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Accueil > Séminaires et conférences > Séminaires, soutenances de thèses et HDR précédents > 2018 > Séminaires 2018 > Le mercredi 21 mars 2018 à 10h

Le mercredi 21 mars 2018 à 10h

The seminar "Laser Diagnostics Using Ultrashort Pulses for Plasmas and Gas Flows" by Dr Tat Loon CHNG

will take place on 21 of March (Wednesday) at 10h00 at LPP Palaiseau, meeting room (Salle) CPHT Ex LIX Aile 0 (GPS 48.714027, 2.207931, the ground
floor).

Laser Diagnostics Using Ultrashort Pulses for Plasmas and Gas Flows
Particularly over the last decade, the growing ease of access to ultrashort pulsed laser sources has driven the development of novel methods for measuring quantities such as gas concentrations, velocities, temperatures and even electric fields. At the same time, the potential benefits of operating in the ultrashort regime (< 100 psec) have also warranted a re‐examination of existing laser‐based techniques, using these short pulses instead as the
source of excitation. In this talk, I will discuss some of these main benefits and describe four such methods – Radar REMPI, Air Lasing, Xenon Flow Tagging Velocimetry and electric field measurements using based upon the field induced second harmonic generation – which could be of interest to the plasma and combustion community. Radar Resonance Enhanced Multiphoton Ionization (REMPI) combines the selective ionization capabilities of REMPI spectroscopy with the high detection sensitivity of coherent microwave scattering (radar) for the measurements of gas concentrations. Part per billion (ppb) concentrations under ambient conditions and part per million (ppm) concentrations in a flame have been successfully demonstrated using this technique. “Air Lasing” or the act of creating a laser from the major constituents of air is an ongoing research topic which has attracted significant interest in the standoff detection community.
The ability to generate a coherent probe beam at a distance, makes such an approach a credible alternative to traditional counterparts such as LIDAR (Light Detection and Ranging). More recently, this approach has been extended to include the minor species of air and may even hold promise for remote detection of trace gases. Xenon Flow Tagging Velocimetry is a method in which a femtosecond pulse is focused into Xe gas, exciting a ‘line’ of Xe atoms along the focal axis. These ‘tagged’ atoms subsequently
fluoresce and the time evolution of this emission may be captured using a time‐gated, intensified camera. The fluorescence is long‐lived, typically on the order of tens of microseconds. Determining the motion of the displaced line thus provides a measure of the flow velocity, and may be of relevance to applications (such as thrusters) which utilize Xe gas. In addition, the laser excitation may be tailored such that the light fluoresces to a metastable state, and affords the possibility of re‐excitation and fluorescence in order to
prolong the signal lifetime. An optical electric field measurement method for use in high pressure plasma discharges is based upon the field induced second harmonic generation technique and can be used for localized electric field measurements with sub‐nanosecond resolution in any gaseous
species. When an external electric field is present, a dipole is induced in the typically centrosymmetric medium, allowing for second harmonic generation with signal intensities which scale by the square of the electric field.
All graduate students and staff with an interest in laser diagnostics for plasmas are most welcome to attend.

Dans la même rubrique :

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Tutelles : CNRS Ecole Polytechnique Sorbonne Université Université Paris Sud Observatoire de Paris Convention : CEA
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Mentions légales
Exploitant du site : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Hébergeur : Laboratoire de Physique des Plasmas, Ecole Polytechnique route de Saclay F-91128 PALAISEAU CEDEX
Directeur de la publication : Pascal Chabert (Directeur)