MHZ-RATE NONLINEAR SPECTROSCOPY AND IMAGING PLATFORM FOR TRANSIENT AND NONEQUILIBRIUM FLOWS

ABSTRACT: The objective of the proposed research effort is to demonstrate the feasibility of 100 kHz to 1 MHz nonlinear spectroscopy for measurements of molecular energy distributions, energy transfer, major species, and temperature in transient combusting and nonequilibrium flows. This will be accomplished, in part, by extending burst-mode laser technology to the fs and ps regimes for greater than three-orders of magnitude improvement in available probe-pulse energy at MHz repetition rates. This laser architecture will also ensure precise synchronization of transform-limited fs and ps pulses for efficient coherent excitation of multi-photon transitions while minimizing interferences such as nonresonant background and collisions. During the Phase I, Spectral Energies and Purdue University will investigate the optimal laser architecture for fs/ps burst-mode laser spectroscopy and demonstrate potential spectroscopic and imaging systems using high-speed ps coherent anti-Stokes Raman scattering (CARS) as a test platform. The Phase II will result in a prototype MHz rate fs/ps CARS system and demonstration in Air Force relevant flows, such as for pulse detonation, scramjet, and gas-turbine combustion. This research program will result in commercial laser spectroscopy and imaging systems that will address critical research needs in areas such as advanced propulsion, munitions, space vehicles, and related industries.; BENEFIT: High-temperature, transient combustion and nonequilibrium conditions in novel propulsion engines, space vehicles, and munitions systems require expensive and time-consuming testing, typically at low data rates. Capturing the relevant timescales in these devices requires measurements at rates of 100 kHz to 1 MHz to track the interaction of flames with hypersonic boundary layers, shockwaves, detonation waves, pulsed plasmas, and fluid dynamic instabilities. The high-speed measurement capabilities proposed in this work will be able to resolve these interactions in both time and space (in a line or a plane) to provide the understanding and predictive models needed to evaluate advanced technologies and meet performance targets of future weapons systems. The prototype instruments proposed in this research program will also fill a gap in commercially available laser technology, offering a greater than three orders of magnitude improvement in probe-pulse energies and repetition rates for a wide range of applications in the aerospace, defense, energy, and manufacturing industries. 1. Immediate benefits to Air Force test facilities and OEMs: The proposed research program will deliver a prototype burst-mode fs and ps laser source and imaging system that is currently not available at Air Force test facilities and OEMs). This will enable investigations of nonequilibrium molecular energy distributions, temperatures, and major species concentrations in test cells for high-speed propulsion systems of interest to the Air Force, including pulse detonation, scramjet, rocket, and gas turbine engines. In high-enthalpy impulse facilities, which have run times on the order of a millisecond, the ability to acquire data at 100 kHz to MHz rates will significantly increase productivity and provide the data bandwidth needed to track the space-time evolution of transient phenomena. This will be invaluable for validating predictive models of molecular energy distributions and improving simulations of hypersonic shocks, boundary layers, detonations, and plasmas. 2. Scientific discovery: Nonlinear spectroscopic techniques, such as coherent anti-Stokes Raman scattering (CARS), are often used for the spectroscopic study of rotational and vibrational nonequilibrium flows and plasmas. However, the effects of nonresonant background and collisions limit accuracy and degrade sensitivity, especially at high pressure. Moreover, low data-acquisition speeds (~10-50 Hz) prevent temporal resolution for highly transient processes. The proposed instrumentation would increase both the probe-pulse energy and repetition rate to allow studies of highly transient processes, such as hypersonic boundary layers, detonation waves, and pulsed plasmas used for combustion enhancement. By extending burst-mode laser technology to the fs and ps regimes, it will also be possible to temporally suppress the effects of nonresonant background and collisions and to identify dominant energy transfer processes controlling vibrational level populations and energy thermalization, measure rotational/vibrational temperature and major-species concentrations, and measure of vibrational-rotational energy transfer rates. This will enable detailed development and validation of accurate numerical models used to predict these phenomena in transient combusting and nonequilibrium flows. 3. Economic security and prosperity: The introduction of low-data-rate amplified fs and ps laser systems enabled a major advance in spectroscopic capability, with these lasers now being standard instruments in chemistry, physics, biomedical, and engineering laboratories throughout the world. The pulse energies of these systems have increased by nearly an order of magnitude within the last decade but are ultimately limited by practical considerations such as the laser footprint and average power. By extending burst-mode laser technology to the fs and ps regimes, the proposed work will enable significantly higher pulse energies in a compact package and with relatively low average power. Such a system has significant commercial potential in a wide range of laboratories focusing on aerospace, defense, energy, and manufacturing, and these industries in turn have an enormous impact on national economic security and prosperity. Spectral Energies is well positioned to be able to commercialize the proposed prototype instrumentation because of past and current investments in burst-mode laser technology, instrumentation for nonlinear spectroscopy, and laser manufacturing capabilities.