H Höft

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.

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.

Impact of gas flow rate on breakdown of filamentary dielectric barrier discharges

The influence of gas flow rate on breakdown properties and stability of pulsed dielectric barrier discharges (DBDs) in a single filament arrangement using a gas mixture of 0.1 vol. % O2 in N2 at atmospheric pressure was investigated by means of electrical and optical diagnostics, accompanied by fluid dynamics and electrostatics simulations. A higher flow rate perpendicular to the electrode symmetry axis resulted in an increased breakdown voltage and DBD current maximum, a higher discharge inception jitter, and a larger emission diameter of the discharge channel. In addition, a shift of the filament position for low gas flow rates with respect to the electrode symmetry axis was observed. These effects can be explained by the change of the residence time of charge carriers in the discharge region—i.e., the volume pre-ionization—for changed flow conditions due to the convective transport of particles out of the center of the gap.

Determination of the electric field strength of filamentary DBDs by CARS-based four-wave mixing

It is demonstrated that a four-wave mixing technique based on coherent anti-Stokes Raman spectroscopy (CARS) can determine the electric field strength of a pulsed-driven filamentary dielectric barrier discharge (DBD) of 1 mm gap, using hydrogen as a tracer medium in nitrogen at atmospheric pressure. The measurements are presented for a hydrogen admixture of 10%, but even 5% H2 admixture delivers sufficient infrared signals. The lasers do not affect the discharge by photoionization or by other radiation-induced processes. The absolute values of the electric field strength can be determined by the calibration of the CARS setup with high voltage amplitudes below the ignition threshold of the arrangement. This procedure also enables the determination of the applied breakdown voltage. The alteration of the electric field is observed during the internal polarity reversal and the breakdown process. One advantage of the CARS technique over emission-based methods is that it can be used independently of emission, e.g. in the pre-phase and in between two consecutive discharges, where no emission occurs at all.

PPPS-2013: This is a sample abstract submission dielectric barrier discharges: Pulsed breakdown, electrical characterization and Chemistry

Summary form only given. The application of atmospheric pressure discharges in new fields like environmental protection, surface treatment or life-sciences requires a profound knowledge on the plasma parameters and properties. This includes (1) the characterization of the breakdown processes triggering plasma chemistry, (2) the proper determination of the electrical parameters and (3) the description of the dominant chemical pathways. The contribution aims to present new approaches regarding these three topics for pulsed driven Dielectric Barrier Discharges in particular. It will be shown by fast electrical, optical and spectroscopic methods that the ignition, breakdown statistics and spatio-temporally resolved development of pulsed DBD microdischarges is controlled by the properties of the power supply (duty cycle, frequency, amplitude varied) as well as the composition of the gas1. In particular the starting point of the microdischarge ignition can be changed which is a new effect in DBDs caused by electric field rearrangement in the gap due to positive ion development. Surface processes at the dielectric barriers influencing this behavior will be discussed, too. The determination of electrical parameters such as discharge current, gas gap voltage, instantaneous power and energy as well as the charge transferred through the gas gap based on a simple equivalent circuit will be presented. The proposed approach accurately accounts the displacement current and key capacitance values, which inexactly determination are a source of experimental errors in particular in case of pulsed driven DBDs. The presented approach is consistent with sinusoidal-voltage driven or miniature pulsed driven DBDs. We believe that these new insights on electrical characterization are an important input for those who are working with DBDs, since the electrical parameters are mandatory information. The characterization of the dominant chemical pathways of advanced plasma processes is usually focused on the volume processes only. This contribution will discuss several examples which shall emphasize, that secondary effects must be considered, too. These shall cover the topics of adsorption-enhanced VOC conversion by DBD plasma treatment, NOx conversion and indirect plasma treatment of liquids for antimicrobial and chemical decontamination.

Dielectric barrier discharges: Pulsed breakdown, electrical characterization and Chemistry

The application of atmospheric pressure discharges in new fields like environmental protection, surface treatment or life-sciences requires a profound knowledge on the plasma parameters and properties. This includes the characterization of the breakdown processes triggering plasma chemistry, the proper determination of the electrical parameters and the description of the dominant chemical pathways. This contribution aims to present new approaches regarding these three topics for pulsed driven Dielectric Barrier Discharges (DBDs) in particular. Fast electrical, optical and spectroscopic methods enable the study of ignition, breakdown statistics and spatio-temporally resolved development of pulsed DBD microdischarges. The determination of electrical parameters such as discharge current, gas gap voltage, instantaneous power and energy as well as the charge transferred through the gas gap is based on a simple equivalent circuit which is consistent with sinusoidal-voltage driven or miniature pulsed driven DBDs. The characterization of the dominant chemical pathways of advanced plasma processes discusses also several examples including secondary effects, such as adsorption-enhanced VOC conversion by DBD plasma treatment.

On the spatio-temporal development of pulsed barrier discharges

Summary form only given. There is an increasing interest in atmospheric pressure discharges for technological applications. Due to their low gas temperature, high electron energy and the presence of active species these discharges have a high potential for plasma chemistry e.g. for ozone production as well as gas and water purification1. Besides corona discharges and micro hollow cathode discharges, dielectric barrier discharges are often used in this field of application. Here, sinusoidal driven ones are basically used in practice. In last years however, there are applications where pulsed driven discharges show a better efficiency in plasma-chemical application (e.g. ozone generation2) than sinusoidal driven ones. This finding forces systematic studies in this matter.