Leanne Pitchford

Dynamics of a guided streamer (‘plasma bullet’) in a helium jet in air at atmospheric pressure

It has been demonstrated experimentally that a non-equilibrium plasma column can be generated by discharge pulses in a helium jet surrounded by atmospheric-pressure air. The ?plasma jet? can be longer than 10?cm and fast imaging shows that most of the light emitted by the plasma jet is produced in a small ?plasma bullet? that propagates along the helium jet at speeds of several tens of km?s?1. With the help of a simple fluid model of the discharge, we show that the plasma jet is very similar to a cathode streamer (ionization wave) guided by the helium jet. We discuss the properties of the helium streamer and of the plasma channel behind the streamer head as a function of parameters such as electrode geometry, voltage pulse waveform and preionization density. The model can reproduce qualitatively and explain most of the features observed experimentally.

Pseudospark discharges via computer simulation

A hybrid fluid-particle (Monte Carlo) model to describe the initiation of pseudospark discharges has been developed. In this model, time-dependent fluid equations for the electrons and positive ions are solved self-consistently with Poissons equation for the electric field in a two-dimensional, cylindrically symmetrical geometry. The Monte Carlo simulation is used to determine the ionization source term in the fluid equations. This model has been used to study the evolution of a discharge in helium at 0.5 torr, with an applied voltage of 2 kV and in a typical pseudospark geometry. From the numerical results, the authors have identified a sequence of physical events that lead to the rapid rise in current associated with the onset of the pseudospark discharge mode. They find that there is a maximum in the electron multiplication at the time which corresponds to the onset of the hollow cathode effect, and although the multiplication later decreases, it is always greater than needed for a steady-state discharge. >

Predicted properties of microhollow cathode discharges in xenon

A fluid model has been developed and used to help clarify the physical mechanisms occurring in microhollow cathode discharges (MHCD). Calculated current-voltage (I-V) characteristics and gas temperatures in xenon at 100 Torr are presented. Consistent with previous experimental results in similar conditions, we find a voltage maximum in the I-V characteristic. We show that this structure reflects a transition between a low-current, abnormal discharge localized inside the cylindrical hollow cathode to a higher-current, normal glow discharge sustained by electron emission from the outer surface of the cathode. This transition, due to the geometry of the device, is a factor contributing to the well-known stability of MHCDs.

Model description of surface dielectric barrier discharges for flow control

This paper presents a study of the development of a surface dielectric barrier discharge in air under conditions similar to those of plasma actuators for flow control. The study is based on results from a 2D fluid model of the discharge in air that provides the space and time evolution of the charged particle densities, electric field and surface charges. The electrohydrodynamic (EHD) force associated with the momentum transfer from charged particles to neutral molecules in the volume above the dielectric layer is also deduced from the model. Results show that the EHD force is important not only during the positive part of the sinusoidal voltage cycle (i.e. when the electrode on top of the dielectric layer plays the role of the anode) but also during the negative part of the cycle (cathode on top of the dielectric layer). During the positive part of the cycle, the EHD force is due to the formation of a positive ion cloud that is periodically interrupted by high current breakdown. The EHD force during the negative part of the cycle is due to the development of a negative ion cloud that continuously grows during the successive high frequency current pulses that form in this regime.

Contribution of positive and negative ions to the electrohydrodynamic force in a dielectric barrier discharge plasma actuator operating in air

We present a parametric study of the electrohydrodynamic force generated by surface dielectric barrier discharge plasma actuators in air for sinusoidal voltage waveforms. The simulation results confirm that momentum is transferred from the charged particles to the neutral species in the same direction during both positive and negative parts of the cycle. The momentum transfer is due to positive ions during the positive part of the cycle (electrode above the dielectric layer is the anode), and to negative ions during the negative part of the cycle. The relative contribution of the positive and negative parts of the cycle depends on the voltage amplitude and frequency. The model predicts that the contribution of negative ions tends to be dominant at low voltage frequencies and high voltage amplitudes.

Calculated gas temperature profiles in argon glow discharges

We use a previously developed model for the electrical properties of glow discharges, coupled to a Monte Carlo simulation of the heavy particles (ions and fast neutrals) in the cathode sheath region, to calculate the gas heating source term in glow discharges in argon with planar electrodes. The gas temperature profile is calculated from the one-dimensional heat transport equation, and we report results for discharge current densities up to 4 mA/cm2 for a value of pd (product of the gas pressure and gap spacing) of 1.5 Torr cm. The gas temperature peaks at a position slightly inside the cathode sheath, and at the highest current density reported here the gas temperature reaches 354 K. A large fraction of the energy gained by the ions in the sheath is transported directly to the cathode (mostly through fast neutrals), and the fractional ion energy converted to thermal motion of the gas decreases with increasing current density. We report some results to show the dependence of the gas temperature on the the...

O2(aΔ1g) production at atmospheric pressure by microdischarge

We report experimental results showing that singlet oxygen O2(aΔ1g) can be efficiently produced at atmospheric pressure in a three-electrode microcathode sustained discharge (MCSD) configuration. This configuration consists of a microhollow cathode discharge (MHCD) acting as a plasma cathode to sustain a stable glow discharge between the MHCD and a third planar electrode placed at a distance of 8 mm. Experiments were performed in He/O2/NO mixtures. O2(aΔ1g) number densities higher than 1016 cm−3 were measured in the MCSD afterglow at total flow rates up to 30 ln/mn, resulting in O2(aΔ1g) fluxes above 10 millimole per hour (mmol/h).

New Insights in the Physics of DBD Plasma Actuators for Flow Control

*We have shown in previous publications that the ElectroHydroDynamic (EHD) force in a surface dielectric barrier discharge takes place in the ion clouds that form and extend above the dielectric surface during the corona phase of the discharge (i.e. before the formation of a quasi-neutral plasma, and between successive current pulses). During the positive phase of the sinusoidal voltage (i.e. when the anode is above the dielectric layer), a positive ion cloud forms and spreads above the surface, as in a transient corona discharge, until breakdown (filamentary glow discharge or streamer breakdown) occurs. During the negative part of the cycle (when the electrode above the dielectric surface plays the role of a cathode), the current is composed of higher frequency lower amplitude pulses. In air, a negative ion cloud forms above the surface and continuously grows as the voltage drop between the electrodes increases. The EHD force during the negative phase of the sinusoidal voltage is in the same direction as during the positive phase, and can be as large as in the positive phase. In this paper we study in detail the surface DBD and the generated EHD force in air. We perform a parametric study (voltage amplitude and frequency) of the EHD force and dissipated power, and compare the model results with available experimental results.

Ion and neutral energy distributions to the MgO surface and sputtering rates in plasma display panel cells

One of the factors limiting the lifetime of plasma display panels (PDPs) is sputtering of the MgO layer deposited on the dielectric covering the electrode. The goal of the work reported here is to evaluate the influence of different operating conditions on the sputtering rate of the MgO layer in PDPs operating with Xe/Ne mixtures. Sputtering is caused by the ions and fast neutral atoms impacting the surface. Starting with results from a fluid model, we use a Monte Carlo simulation to calculate energy distributions of the ion and fast neutrals arriving on the MgO coated surface for a set of baseline conditions. Estimates of the sputtering yield of MgO are then combined with the energy distributions to evaluate the sputtering rates for matrix and coplanar geometries. Parametric calculations in a simple geometry were performed to examine the effect of gas mixture, gas pressure, cell capacitance, and applied voltage on the sputtering rate of the MgO layer. Our main result is the prediction of longer lifetime for mixtures containing about 20%Xe for both matrix and coplanar geometries.

Ignition of Microcathode Sustained Discharge

Microdischarges sustained at high gas pressure in specific geometries can be very stable. A particular type of microdischarge, which is the microhollow cathode discharge (MHCD), can be used as an electron source to sustain a larger volume discharge. In this paper, we present results from a numerical study of a microcathode sustained discharge (MCSD) as part of a three-electrode system consisting of an MHCD and an MCSD. The time evolution of the plasma in the MCSD region is shown. Our discussion is focused on the ignition of the MCSD assisted by the electrons coming from the MHCD.