperature (Te) in low density (~1010-1011 cm-3) plasmas would greatly benefit the study of processing plasmas, electric propulsion thrusters, atmospheric pressure plasmas for combustion and flow control, and other weakly ionized systems. Electron properties can be measured via physical probes or with a non-intrusive optical technique, such as laser Thomson scattering (LTS). LTS has seen widespread use for measurements in high-density plasmas but extension of the technique to the low-density regime has been extremely challenging owing to weak LTS signals and interferences from Rayleigh and elastic background scattering. We present the development of a cavity enhanced Thomson scattering (CETS) diagnostic that seeks to capitalizes on a high-power (10-100 kW) intra-cavity beam achieved by frequency locking a narrow line width laser source to a high finesse optical cavity. The technique should result in a much higher average power light source for LTS measurements and much lower peak power in comparison to pulsed laser sources, which may be advantageous for reducing plasma perturbations. The initial proof-of-concept CETS instrument has been built using a low-power (20 mW) fiber laser and a moderate cavity finesse of F ≈21,000. The Pound-Drever-Hall locking technique is used to maintain a frequency overlap between the laser and cavity. An intra-cavity power of 7 W is generate from an incident laser power of 5 mW, which corresponds to a power build-up factor of 1400. The instrument has been used to measure Rayleigh scattering and rotational Raman spectra in N2, O2, and CO2. Current work is moving to a higher power configuration based on a 5 W laser source and cavity with finesse of F>100,000. The status and challenges of the high power system are discussed.