M Kettlitz

On the spatio-temporal development of pulsed barrier discharges: influence of duty cycle variation

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 1?mm gap in a gas mixture of 0.1?vol% O2 in N2 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.

Novel insights into the development of barrier discharges by advanced volume and surface diagnostics

The comprehensive characterization of microdischarges (MDs) requires complementary diagnostics of volume and surface processes at the same discharge configuration under identical conditions. This contribution summarizes the results from optical, spectroscopic and electric investigations as well as the determination of surface charges and metastable nitrogen molecules in filamentary and diffuse barrier discharges. The feasibility of such an approach is demonstrated on selected examples.Fast optical and spectroscopic methods are reviewed for the example of a pulsed driven single filament dielectric barrier MD. It is demonstrated that the methods of streak recording and cross-correlation spectroscopy can complement one another for a comprehensive study of the MD development. Using these techniques it is shown that the so-called prephase is present also in sub-microsecond pulsed barrier discharges. The excitation starts directly with the voltage increase. In the case of diffuse barrier discharges in nitrogen, the combination of spectroscopic and electrical characterization, surface charge measurement by the Pockels effect, and the determination of nitrogen metastables N2(A) by laser-induced fluorescence provides detailed knowledge about the time-integrated surface charge which correlates with the discharge current for each half cycle, whereas the temporal maximum of the metastables of the order of few 1013?cm?3 is delayed in relation to the current maximum. The spatial (axial) maximum of the metastable density is located near the anode like the emission maximum from N2 second positive system at ??=?337?nm. Furthermore, the lifetime of surface charges beyond a typical discharge period has been investigated.

Breakdown characteristics in pulsed-driven dielectric barrier discharges: influence of the pre-breakdown phase due to volume memory effects

The pre-phase of the breakdown of pulsed-driven dielectric barrier discharges (DBDs) was investigated by fast optical and electrical measurements on double-sided DBDs with a 1 mm gap in a gas mixture of 0.1 vol% O2 in N2 at atmospheric pressure. Depending on the pulse width (the pause time between subsequent DBDs), four different breakdown regimes of the following discharge were observed. By systematically reducing the pulse width, the breakdown characteristics could be changed from a single cathode-directed propagation (positive streamer) to simultaneous cathode- and anode-directed propagations (positive and negative streamer) and no propagation at all for sub-μs pulse times. For all cases, different spatio-temporal emission structures in the pre-phase were observed. The experimental results were compared with time-dependent, spatially one-dimensional fluid model calculations. The modelling results confirmed that different pre-ionisation conditions, i.e. considerably high space charges in the volume created by the residual electrons and ions from the previous discharge, are the reason for the observed phenomena.

Barrier discharges driven by sub-microsecond pulses at atmospheric pressure: Breakdown manipulation by pulse width

Barrier discharges at atmospheric pressure in nitrogen-oxygen mixture powered by high voltage pulses of widths between 10 μs and 200 ns were investigated. The development of the microdischarges on rising and falling slopes was recorded by streak and intensified CCD cameras simultaneously. The breakdown on the falling slope strongly depends on the pulse width. As a result of pulse width variation the starting point of ignition changes and positive and negative streamers occur simultaneously in the falling slope. The observed effect is caused by the electric field rearrangement in the gap due to the different positive ion densities related to their gap crossing times.

Breakdown characteristics of high pressure xenon lamps

An investigation of the breakdown of high intensity discharge (HID) lamps filled with xenon at pressures from 0.1 to 5 bar is presented. Three power supplies were used in order to provide voltage rates of increase covering about four orders of magnitude from 5 mV ns−1 to 100 V ns−1, the latter being typical for electronic ballasts driving commercial HID lamps. Customized lamps ensure a volume breakdown between the tungsten tip electrodes of the lamp. Voltage and current waveforms were measured by means of electrical probes and the transient optical radiation was captured by a fast camera system. The breakdown voltage increases with growing pressure and voltage rate up to several 10 kV. Additional UV illumination decreases the breakdown voltage and reduces its mean variation. The experimental results were reproduced with good agreement by a fluid model taking into account the electron energy balance. The model shows an ionization front propagating towards the cathode. The front moves due to continuous field compression and relies on electron avalanches initiated by secondary electrons at the cathode.

Plasma diagnostics in Hg-free short-arc lamps for automotive lighting

In the context of developing a mercury-free lamp for automotive lighting an optimization of lamp design and plasma radiation was performed. In contrast to existing quartz lamps, a new lamp design was chosen consisting of a sapphire capillary combined with ceramic parts and pure tungsten electrodes. The lamps consist of metal halides and between 1 and 20 bar of xenon. They were operated with an electronic ballast at input powers between 20 and 40 W. Besides an optimization of the tube materials, thermography and plasma diagnostics were performed to understand the processes inside the lamp and to find optimal operation conditions. In addition to experimental diagnostics, a simulation of self-reversed sodium spectral lines was performed to verify plasma parameters, particularly the xenon pressure which could not be determined from the experiment. Additionally, tendencies of the influence of single components could be estimated by modelling.

Temperature profiles of an ablation controlled arc in PTFE: I. Spectroscopic measurements

A wall-stabilized arc in a polytetrafluoroethylene nozzle was investigated. Very similar arc properties as in the ablation dominated high-current phase of SF6 circuit breakers were expected. Spectroscopic measurements of arc temperature profiles were performed. Spectral lines of fluorine and ionized carbon were observed. The side-on intensities of optically thin spectral lines were measured. Abel inversion was applied and the emission coefficient was determined. Temperatures of 16 000 to 18 000 K in the arc centre were obtained at various times of the current pulse. The pressure values were in the range 10–30 bar and arc currents varied from 10 to 20 kA. In the framework of the model, radial temperature profiles were determined with an accuracy of ±4%.

Pyrometric cathode temperature measurements in metal halide lamps

Time-averaged temperature distributions along the electrodes of vertically operated high-intensity discharge lamps with cylindrical quartz burners filled with mercury and additives of NaI, TlI and DyI3 have been measured. The lamps have been driven by 120?Hz switched-dc currents between 0.4 and 1.78?A and the measurements have been performed during the cathodic phase at the lower electrode. All considered currents are characterized by a diffuse arc attachment. For the correction of disturbing effects the measured distributions have been fitted with solutions of the quasi-one-dimensional and steady-state energy balance of a rod-shaped tungsten cathode. A model of the near-cathode layer in a multi-species plasma has been applied for the determination of the boundary layer characteristics where the work function has been treated as a free parameter. The required plasma component concentrations have been estimated from spectroscopic measurements in the arc column. The fit procedure includes the adjustment of the extension of the lateral arc attachment region which has a distinct impact on the determined tip temperature, power input from the plasma into the cathode and the work function. For the latter the tungsten value has been verified in the pure Hg lamp, but strong deviations result in the case of a TlI admixture. The lamp with DyI3 clearly shows the gas-phase emitter effect of lowering the work function induced by Dy.

Dimming Characteristics of Metal-Halide Plasma Lamps

The dimming of metal-halide plasma lamps means leaving the optimized technical lamp performance. As a consequence, the discharge behavior changes, which affects the photometric properties, the efficiency, and the lifetime of the lamps. In order to understand the plasma physical mechanisms of the dimming process, a simplified lamp system was studied, providing good comparability between the experiment and the model.

The bidirectional character of O2 concentration in pulsed dielectric barrier discharges in O2/N2 gas mixtures

This paper presents experimental results on the influence of O2 on the characteristics of dielectric barrier discharges (DBDs) at one and at half atmospheric pressure. Gas mixtures of 0.1–10 vol % O2 in N2 were investigated, as well as in virtually pure N2. Electrical data, simultaneous streak and intensified charge-coupled device images were recorded in pulsed driven dielectric barrier discharges of 0.8 mm gap in a single filament arrangement. The O2 concentration is shown to have a significant impact on the electrical characteristics, the temporal DBD development and its breakdown inception. Higher O2 concentrations (above 0.1 vol %) led to an ignition delay, a shorter discharge duration, increased discharge radius, higher discharge current maxima and larger velocities of the cathode directed streamers. For O2 concentrations below 0.01 vol %—i.e. nearly pure nitrogen—some of these effects were reversed. Moreover, the effects were more pronounced at a pressure of 0.5 bar compared to 1 bar. This result can be explained by the pressure dependent decay and recombination processes of positive nitrogen and oxygen ions.