Greifswald, Germany
Greifswald, Germany

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Ropcke J.,INP Greifswald | Davies P.B.,University of Cambridge | Lang N.,INP Greifswald | Rousseau A.,University Paris - Sud | Welzel S.,TU Eindhoven
Journal of Physics D: Applied Physics | Year: 2012

Over the past few years mid-infrared absorption spectroscopy based on quantum cascade lasers operating over the region from 3 to 12m and called quantum cascade laser absorption spectroscopy or QCLAS has progressed considerably as a powerful diagnostic technique for in situ studies of the fundamental physics and chemistry of molecular plasmas. The increasing interest in processing plasmas containing hydrocarbons, fluorocarbons, nitrogen oxides and organo-silicon compounds has led to further applications of QCLAS because most of these compounds and their decomposition products are infrared active. QCLAS provides a means of determining the absolute concentrations of the ground states of stable and transient molecular species at time resolutions below a microsecond, which is of particular importance for the investigation of reaction kinetics and dynamics. Information about gas temperature and population densities can also be derived from QCLAS measurements. Since plasmas with molecular feed gases are used in many applications such as thin film deposition, semiconductor processing, surface activation and cleaning, and materials and waste treatment, this has stimulated the adaptation of QCLAS techniques to industrial requirements including the development of new diagnostic equipment. The recent availability of external cavity (EC) QCLs offers a further new option for multi-component detection. The aim of this paper is fourfold: (i) to briefly review spectroscopic issues arising from applying pulsed QCLs, (ii) to report on recent achievements in our understanding of molecular phenomena in plasmas and at surfaces, (iii) to describe the current status of industrial process monitoring in the mid-infrared and (iv) to discuss the potential of advanced instrumentation based on EC-QCLs for plasma diagnostics. © 2012 IOP Publishing Ltd.

Kettlitz M.,INP Greifswald | Hoft H.,INP Greifswald | Hoder T.,INP Greifswald | Weltmann K.-D.,INP Greifswald | Brandenburg R.,INP Greifswald
Plasma Sources Science and Technology | Year: 2013

Experiments on the spatial and temporal structure of the breakdown process of sinusoidal- and pulsed-operated dielectric barrier microdischarges (MDs) are compared. Three different waveforms are considered: a sinusoidal waveform at 20 kHz and pulsed-bipolar and unipolar-voltage profiles at 10 kHz with varying duty cycles (asymmetric pulse). Electrical data and simultaneous streak and iCCD images of individual MDs in dielectric barrier discharges (DBDs) with 1 mm gap in a gas mixture of 0.1 vol% O2 in N2 at atmospheric pressure are recorded. For sinusoidal-operated DBDs there are no significant differences between the MDs at positive and negative half-periods. Sinusoidal operation corresponds to pulsed-bipolar operation with symmetrical pulses, but with lower streamer velocities and different spatio-temporal emission distribution. The development of pulsed-driven MDs is determined by the voltage between both electrodes and not by the polarity of the driven electrode, resulting in nearly the same behavior of bipolar- and unipolar-pulsed-driven MDs. DBDs operated with asymmetric pulses show a significant difference in the spatial structure and in the temporal behavior between the rising and falling slopes of the high voltage pulse. © 2013 IOP Publishing Ltd.

Zalach J.,INP Greifswald | Franke S.,INP Greifswald
LEUKOS - Journal of Illuminating Engineering Society of North America | Year: 2013

The analysis of plasma temperatures by self-reversed mercury lines is well established in the diagnostics of HID lamps. In mercury-free HID lamps or mercury containing lamps including a complex fill, however, the analysis of mercury lines could be impossible or difficult because additives may disturb the spectral line shape. This work analyzes the suitability of self-reversed scandium lines for temperature determination using Bartels method. Temperatures obtained by the three scandium lines analyzed here were found to agree very well with results from mercury lines, demonstrating their applicability as a diagnostic tool in multicomponent HID lamps. © 2013 The Illuminating Engineering Society of North America.

Zalach J.,INP Greifswald | Franke S.,INP Greifswald
Journal of Applied Physics | Year: 2013

The Boltzmann plot method allows to calculate plasma temperatures and pressures if absolutely calibrated emission coefficients of spectral lines are available. However, xenon arcs are not very well suited to be analyzed this way, as there are only a limited number of lines with atomic data available. These lines have high excitation energies in a small interval between 9.8 and 11.5 eV. Uncertainties in the experimental method and in the atomic data further limit the accuracy of the evaluation procedure. This may result in implausible values of temperature and pressure with inadmissible uncertainty. To omit these shortcomings, an iterative scheme is proposed that is making use of additional information about the xenon fill pressure. This method is proved to be robust against noisy data and significantly reduces the uncertainties. Intentionally distorted synthetic data are used to illustrate the performance of the method, and measurements performed on a laboratory xenon high pressure discharge lamp are analyzed resulting in reasonable temperatures and pressures with significantly reduced uncertainties. © 2013 American Institute of Physics.

Welzel S.,INP Greifswald | Welzel S.,TU Eindhoven | Ropcke J.,INP Greifswald
Applied Physics B: Lasers and Optics | Year: 2011

Quantum cascade lasers (QCLs) have attracted considerable interest as an alternative tuneable narrow bandwidth light source in the mid-infrared spectral range for chemical sensing. Pulsed QCL spectrometers are often used with short laser pulses and a bias current ramp similar to diode laser spectroscopy. Artefacts in the recorded spectra such as disturbed line shapes or underestimated absorption coefficients have been reported. A detailed time-resolved high-bandwidth analysis of individual pulses during a laser sweep has been performed. Quantitative results for CH4 absorption features around 1347 cm-1 (7.42 μm) fell short of the expected values for reasonable operating conditions of the QCL. The origin of the artefacts using short pulses was identified to be partly of the same nature as in the case of long laser pulses. A complex combination with the tuning principle was found, leading to an apparently increased instrumental broadening (effective line width) and underestimated concentrations at low-pressure conditions. © 2010 The Author(s).

Schafer J.,INP Greifswald
The Review of scientific instruments | Year: 2012

The heat convection generated by micro filaments of a self-organized non-thermal atmospheric pressure plasma jet in Ar is characterized by employing laser schlieren deflectometry (LSD). It is demonstrated as a proof of principle, that the spatial and temporal changes of the refractive index n in the optical beam path related to the neutral gas temperature of the plasma jet can be monitored and evaluated simultaneously. The refraction of a laser beam in a high gradient field of n(r) with cylindrical symmetry is given for a general real refraction index profile. However, the usually applied Abel approach represents an ill-posed problem and in particular for this plasma configuration. A simple analytical model is proposed in order to minimize the statistical error. Based on that, the temperature profile, specifically the absolute temperature in the filament core, the FWHM, and the frequencies of the collective filament dynamics are obtained for non-stationary conditions. For a gas temperature of 700 K inside the filament, the presented model predicts maximum deflection angles of the laser beam of 0.3 mrad which is in accordance to the experimental results obtained with LSD. Furthermore, the experimentally obtained FWHM of the temperature profile produced by the filament at the end of capillary is (1.5 ± 0.2) mm, which is about 10 times wider than the visual radius of the filament. The obtained maximum temperature in the effluent is (450 ± 30) K and is in consistence with results of other techniques. The study demonstrates that LSD represents a useful low-cost method for monitoring the spatiotemporal behaviour of microdischarges and allows to uncover their dynamic characteristics, e.g., the temperature profile even for challenging diagnostic conditions such as moving thin discharge filaments. The method is not restricted to the miniaturized and self-organized plasma studied here. Instead, it can be readily applied to other configurations that produce measurable gradients of refractive index by local gas heating and opens new diagnostics prospects particularly for microplasmas.

Hoft H.,INP Greifswald | Huiskamp T.,TU Eindhoven | Kettlitz M.,INP Greifswald | Pemen A.J.M.,TU Eindhoven
IEEE Transactions on Plasma Science | Year: 2014

Dielectric barrier discharges generated in a coaxial tip to cylinder configuration at atmospheric pressure by a 5-ns high-voltage pulse with sub-ns rise time were investigated with a fast intensified CCD camera. Different discharge characteristics were found for the breakdown during the pulse itself and in the subsequent reflections of the pulse. There is a transition from a complete diffuse regime to a breakdown in constricted channels, which finally cross the whole gap. © 2014 IEEE.

Sigeneger F.,INP Greifswald | Loffhagen D.,INP Greifswald
Plasma Sources Science and Technology | Year: 2016

The filaments occurring in an RF argon atmospheric-pressure plasma jet are investigated by means of numerical modelling. The special setup of the jet leads to the establishment of filaments in very regular modes under certain conditions. Such a single filament generated in the active volume between the powered and grounded electrode is described by a time-dependent, spatially two-dimensional fluid model. This self-consistent model includes those mechanisms which can lead to constriction and stratification such as the heat balance equation and the dependence of electron collision rate coefficients on the ionization degree. A curved filament with a contracted radial profile of particle densities and very pronounced striations along its trace has been obtained by the model calculation for a typical discharge parameter condition of the plasma jet. The resulting calculated electron density and mean energy in the filament as well as the period length of the striations agree qualitatively with recent experimental observations. The analysis of the ionization budget makes clear that the constriction and stratification is mainly caused by the different nonlinear dependences of ionization and recombination rates on the electron density. © 2016 IOP Publishing Ltd.

Gnybida M.,TU Eindhoven | Uhrlandt D.,INP Greifswald | Loffhagen D.,INP Greifswald
Journal of Physics D: Applied Physics | Year: 2012

The pulsed discharge in xenon at pressures between 10 and 50Torr and a peak current of 130mA has been analysed by means of a time-dependent, spatially one-dimensional fluid model. Main features of the model of the radially inhomogeneous discharge plasma in a discharge tube with an inner diameter of 6.5mm are given. The comparison of results of model calculations with experimental data shows good agreement for the axial electric field, and the qualitative behaviour of measured axis densities of low-lying excited states is reproduced well by the model. The analysis of the spatiotemporal variation of the pulsed discharge shows the formation of a constricted column plasma at increasing pressure. In particular, a pronouncedly nonlocal behaviour of the electron component is found, and the densities of the metastable and resonance atoms are predicted to have a radial profile with a maximum out of the axis during the discharge phase. © 2012 IOP Publishing Ltd.

Kettlitz M.,INP Greifswald | Hoft H.,INP Greifswald | Hoder T.,INP Greifswald | Reuter S.,INP Greifswald | And 2 more authors.
Journal of Physics D: Applied Physics | Year: 2012

The paper presents experiments on the spatial and temporal structure of the breakdown process of microdischarges (MD). For the first time simultaneous streak and iCCD images of individual filaments in a pulsed driven dielectric barrier discharge (DBD) with 1mm gap in a gas mixture of 0.1vol% O 2 in N 2 at atmospheric pressure are recorded. Furthermore current and voltage measurements with fast probes give access to the electrical discharge characteristics such as transferred charge and power. For asymmetric pulses there is a significant difference in the spatial structure as well as in the temporal behaviour of the MDs between the rising and the falling slopes of the high voltage pulse. If the time between rising and falling slopes is reversed all effects reverse as well. For symmetric pulses there are no significant differences between the MD at rising and falling slopes which is in accordance with a sinusoidal DBD operation. © 2012 IOP Publishing Ltd.

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