Recent advances in ultrafast-laser-based spectroscopy and imaging for reacting plasmas and flames

Abstract

Reacting flows and plasmas are prevalent in a wide array of systems involving defense, commercial, space, energy, medical, and consumer products. Understanding the complex physical and chemical processes involving reacting flows and plasmas requires measurements of key parameters, such as temperature, pressure, electric field, velocity, and number densities of chemical species. Time-resolved measurements of key chemical species and temperature are required to determine kinetics related to the chemical reactions and transient phenomena. Laser-based, noninvasive linear and nonlinear spectroscopic approaches have proved to be very valuable in providing key insights into the physico-chemical processes governing reacting flows and plasmas as well as validating numerical models. The advent of kilohertz rate amplified femtosecond lasers has expanded the multidimensional imaging of key atomic species such as H, O, and N in a significant way, providing unprecedented insight into preferential diffusion and production of these species under chemical reactions or electric-field driven processes. These lasers not only provide 2D imaging of chemical species but have the ability to perform measurements free of various interferences. Moreover, these lasers allow 1D and 2D temperature-field measurements, which were quite unimaginable only a few years ago. The rapid growth of the ultrafast-laser-based spectroscopic measurements has been fueled by the need to achieve the following when measurements are performed in reacting flows and plasmas. They are: (1) interference-free measurements (collision broadening, photolytic dissociation, Stark broadening, etc), (2) timeresolved single-shot measurements at a rate of 1-10 kHz, (3) spatially-resolved measurements, (4) higher dimensionality (line, planar, or volumetric), and (5) simultaneous detection of multiple species. The overarching goal of this article is to review the current state-of-the-art ultrafast-laserbased spectroscopic techniques and their remarkable development in the past two decades in meeting one or all of the above five goals for the spectroscopic measurement of temperature, number density of the atomic and molecular species, and electric field.

Spectroscopy techniques and the measurement of molecular radical densities in atmospheric pressure plasmas

“…127,128 Recently however, even these techniques have been extended with 'resonant variants' and ultrafast capability (i.e. with ps or fs laser sources) and have been shown suitable for the detection of minor species (OH and NO) 129,130 . A recent review covers this material comprehensively including a perspective on the future opportunities for flame and plasma diagnostics using ultrafast sources.…”

Thermométrie Raman cohérente pour l'étude de la combustion

“…Forte de ces atouts, la spectroscopie CARS trouve de nombreuses applications [4][5][6] pour le diagnostic en phase gazeuse permettant d'étudier des milieux réactifs complexes tels que la combustion, les plasmas et les écoulements supersoniques, mais aussi la microscopie des milieux condensés, et les sciences de la vie (analyse de tissus, cellules et bactéries, spectroscopie in vivo, endoscopie…).…”

Ultrafast background-free ro-vibrational fs/ps-CARS thermometry using an Yb:YAG crystal-fiber amplified probe

“…In particular, coherent anti-Stokes Raman scattering (CARS) [3,4] is widely used to retrieve local information. It is applied to microscopy in condensed media [5,6], life sciences [7], metrology [8,9], but also to diagnose gaseous flows [10] such as combustion [1,11], plasma [12][13][14][15] and supersonic flows [16,17]. Especially in the case of reacting flows, these measurements are essential for the understanding and the reliable simulation of the physical processes taking place in presence of high temperature, pressure and turbulence.…”

“…Especially in the case of reacting flows, these measurements are essential for the understanding and the reliable simulation of the physical processes taking place in presence of high temperature, pressure and turbulence. Among others, the temperature is a key parameter to describe these flows, since it governs the kinetic of chemical reactions [15]. Thanks to its ability to provide broadband ground state spectroscopy in a single shot, CARS thermometry is widely used [10], its precision and accuracy being still unrivalled.…”

“…This NR signal is an important limitation of traditional CARS diagnostics in reactive media since it induces complex interferences within the CARS spectrum, perturbing its interpretation. CARS thermometry has traditionally used for long time ns-lasers operating at few Hz repetition rate [15]. Thanks to the recent development of ultrafast laser sources, ultra-short pulses regime (femtosecond or picosecond duration) as well as high repetition rates (MHz-kHz) are now available, allowing high speed measurements [18][19][20][21][22] in turbulent environments.…”