Session GT3: Atmospheric Pressure Plasmas

Ionization wave Propagation in Nanosecond Pulsed Discharge and its Application

Surface dielectric barrier discharge (SDBD) at atmospheric pressure driven by high-voltage nanosecond pulses has been a promising discharge form, which has its potential application in plasma flow control and the plasma assisted ignition and combustion. The breakdown process of the nanosecond pulsed SDBD is generally accompanied by a discharge propagation along the dielectric surface, which is usually called the surface ionization wave (SIW). An investigation of the SIW is valuable for both a deeper understanding of the atmospheric pressure discharge and an improvement of its plasma application.

In this work, the propagation of the surface ionization wave in the nanosecond pulsed surface dielectric barrier discharge with different dielectric materials and pulse repetition rates is investigated. The current waveforms at different locations along the route of the SIW propagation are obtained, based on a specially designed ground strip array geometry. The temporal evolution and spatial distribution of the electric field during the SIW propagation are measured by using the electric field induced second harmonic (EFISH) generation method. It is found that with the dielectric material on which the surface charges decay faster, there are the well-pronounced primary and secondary SIWs with a higher velocity on the voltage rising edge and both the peak current and the peak electric field are also higher, with a less spatial attenuation along the SIW propagation route. It is demonstrated that the residual surface charges with the same polarity as the high-voltage pulse suppress the development of the surface ionization wave.   

Strong Correlation Effects in Atmospheric Pressure Plasmas

Atmospheric pressure plasmas have been widely tested for numerous applications showing promising results and an increasing interest due to the reduced running cost and operational simplicity. In this work, we show that the ion species are strongly coupled in atmospheric pressure plasmas and this leads to strongly correlated effects that currently are not accounted for in standard modeling techniques. Using first principles Molecular Dynamics simulations, we observed that the ion temperature is set by Disorder Induced Heating (DIH), ion-neutral temperature relaxation through collisions and ion-neutral 3-body recombination. We show that the maximum and equilibrium ion temperatures increase with the ionization fraction and that effect can be correctly predicted using energy conservation arguments and accounting for the DIH. We also show that the ion-ion interactions are not screened by the presence of neutral atoms. The observed effects show that atmospheric pressure plasmas are sufficiently dense that they are influenced by strong correlation effects associated with many-body interactions that are not treated in the dilute limit.   

The effect of humidity on streamer propagation in long air gaps

A 2D numerical simulation of the positive streamer properties was performed in 9-12 cm plane-to-plane air gaps for various pressures and water vapor contents. It was shown that an increase in air humidity leads to hampering the streamer development and to increasing the average critical electric field required for bridging the discharge gap. The effect of humidity was most profound at atmospheric pressure and decreased with decreasing pressure. The influence of water content on the streamer properties was explained by a decrease in the streamer channel conductivity due to dissociative recombination of electrons with positive hydrated ions and enhanced three-body electron attachment to O₂ molecules. The calculated critical electric field in humid air gaps was compared with available experimental data.

    

Effects of humidity on the dynamics and electron recombination of a pin-to-pin discharge in He + H2O at atmospheric pressure

The effects of the humidity of the feed gas on the discharge chemistry need to be considered to control the plasma chemistry. Detailed studies are scarce and often dominated by surface interactions, obscuring any volume effects. Here, volume kinetics are studied in a negative nanosecond pulsed discharge generated in a pin–pin 3 mm gap geometry in He + H₂O. The effect of humidity on the discharge development, electric field and electron density is investigated through experiments and modelling. It is found that the presence of water vapour affects both the electron density at the start of the pulse (remaining from the previous pulse) and the ionisation rates during the ignition phase, leading to a complex dependence of the discharge development speed depending on the water concentration. The electron decay is studied using the 0D global kinetics model GlobalKin. The dominant reactions responsible for the electron decay are determined by comparing experimental and simulated results and these reactions are grouped in simplified kinetic models. It is found that with water concentrations increasing from 0 to 2500 ppm, the complexity of the dominant reactions increases with in particular O₂ + H₂O₃ and water clusters becoming important for high water concentrations.   

Simulation of Nonthermal Plasma Discharges in Air and CO2 in Sub-millimetre Needle-Plane Gaps Under Fast-Rising Voltages

Interest in the use of high voltage (HV) pulsed power technologies has grown due to the numerous fields in which these technologies have proven effective. Applications range from the development of railgun technology and plasma closing switches (PCS); to plasma medicine, pulsed electric field (PEF) treatment of foodstuffs, and air purification via pulsed plasma discharges. In such applications, the understanding of the pre-breakdown dynamics under fast-rising voltages in various gases is often critical to system design and performance. In the interests of continued component miniaturisation and for the development of novel pulsed micro-electromechanical systems, this work presents the simulation of nonthermal plasma discharges initiated in a gas-filled, 250-micrometre needle-plane electrode gap, under ramp voltages of varying rates of rise. Using the hydrodynamic approach, discharges in atmospheric air and CO₂ have been modelled. The results have allowed the effects of voltage polarity and rise rate on the plasma front morphology, propagation characteristics, and plasma composition to be analysed. Differences in the evolution of the plasma front between air and CO₂ have also been studied, which may help to inform the choice of gas for use in gas-insulated pulsed power systems.   

Influence of water vapor and negative ions on self-organized luminous pattern formation in an atmospheric-pressure dc glow discharge

An atmospheric-pressure dc glow discharge with a liquid anode sometimes forms a self-organized luminous pattern on the liquid anode surface. However, the mechanism of this interesting phenomenon has not been understood yet. In general, the self-organized pattern is described mathematically by ''the reaction-diffusion system'', which is simultaneous partial differential equations with two variables.We expect that the two variables in the formation of the plasma self-organized pattern are the densities of electrons and negative ions. This is because the presence of molecular oxygen or water vapor is the key to the pattern formation. In this work, we examined the densities of OH radicals and negative ions using laser-induced fluorescence spectroscopy and laser photodetachment, respectively. We observed higher densities of OH and water vapor when the self-organized luminous pattern was formed on the liquid anode surface. The density of water vapor was estimated from the collisonal quenching frequency of OH(A²Σ⁺). In addition, the negative ion density was also higher when the self-organized luminous pattern was formed.   

Striations in Atmospheric Pressure AC Driven Helium Glow Discharge

Plasma stratification has been studied for more than a century. Despite the many studies reported on this topic,  a complete understanding of the phenomena is still missing. In this work, striations at atmospheric pressure AC driven helium glow discharge in is studied for a pin-to -plate electrode geometry with an electrode spacing of 8 mm. The striations are observed by several bright and dark regions in the positive column. The AC power supply was operated at 25 kHz frequency with the pin electrode powered and the plate electrode grounded. Voltage current characteristics were determined, and the striation patterns were correlated to a range of current densities from ~0.3 to 1.6 A cm^-2. Visualization of the discharge indicate that the striations appear not during the entire voltage rise and decay but during the second half of the voltage rise and first of the voltage decay. It is also found that the striation intensity also changes temporally and has the maximum intensity when a distinct and high intensity negative glow is formed near the electrode surface.  The voltage current measurements clearly indicate the existence of a hysteresis regime pertaining to the onset and ending of the striated behavior of the positive column.  Dependence of the striation on input power and electrode geometry are also investigated. High speed imaging revealed distinct behaviors of theses striations during the positive and negative cycle of the alternating waveform. The measurements indicate that the striations travel in the velocity range of 200 – 700 m/s.