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Projekt Druckansicht

Investigation of transient phenomena in microplasmas by absorption spectroscopy and by laser-assisted "pump-probe"-experiments

Antragsteller Dr. Marc Böke
Fachliche Zuordnung Optik, Quantenoptik und Physik der Atome, Moleküle und Plasmen
Förderung Förderung von 2009 bis 2017
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 74729252
 
Erstellungsjahr 2016

Zusammenfassung der Projektergebnisse

The project A2 has contributed significantly to the broadening of understanding of atmospheric pressure microplasmas. Both, quantitative Tunable Laser Absorption Spectroscopy (TDLAS) and Broadband Absorption Spectroscopy (BBAS), together with electric and spectroscopic methods were employed to unravel in particular the role of metastable Hem and Arm atoms and of gaseous impurities. Some of the most important developments and findings are summarised below. The technique of TDLAS was significantly improved over conventional methods, increasing the sensitivity by about one order of magnitude, by employing lock-in techniques. A novel approach has been developed for BBAS involving a custom-designed femtosecond laser ’FemtoProbe’ (in cooperation with of Prof. Ilday, Bilkent University, Ankara). The laser delineates from one master oscillator two channels: a) one medium power channel with a nonlinear fiber delivers a broadband supercontinuum spectrum for BBAS and b) a high power channel allows ignition of the plasma by the laser pulse at the same time. Ignition of an rf-driven microplasma jet (μ-APPJ) was achieved at the sustainment voltage, well below the ignition voltage given by the Paschen criterion. Mapping the (absolute) concentrations of Hem and Arm over the full cross section of (μ-APPJ) revealed localisation of the metastables in the sheath of the plasma due to excitation by fast electrons. Details of the Hem and Arm profiles can be well-understood by the different excitation energies of Hem and Arm. Modelling (B1) and measurements in which the gap width of the discharge was decreased to 0.1 mm (a sheath-sheath plasma) shows good agreement of the profiles and points to the onset of a non neutral plasma at the lowest gap distance. Space- and time-resolved Hem densities have been measured in an rf-driven self-pulsing atmospheric pressure microplasma jet (SP μ-APPJ, with A1). The plasma exhibits a bright propagating constricted discharge during every pulse, co-existing with a homogeneous glow-mode (α-mode). In the sheath region of the constricted mode, densities reach the order of 10^13cm^−3, three orders of magnitude higher than in the homogeneous glow-mode. A significant concentration of Hem is also found well behind the constricted head of the discharge, instrumental for the re-ignition of the discharge at the next pulse and for its propagation. In a coaxial rf –driven jet (C1) operated in Ar single filaments with a diameter of 125 μm and lifetimes of a few 100 ns were observed with metastable densities of about 10^13cm^−3. Maps of the Arm evolution in the ignition phase of the discharge were determined showing highest densities about 3.5 μs after ignition. In the filamentary pulsed discharge (∼20 kHz) in a quartz tube of C2 the discharge current oscillates with a damped amplitude between intense positive pulses. The electric current flows not only through a plasma filament but also through the whole cross section of the quartz tube as a diffuse plasma. Inside the channels Arm densities of∼ 10^14cm^−3 were determined. Measuring the lifetime of Hem and Arm was used to quantify the level of impurities in the plasma. In a set-up with controlled atmosphere, a “titration” method was developed, quantifying e.g. the amount of air backstreaming into the jet when operated ambient air, or the amount of water vapour desorbing from surfaces in a micro parallel plate discharge (A4). Even minute concentrations of impurities have a profound influence on the stability of the discharges predominantly by quenching of the metastables. In cooperation with other projects of FOR 1123, a detailed and rather comprehensive picture of several discharge types was achieved. Hem and Arm atoms and impurities play a decisive role for the dynamics, stability and power balance of the discharges. The level of understanding has been significantly extended over the status before FOR 1123.

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