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Laser spectroscopic determination of electric fields, electron densities, and electron temperatures in atmospheric pressure microdischarges

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

Laser electric field measurement based on four wave mixing on diatomic molecules at atmospheric pressures was established in hydrogen and nitrogen, the latter as a novelty. Temporal resolution of about 4 ns with a dye laser and a few hundred ps with a ps-laser and a Raman cell was achieved. The former case was done in collaboration with Osaka University / Japan and the latter case was realised in collaboration with Ohio State University / USA. The technique was applied to the investigation of various ns-pulsed discharges. In combination with a simple model and current measurements also plasma densities could be obtained. It could be shown that in ns-pulsed discharges effectively all ionisation is during the fast ignition phase when the reduced electric field strongly exceeds the quasi DC value in the later semi-stationary phase. In dielectric surface wave discharges the spatial-temporal evolution of the electric field vector of the ionisation wave could be traced for the first time. This work was carried out together with Ohio State University / USA. In collaboration with the INP in Greifswald the electric field in a single streamer was measured temporally resolved and the surface charge density could be inferred. Finally, in collaboration with Technical University Eindhoven the electric field in the head of a so called “plasma bullet” was measured for the first time temporally and spatially resolved and correlated to the optical emission. A ns-second pulsed jet in helium has been developed. The jet serves as a model discharge for ns-discharges in helium. The evolution of the entire discharge was traced by Thomson scattering, laser absorption measurements, and emission spectroscopy with high spatial and temporal resolution. This is complemented by electrical measurements. Thomson scattering allowed for the first time an experimental determination of the EVDF in the discharge. Further, a novel technique was developed that provided also for the first time absolute densities of Rydberg states of the helium excimer molecule. Based on rate coefficients from the literature, it was possible to calculate ab initio all plasma parameters and excited state population by a model. Excellent agreement with the measurements was found. Last but not least, a ns-self-pulsing micro-thin cathode discharge was developed and investigated by emission spectroscopy, laser absorption spectroscopy, and electrical measurements. With less than 1 kV DC voltage applied, the discharge produces current pulses of about 30 A in 2 ns pulses which are discharged over a narrow channel of only a few 10 micrometer diameter. It could be shown that the plasma is fully ionised at the end of the current pulse. In the optical emission, a broad continuum from the UV to the near IR is visible directly after ignition. Bremsstrahlung could be excluded as the origin of this radiation but the physical cause for the continuum is still unclear.

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