Abstract:

A brief review of a new advanced laser-based method for gas and gas flow nonintrusive diagnostics will be presented. The presentation will be based on recent results of experimental and theoretical studies of the coherent microwave radiation from a laser generated, collisional, weakly ionized plasma. For these studies we are working with plasma dimensions that are relatively small compared with the microwave wavelength. In this case the microwave scattering signal strength can be modeled as scattering from an oscillating plasma dipole, similar to the scattering of an atom in light. If the skin-layer at microwave frequency is greater than the characteristic size of the plasma, the scattering of the microwaves falls in the far field into the Rayleigh scattering regime. The rate of increase and decay of the microwave scattered signal reflect the time evolution of the laser induced plasma – from breakdown to post-breakdown plasma decay. Then, we suggested a new powerful method of remote detection called Radar-REMPI in which the microwave was scattered from the plasma that was selectively produced by the Resonance Enhanced Multiphoton Ionization. We demonstrated capabilities of our approach for the high accurate direct in-situ measurements of the electron loss rate in air and in arbitrary gas mixtures. As an example, the attachment rate for electrons to molecular oxygen in room temperature, atmospheric pressure air was determined at different levels of humidity. In a gas mixture the REMPI of a relatively small density component can catalyze the avalanche ionization process in a buffer gas created by a laser beam at very low intensity, which is not enough for initiation of the avalanche breakdown by itself. This additional ionization in the bulk gas results as amplifier for the intensity of microwave scattering radiation. If gas is moving, the Doppler shift of the Radar-REMPI or Radar-Laser Spark signal can give information about the bulk gas velocity.

About the speaker:

Dr. Mikhail N. Shneider received a master’s degree in theoretical physics (with honors) from the Kazan State University, Russia, a Ph.D. in Plasma physics and Chemistry from All-Union Electrotechnical Institute, Moscow and Doctor of Sciences (highest scientific degree in Russia) in Plasma physics and Chemistry from Institute for High Temperatures, Russian Academy of Sciences, Moscow. Since 1998 until the present, Dr. Shneider has been working at the Mechanical and Aerospace Engineering Department, Princeton University. At present he is a Senior Scientist in the Applied Physics Group. His research interests are in the theoretical study of gas discharge physics; physical gas dynamics; biophysics, atmospheric electrical phenomena; non-linear optics and laser-matter interaction. Dr. Shneider was invited many times as a guest professor to universities in Austria, China, France, Germany, Great Britain and Russia. He has about 200 papers in refereed journals (9 review papers), 3 US patents and two books.