MICROWAVE SCATTERING FOR DIAGNOSTICS OF LASER-INDUCED PLASMAS AND DENSITIES OF SPECIES IN COMBUSTION MIXTURES

Laser-induced plasmas since their discovery in the 1960’s have found numerous applications in laboratories and industries. Their uses range from soft ionization source in mass spectroscopy, development of compact particle accelerator, and X-ray and deep UV radiation sources to diagnostic techniques such as laser-induced breakdown spectroscopy and laser electronic excitation tagging. In addition, the laser-induced plasma is important for studying of various nonlinear effects at beam propagation, such as laser pulse filamentation.

This work deals with two challenging aspects associated with laser-induced plasmas. First is the study of Multi-Photon Ionization (MPI) as a fundamental first step in high-energy laser-matter interaction critical for understanding of the mechanism of plasma formation. The second is application of laser induced plasma for diagnostics of combustion systems.

Numerous attempts to determine the basic physical constants of MPI process in direct experiments, namely photoionization rates and cross-sections of the MPI, were made; however, no reliable data was available until now, and the spread in the literature values often reached 2–3 orders of magnitude. This work presents the use of microwave scattering in quasi-Rayleigh regime off the electrons in the laser-induced plasma as method to measure the total number of electrons created due to the photoionization process and subsequently determine the cross-sections and rates of MPI. Experiments were done in air, O₂, Xe, Ar, N₂, Kr, and CO at room temperature and atmospheric pressure and femtosecond-laser pulse at 800 nm wavelength was utilized. Rayleigh microwave scattering (RMS) technique was used to obtain temporally resolved measurements of the electron numbers created by the laser. Numbers of electrons in the range 3 × 10⁸–3 × 10¹² were produced by the laser pulse energies 100–700 μJ and corresponding electron number densities down to about 10¹⁴ cm^-3 in the center of laser-induced spark were observed. After the laser pulse, plasma decayed on the time scale from 1 to 40 ns depending on the gas type and governed by two competing processes, namely, the creation of new electrons from ionization of the metastable atoms and loss of the electrons due to dissociative recombination and attachment to oxygen.

Diagnostics of combustion at high pressures are challenging due to increased collisional quenching and associated loss of acquired signal. In this work, resonance enhanced multiphoton photon ionization (REMPI) in conjunction with measurement of generated electrons by RMS technique were used to develop diagnostics method for measuring concentration of a component in gaseous mixture at elected pressure. Specifically, the REMPI-RMS diagnostics was developed and tested in the measurements of number density of carbon monoxide (CO) in mixtures with nitrogen (N₂) at pressures up to 5 bars. Number of REMPI-induced electrons scaled linearly with CO number density up to about 5×10¹⁸ cm^-3 independently of buffer gas pressure up to 5 bar, and this linear scaling region can be readily used for diagnostics purposes. Higher CO number densities were associated laser beam energy loss while travelling through the gaseous mixture. Four (4) energy level model of CO molecule was developed and direct measurements of the laser pulse energy absorbed in the two-photon process during the passage through the CO/N₂ mixture were conducted in order to analyze the observed trends of number of REMPI-generated electrons with CO number density and laser energy.