Hans-Erich Wagner

An investigation of dielectric barrier discharge in Ar and Ar/NH3 mixture using cross-correlation spectroscopy

Dielectric barrier discharges (BDs) are known to operate in two distinctive modes. The filamentary mode of BD is characterized by a large number of short lasting spatially bounded microdischarges. This type of discharge is typical for most cases of BDs at atmospheric pressure. Under some specific conditions another form of BD may arise. In this mode plasma uniformly covers the whole electrode area. This mode is usually referred to as the diffuse or homogeneous mode of BD.This work presents studies of the filamentary mode of BD in argon and its transition to the diffuse mode by ammonia addition. The discharges were investigated by means of cross-correlation spectroscopy. Particularly, the influence of electrode shape, discharge gap and the influence of ammonia admixture on discharge development were studied. The measurement offers results with high temporal and spatial resolution which are useful for comparison with results of numerical models.The obtained results include the electrical current measurement of discharge in several ammonia admixtures to argon. The diffuse discharge appeared at ammonia admixture above 3 vol%. The observed propagation of streamer in discharge in pure argon for two different electrode configurations is presented. When compared with discharge in pure argon the streamer velocity is decreased for 0.1 vol% ammonia admixture. With increasing ammonia concentration the streamer velocity increases again. The behaviour of presented impurities was observed and the results are included in this report. The basic model of the discharge kinetic is included. For reduced electric field below 10 Td the electron drift is much higher in argon–ammonia mixture than in pure argon.

Progress in the Visualization of Filamentary Gas Discharges. Part 1: Milestones and Diagnostics of Dielectric-barrier Discharges by Cross-correlation Spectroscopy

Abstract Filamentary gas discharges at atmospheric pressure, namely dielectric-barrier discharges and coronas, produce highly non-equilibrium plasmas in a controllable way at a moderate gas temperature. They provide the effective generation of atoms, radicals and excited species by energetic electrons. Consequently, they have found many applications, among others in environmental protection. The controlled application of these discharges requires a good understanding of their physical properties and the knowledge of the basic plasma parameters. Milestones on this way and the progress in the plasma diagnostics are listed of. Recently, the technique of spatially resolved cross-correlation spectroscopy (CCS) (with a resolution in the sub-ns and sub-mm range) has been used to contribute to a better understanding of these discharges. The application of the CCS method is described in more detail.

Investigations on the stability of the low pressure positive column in oxygen

The stability of the low pressure positive column in oxygen was investigated in a pressure range from 0.5 to 0.9?Torr within a discharge current interval from 0.5 to 90?mA. The transition between the well-known T- and H-modes has been studied. The H- to T-mode transition showed a marked hysteresis in the E(I) characteristic which is affected by wall processes. For the first time temporally resolved electric field measurements were realized. At small discharge current the electric field showed a significant modulation, characterized by incoherent fluctuations with a broadband Fourier spectrum. With increasing current the discharge operates in the T-mode, where a mode selection with high modulation degree occurred, resulting in a periodic oscillation of the electric field at a discrete frequency spectrum. The dynamic state in the T-mode is expressed by T-waves moving from the cathode to the anode. It seems that they were excited by oscillations in the cathode region. The waves were damped in the direction of the anode and show no dispersion.The discharge stability was studied using a hydrodynamic model considering electrons, positive and negative ions as well as metastable O2(a?1?g) molecules. Here the negative O?-ions play a crucial role. In good agreement with the experiments the transition between the H-and T-modes was explained as a linear unstable equilibrium state where the energy dependence of the corresponding rate coefficients is the driving mechanism (attachment-induced instability).

Properties of the dielectric barrier discharge in mixtures of methane with helium

The influence of the chemical composition of the feeding gas mixtures CH4-He on the physical properties of dielectric barrier discharge was investigated experimentally as well as theoretically. Certain changes in the shape of charge-voltage characteristics (Lissajous figures) were observed when the content of He was varied within the range 0–90%. An increase of the He concentration was found to cause monotonous but not linear decrease of the ignition voltage as well as of the burning voltage of the discharge. Measurements of the burning voltage were used to estimate the characteristic energy of electrons in the mixtures under consideration. These estimates were compared with experimentally determined values of the energy consumption for the synthesis of C2H2, C2H4 and C2H6, respectively. It was demonstrated that the observed dependencies of the energy consumption upon He content can be explained taking into consideration the influence of the chemical composition of feeding gas mixture upon the characteristic energy of electrons in the dielectric barrier discharge.

Dynamic behaviour of an oxygen dc discharge

The dynamic behaviour of a positive column oxygen plasma is investigated both experimentally and theoretically. In the experiment the response of the discharge current to small external perturbations of the electrical field is measured. Using these data the electrical impedance of the plasma is obtained dependent on the frequency of the external perturbation. A drift approximation model is presented, which covers the experimental findings over a wide range of the perturbation frequency. It appears that the influence of the fluctuations of the neutral species is negligible and the dynamic behaviour of the plasma is mostly governed by the charged particles. This provides a simple expression for the plasma impedance which allows the reaction rate and transport coefficients, as well as discharge parameters. to be validated.

Spatio-temporally Resolved Investigation of Surface Charges, N2(A3∑u+) Metastables and Discharge Development in Barrier Discharges

Abstract To study the interaction of an insulated surface with the volume of a barrier discharge, a discharge cell was developed to combine for the first time three diagnostic techniques in ONE discharge cell configuration. The discharge development was investigated by the cross-correlation-spectroscopy (CCS), the surface charge measurement on the dielectrics by the application of the optoelectronic Pockels effect in combination with a CCD camera, and the determination of the metastable N2(A3∑u+) molecule by laser induced fluorescence spectroscopy. Under the conditions studied, in pure N2 and He the discharge usually operates in the diffuse Townsend-like mod. At over voltages in nitrogen as well as at small admixtures of N2 (in percent range) to He the filamentary mode appears. The time dependent surface charge density in the helium diffuse mode of about | σ |~ 0.5 nC/cm2 is in good agreement with the time integrated current pulses. The measurements in the filamentary mode show remaining charges from the previous half-cycle as well as a memory effect for the microdischarge ignition. The detection of the nitrogen metastables was performed in the filamentary mode, resulting in a density of about 1013 cm-3 . The maximal density is reached clearly after the microdischarge and a difference between the anode and cathode phase is obvious.

Diagnostics of APG discharge in neon with a small admixture of hydrogen

The aim of this work is to study the atmospheric pressure glow discharge (APG discharge) in neon. The discharge was generated between two metal electrodes, which were covered with an alumina layer. A small admixture of hydrogen could be added to neon. The type of the discharge (as concerning filamentary or glow discharge) was mainly derived from electrical measurements. The light emitted by the discharge was studied by means of optical emission spectroscopy (OES) with a temporal and spatial resolution.The results confirm that in neon the same type of diffuse dielectric-barrier discharge can be generated as in helium, namely a transient sub-normal glow discharge. This regime was also observed if up to 1% of hydrogen was added to neon. In general the recorded spectra consisted of atomic spectral lines of neon, as well as hydrogen atomic lines and molecular bands of N2 and OH due to external admixture or impurities in the carrier gas. When the amount of impurities was low enough, a continuous spectrum probably belonging to bremsstrahlung of electrons on neutral atoms was observed.

Stepped form of negative corona current pulses in hydrogen

Current wave forms of initial stages of discharge formation in a short negative point-to-plane gap have been measured with a nanosecond time resolution in hydrogen at pressures (12.5–76) kPa and for various overvoltages and cathode point radii. The measurements revealed the existence of a stepped form of negative corona current pulses in hydrogen. To test existing models for negative corona pulse formation, effects of changing cathode secondary electron emission were studied using copper and brass cathodes coated by CuI and graphite. It is concluded that a negative corona pulse is associated with the ignition of a cathode-directed streamer in the vicinity of the cathode. We report what we believe are the first experimental observations of non-Trichel oscillations of negative corona current with a frequency of (1–10) MHz.