Sebastian Nemschokmichal

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.

Laser-induced fluorescence spectroscopy of {\rm N}_2(A\,{}^3\Sigma_{\rm u}^+) and absolute density calibration by Rayleigh scattering in capacitively coupled rf discharges

Laser-induced fluorescence spectroscopy (LIF) was applied to measure absolute densities of the first metastable state in capacitively coupled rf discharges at 13.56 MHz. The metastable state was transferred to the B 3Πg, v = 3 state with a dye laser at 687.44 nm and the fluorescence of the transition to the state was observed at 762 nm. The time-depending LIF signal was investigated and the effective lifetime of the B 3Πg, v = 3 state was determined by eliminating the delay due to the photomultiplier circuit. The absolute density calibration was done by comparison with Rayleigh scattering at the laser wavelength of 687 nm. Therewith, axial density profiles were measured and their dependence on pressure and power was investigated. The metastable densities vary between 1012 and 1013 cm−3 for pressures from 40 to 1000 Pa. Furthermore, small amounts of oxygen (0.5–2%) were added and the influence on the metastable density is discussed.

Spatial and Phase-Resolved Optical Emission Patterns in Capacitively Coupled Radio-Frequency Plasmas

Phase-resolved optical emission spectroscopy is a powerful tool to investigate the excitation mechanisms within the radio-frequency (RF) cycle of capacitively coupled RF plasmas. In this paper, the optical emission of oxygen, hydrogen, and argon atoms is investigated over a wide range of process parameters. The spatially (axial) and temporally resolved spectroscopy reveals four prominent emission patterns.

The influence of negative ions in helium–oxygen barrier discharges: I. Laser photodetachment experiment

This work is the experimental part of a comprehensive study that aims to understand the influence of negative ions on the development of atmospheric pressure barrier discharges in electronegative systems. The investigations will be complemented by a 1D numerical fluid simulation. Laser photodetachment experiments were performed in a glow-like barrier discharge operated in helium with admixtures of oxygen up to 1 vol.% at a gas pressure of 500 mbar. The discharge gap between the glass-coated electrodes was 3 mm. The discharge properties were characterized by electrical measurements and optical emission spectroscopy. Laser photodetachment of , , and was studied using the fundamental and second harmonic wavelength of a Nd-YAG laser. The laser photodetachment of negative ions influences the breakdown characteristics when the laser is fired during the prephase of the discharge only. The breakdown voltage is reduced, which indicates an enhanced pre-ionization initiated by the detached electrons. Systematic variations in the laser pulse in time, the axial laser beam position, the laser pulse energy, and the laser wavelength provided detailed knowledge on this process. The investigation underlines the importance of the discharge prephase in general and aims to differentiate between the negative ion species , , and .

The influence of negative ions in helium–oxygen barrier discharges: II. 1D fluid simulation and adaption to the experiment

A 1D fluid simulation was developed to investigate the influence of negative ions in a helium–oxygen barrier discharge between two glass plates at a distance of . The paper describes setting up the simulation for a pressure of and an admixture of oxygen to helium. In order to enable the comparison with laser photodetachment experiments, the simulation is adapted to the experimentally observed discharge current and gap voltage by varying gas temperature, flux of thermally desorpted electrons and secondary electron emission coefficients. The discharge is characterized by evaluation of the most important elementary collision processes as well as the kinetics of the charged species. Besides, the influence of long-living species on the discharge behavior is taken into account by long-time simulations. The negative ions are characterized by their spatio-temporal distribution in the gap and their production and loss processes. The comparison between simulations without and with consideration of negative ions reveals the importance of negative ions on the discharge development.

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.