Spatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges in helium with nitrogen admixtures

This work reports on the spatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges driven by sine-wave feeding voltage at a frequency of 2 kHz in helium with small nitrogen admixtures. The discharge gap of 3 mm is bounded by glass plates on both plane electrodes. Priority is given to the lateral discharge inhomogeneities, underlying volume- and surface-memory effects, and the breakdown mechanism. Therefore, relevant processes in the discharge volume and on the dielectric surfaces were investigated by ICCD camera imaging and optical emission spectroscopy in combination with electrical measurements and surface charge diagnostics using the electro-optic Pockels effect of a bismuth silicon oxide crystal. The number of current pulses per half-cycle of the sine-wave voltage rises with increasing nitrogen admixture to helium due to the predominant role of the Penning ionization. Here, the transition from the first glow-like breakdown to the last Townsend-like breakdown is favored by residual species from the former breakdowns which enhance the secondary electron emission during the pre-phase of the later breakdowns. Moreover, the surface charge measurements reveal that the consecutive breakdowns occur alternately at central and peripheral regions on the electrode surface. These spatial inhomogeneities are conserved by the surface charge memory effect as pointed out by the recalculated spatio-temporal development of the gap voltage

Temporal and spatial characterisation of nitrogen metastables in atmospheric pressure barrier discharges

The absolute density of the metastable N2(A,v=0) molecule was extensively studied in nitrogen barrier discharges at 500 mbar. For the detection of the metastables laser-induced fluorescence spectroscopy (LIF) was used, at which for the calibration of the absoute metastables density a comparison with Rayleigh scattering was performed. To get the ratio of the LIF signal to the Rayleigh signal it is shown that the LIF signal is the convolution of the Rayleigh signal with an exponential decay. Besides, the different cross sections are calculated and the ratio of the detection sensitivities at the laser and fluorescence wavelength is determined. As a first step on the way to atmospheric pressure barrier discharges, the laser-induced fluorescence spectroscopy was implemented in low pressure capacitively coupled radio-frequency discharges. The determined metastables density in the capacitively coupled radio-frequency discharge is somewhat below 10^12 cm^(-3) at 40 Pa and somewhat below 10^13 cm^(-3) at 1000 Pa. The axial density profiles show a nearly symmetric shape due to the long lifetime of the metastable state. At a pressure of 500 mbar the two discharge modes of the barrier discharge, the filamentary and the diffuse mode, were analysed. The filamentary mode was mainly investigated in an asymmetric discharge configuration. Typical densities in the detection volume are in the range of 10^13 cm^(-3), resulting in maximal densities of up to 10^15 cm^(-3) in the microdischarge channel. Such large densities are in agreement with the fast decay by the pooling reaction after the maximum of the metastables density in the afterglow of the discharge pulse. The time dependent measurements in the afterglow of single microdischarges offer a delay of the metastables production with respect to the discharge current. This delay indicates that the metastables production takes place mostly by cascades from higher triplet states, which are in turn excited by electron impact. The axial density profiles show a maximum in metastables density in front of the anode in agreement with optical emission spectroscopy, but which cannot be clearly identified because of the asymmetric discharge configuration. The measurements for the diffuse discharge mode were performed in a symmetric discharge configuration. The metastables density is in the range of 10^13 cm^(-3). It increases during the current pulse of the discharge and decays afterwards. The maximum of the metastables density is delayed with respect to the maximum of the discharge current. The depletion of metastables in the early discharge afterglow is dominated by the pooling reaction, afterwards quenching by nitrogen atoms becomes important assuming a nitrogen atom density in the order of 10^14 cm^(-3). As for the filamentary mode, the losses by diffusion are negligible for the measurement positions. The measured axial density profiles show an accumulation of metastables in front of the anode, whereas the density in front of the cathode is below the detection limit. To calculate the metastables current density to the dielectrics after the discharge pulse a simulation is developed including the dominant volume processes for the depletion of metastables and the axial diffusion. Starting point for the simulation is the axial metastables density distribution at the end of the discharge pulse. The calculated metastables current density at the dielectrics is in the range of 10^14 cm^(-2)s^(-1). With the use of recently calculated secondary electron emission coefficients a comparison of the secondary electron emission by metastables with the discharge current is done. It is figured out that the secondary electron emission current is large enough to be important during the discharge ignition. To expand the simulation to the whole voltage cycle, the excitation of metastables is assumed to be proportional to the discharge current and electron density. Using this model, the measured time dependences of the metastables density are well reproduced for the investigated parameter variations. This is not the case for the axial profiles, where a metastables loss process is missed to explain the formation of a density plateau in front of the anode during the discharge pulse. The intended calculation of the metastables current density shows that the delay of the metastables production with respect to the discharge current might be responsible for the ignition of microdischarges at the beginning of the discharge pulse.Die absolute Dichte des metastabilen N2(A,v=0) Moleküls wurde umfassend in Stickstoffbarrierenentladungen bei 500 mbar untersucht. Zur Detektion der Metastabilen wurde die laserinduzierte Fluoreszenzspektroskopie (LIF) genutzt, wobei zur Kalibrierung der absoluten Metastabilendichte ein Vergleich mit Rayleighstreuung durchgeführt wurde. Um das Verhältnis vom LIF-Signal zum Rayleighsignal zu bestimmen wird gezeigt dass das LIF-Signal die Faltung aus dem Rayleighsignal mit einem exponentiellen Abfall ist. Daneben werden die verschiedenen Querschnitte berechnet und das Verhältnis der Detektorempfindlichkeiten bei der Laser- und der Fluoreszenzwellenlänge bestimmt. Als erster Schritt hin zu Atmosphärendruckentladungen wurde die laserinduzierte Fluoreszenzspektroskopie an kapazitiv gekoppelten Radiofrequenzentladungen bei Niederdruck realisiert. Die bestimmte Metastabilendichte in der kapazitiv gekoppelten Radiofrequenzentladung liegt etwas unter 10^12 cm^(-3) bei 40 Pa und etwas unter 10^13 cm^(-3) bei 1000 Pa. Die axialen Dichteprofile zeigen eine nahezu symmetrische Form aufgrund der langen Lebensdauer des metastabilen Zustandes. Bei einem Druck von 500 mbar wurden die beiden Entladungsmodi der Barrierenentladung, der filamentierte und der diffuse Modus, analysiert. Der filamentierte Modus wurde hauptsächlich in einer asymmetrischen Entladungskonfiguration untersucht. Typische Dichten im Detektionsvolumen sind im Bereich von 10^13 cm^(-3), was maximale Dichten von bis zu 10^15 cm^(-3) im Mikroentladungskanal ergibt. Solch große Dichten sind in Übereinstimmung mit dem schnellen Abfall durch die Poolingreaktion nach dem Maximum der Metastabilendichte im Nachleuchten des Entladungspulses. Die zeitabhängigen Messungen im Nachleuchten einzelner Mikroentladungen zeigen eine Verzögerung der Metastabilenproduktion in Bezug auf den Entladungsstrom. Diese Verzögerung deutet an dass die Metastabilenproduktion hauptsächlich durch Kaskaden von höheren Tripletzuständen stattfindet, welche wiederum durch Elektronenstoß angeregt werden. Die axialen Dichteprofile zeigen ein Maximum der Metastabilendichte vor der Anode in Übereinstimmung mit optischer Emissionsspektroskopie, welches aber aufgrund des asymmetrischen Entladungsaufbaus nicht eindeutig nachgewiesen werden kann. Die Messungen für den diffusen Entladungsmodus wurden in einer symmetrischen Entladungskonfiguration durchgeführt. Die Metastabilendichte liegt im Bereich von 10^13 cm^(-3). Sie steigt während des Strompulses der Entladung und fällt anschließend ab. Das Maximum der Metastabilendichte ist bezüglich des Maximums des Entladungsstroms verzögert. Der Abbau der Metastabilen im frühen Entladungsnachleuchten ist durch die Poolingreaktion dominiert, anschließend wird die Stoßabregung durch Stickstoffatome wichtig, unter Annahme einer Stickstoffatomdichte in der Größenordnung von 10^14 cm^(-3). Wie für den filamentierten Modus sind die Verluste durch Diffusion vernachlässigbar für die Messpositionen. Die gemessenen axialen Dichteprofile zeigen eine Ansammlung von Metastabilen vor der Anode, wohingegen die Dichte vor der Kathode unterhalb der Detektionsgrenze liegt. Um die Metastabilenstromdichte zu den Dielektrika nach dem Entladungspuls zu berechnen wurde eine Simulation entwickelt, welche die dominanten Volumenprozesse für den Abbau der Metastabilen und die axiale Diffusion beinhaltet. Ausgangspunkt für die Simulation ist die axiale Metastabilendichteverteilung am Ende des Entladungspulses. Die berechnete Metastabilenstromdichte an den Dielektrika ist in der Größenordnung von 10^14 cm^(-2)s^(-1). Unter Benutzung kürzlich berechneter Sekundärelektronenemissionskoeffizienten wird ein Vergleich der Sekundärelektronenemission durch Metastabile mit dem Entladungsstrom durchgeführt. Es wird herausgearbeitet dass der Sekundärelektronenemissionstrom groß genug ist um eine wesentliche Rolle bei der Zündung der Entladung zu spielen. Um die Simulation auf den ganzen Entladungszyklus auszubauen wird die Anregung der Metastabilen als proportional zum Entladungsstrom und zur Elektronendichte angenommen. Unter Benutzung dieses Modells wird die gemessene Zeitabhängigkeit der Metastabilendichte für die untersuchten Parametervariationen gut reproduziert. Dies ist bei den axialen Profilen nicht der Fall, bei denen ein Metastabilenverlustprozess fehlt um die Herausbildung eines Dichteplateaus vor der Anode während des Entladungspulses zu erklären. Die beabsichtete Berechnung der Metastabilenstromdichte zeigt dass die Verzögerung der Metastabilenproduktion in Bezug auf den Entladungsstrom für die Zündung von Mikroentladungen am Anfang des Entladungspulses verantwortlich sein könnte

The influence of negative ions in helium–oxygen barrier discharges: III. Simulation of laser photodetachment and comparison with experiment

Abstract The laser photodetachment experiment in a diffuse helium–oxygen barrier discharge is evaluated by a 1D fluid simulation. As in the experiment, the simulated discharge operates in helium with 400 ppm oxygen admixture at 500 mbar inside a discharge gap of 3 mm . The laser photodetachment is included by the interaction of negative ions with a temporally and spatially dependent photon flux. The simulation with the usually applied set of reactions and rate coefficients provides a much lower negative ion density than needed to explain the impact on the discharge characteristics in the experiment. Further processes for an enhanced negative ion formation and their capabilities of reproducing the experimental results are discussed. These further processes are additional attachment processes in the volume and the negative ion formation at the negatively charged dielectric. Both approaches are able to reproduce the measured laser photodetachment effect partially, but the best agreement with the experimental results is achieved with the formation of negative ions at the negatively charged dielectric

Influence of released surface electrons on the pre-ionization of helium barrier discharges: laser photodesorption experiment and 1D fluid simulation

Abstract The presented work highlights the role of residual weakly-bound surface electrons acting as an effective seed electron reservoir that favors the pre-ionization of diffuse barrier discharges (BDs). A glow-like BD was operated in helium at a pressure of 500 mbar in between two plane electrodes each covered with float glass at a distance of 3 mm.The change in discharge development due to laser photodesorption of surface electrons was studied by electrical measurements and optical emission spectroscopy. Moreover, a 1D numerical fluid model of the diffuse discharge allowed the simulation of the laser photodesorption experiment, the estimation of the released surface electrons, and the understanding of their impact on the reaction kinetics in the volume. The breakdown voltage is clearly reduced when the laser beam at photon energy of 2.33 eV hits the cathodic dielectric that is charged with residual electrons during the discharge pre-phase. According to the adapted simulation, the laser releases only a small amount of surface electrons in the order of 10 pC. Nevertheless, this significantly supports the pre-ionization. Using a lower photon energy of 1.17 eV, the transition from the glow mode to the Townsend mode is induced due to a much higher electron yield up to 1 nC. In this case, both experiment and simulation indicate a retarded stepwise release of surface electrons initiated by the low laser photon energy

Spatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges in helium with nitrogen admixtures

Abstract This work reports on the spatio-temporal characterization of the multiple current pulse regime of diffuse barrier discharges driven by sine-wave feeding voltage at a frequency of 2 kHz in helium with small nitrogen admixtures. The discharge gap of 3 mm is bounded by glass plates on both plane electrodes. Priority is given to the lateral discharge inhomogeneities, underlying volume- and surface-memory effects, and the breakdown mechanism. Therefore, relevant processes in the discharge volume and on the dielectric surfaces were investigated by ICCD camera imaging and optical emission spectroscopy in combination with electrical measurements and surface charge diagnostics using the electro-optic Pockels effect of a bismuth silicon oxide crystal. The number of current pulses per half-cycle of the sine-wave voltage rises with increasing nitrogen admixture to helium due to the predominant role of the Penning ionization. Here, the transition from the first glow-like breakdown to the last Townsend-like breakdown is favored by residual species from the former breakdowns which enhance the secondary electron emission during the pre-phase of the later breakdowns. Moreover, the surface charge measurements reveal that the consecutive breakdowns occur alternately at central and peripheral regions on the electrode surface. These spatial inhomogeneities are conserved by the surface charge memory effect as pointed out by the recalculated spatio-temporal development of the gap voltage

Self-stabilized discharge filament in plane-parallel barrier discharge configuration: formation, breakdown mechanism, and memory effects

Abstract Single self-stabilized discharge filaments were investigated in the plane-parallel electrode configuration. The barrier discharge was operated inside a gap of 3 mm shielded by glass plates to both electrodes, using helium-nitrogen mixtures and a square-wave feeding voltage at a frequency of 2 kHz. The combined application of electrical measurements, ICCD camera imaging, optical emission spectroscopy and surface charge diagnostics via the electro-optic Pockels effect allowed the correlation of the discharge development in the volume and on the dielectric surfaces. The formation criteria and existence regimes were found by systematic variation of the nitrogen admixture to helium, the total pressure and the feeding voltage amplitude. Single self-stabilized discharge filaments can be operated over a wide parameter range, foremost, by significant reduction of the voltage amplitude after the operation in the microdischarge regime. Here, the outstanding importance of the surface charge memory effect on the long-term stability was pointed out by the recalculated spatio-temporally resolved gap voltage. The optical emission revealed discharge characteristics that are partially reminiscent of both the glow-like barrier discharge and the microdischarge regime, such as a Townsend pre-phase, a fast cathode-directed ionization front during the breakdown and radially propagating surface discharges during the afterglow

Surface charge measurements on different dielectrics in diffuse and filamentary barrier discharges

Abstract Previously, we reported on the measurement of surface charges during the operation of barrier discharges (BDs) using the electro-optic Pockels effect of a bismuth silicon oxide (BSO) crystal. With the present work, the next milestone is achieved by making this powerful method accessible to various dielectrics which are typically used in BD configurations. The dynamics and spatial distribution of positive and negative surface charges were determined on optically transparent borosilicate glass, mono-crystalline alumina and magnesia, respectively, covering the BSO crystal. By variation of the nitrogen admixture to helium and the pressure between 500 mbar and 1 bar, both the diffuse glow-like BD and the self-stabilized discharge filaments were operated inside of a gas gap of 3 mm. The characteristics of the discharge and, especially, the influence of the different dielectrics on its development were studied by surface charge diagnostics, electrical measurements and ICCD camera imaging. Regarding the glow-like BD, the breakdown voltage changes significantly by variation of the cathodic dielectric, due to the different effective secondary electron emission (SEE) coefficients. These material-specific SEE yields were estimated using Townsend’s criterion in combination with analytical calculations of the effective ionization coefficient in helium with air impurities. Moreover, the importance of the surface charge memory effect for the self-stabilization of discharge filaments was quantified by the recalculated spatio-temporal behavior of the gap voltage

Impact of volume and surface processes on the pre-ionization of dielectric barrier discharges: advanced diagnostics and fluid modeling

The phenomenology and breakdown mechanism of dielectric barrier discharges are strongly determined by volume and surface memory effects. In particular, the pre-ionization provided by residual species in the volume or surface charges on the dielectrics influences the breakdown behavior of filamentary and diffuse discharges. This was investigated by advanced diagnostics such as streak camera imaging, laser photodetachment of negative ions and laser photodesorption of electrons from dielectric surfaces in correlation with 1D fluid modeling. The streak camera images show that an increasing number of residual charges in the volume changes the microdischarge breakdown in air-like gas mixtures from a cathode-directed streamer to a simultaneous propagation of cathode- and anode-directed streamers. In contrast, seed electrons are important for the pre-ionization if the density of residual charges in the volume is low. One source of seed electrons are negative ions, whose density exceeds the electron density during the pre-phase of diffuse helium–oxygen barrier discharges as indicated by the laser photodetachment experiments. Electrons desorbed from the cathodic dielectric have an even larger influence. They induce a transition from the glow-like to the Townsend-like discharge mode in nominally pure helium. Apart from analyzing the importance of the pre-ionization for the breakdown mechanism, the opportunities for manipulating the lateral structure and discharge modes are discussed. For this purpose, the intensity and diameter of a diffuse discharge in helium are controlled by an illuminated semiconducting barrier