| AVS 60th International Symposium and Exhibition | |
| Plasma Science and Technology | Tuesday Sessions |
| Session PS1-TuA |
| Session: | Plasma Diagnostics, Sensors and Control |
| Presenter: | S. Welzel, Eindhoven University of Technology, Netherlands |
| Authors: | S. Welzel, Eindhoven University of Technology, Netherlands F. Brehmer, Eindhoven University of Technology, Netherlands; AFS GmbH, Germany M.C.M. van de Sanden, Eindhoven University of Technology; DIFFER, Netherlands R. Engeln, Eindhoven University of Technology, Netherlands |
| Correspondent: | Click to Email |
The detection of stable and transient species along with gas temperature measurements remains a challenge for the majority of molecular (complex) plasmas. Considering particularly plasmas at atmospheric pressure with inherently small discharge volumes, (optical) access to the active plasma is often hampered. On the other hand, phase- and time-resolved in-situ measurements are a valuable tool (i) to establish heavy particle temperatures, (ii) to identify excitation mechanisms in the plasma, (iii) to discriminate gas phase and surface reactions as they occur on significantly different time scales, and (iv) to unravel particularly temperature-dependent reaction mechanisms. Modern mid-infrared laser sources, known as quantum cascade lasers (QCLs), provide a means for highly time-resolved absorption spectroscopy in the molecular "fingerprint" region. The time-resolution can be thereby as good as a few tens of nanoseconds. Although continuous-wave QCLs are increasingly being applied for conventional monitoring purposes, pulsed distributed feedback QCLs are perfectly suited for diagnostic studies on transient plasmas.
CO2 containing dielectric barrier discharges (DBDs) operated in the mid-frequency (kHz) range were studied by means of in-situ time-resolved QCL absorption spectroscopy. Special beam shaping optics was used to accommodate the laser beam diameter to typical gap widths of ~ 1 mm in single and multiple-pass configuration. Different synchronisation schemes were applied to achieve phase-resolved measurements during individual AC cycles as well as to monitor molecular absorption signals during pulsed discharge operation. Mixing ratios of CO in its electronic and vibrational ground state were of the order of a few percent and thus confirmed earlier ex-situ studies of the effluent. More importantly, the concentrations levels were changing only slowly in time, i.e. of the order of the residence time. A direct CO2-to-CO dissociation through electron impact appears very unlikely under these conditions. The kinetics of low-lying ro-vibrational states of CO2 along with the evolution of the CO concentration were studied on a sub-millisecond time-scale to (i) establish (rotational) gas temperatures, and (ii) to estimate the influence of vibrational-vibrational and vibrational-translational energy transfer processes.