L. C. Pitchford

Electrohydrodynamic force and aerodynamic flow acceleration in surface dielectric barrier discharge

Surface discharges created in dielectric barrier discharge (DBD) configurations have been proposed as actuators for flow control in aerodynamic applications. We focus on DBDs operating in a glow regime, i.e., where the discharge is sustained by ion-induced secondary electron emission from the surface and volume ionization. After a brief discussion of the force per unit volume acting on the flow and due to the momentum transfer from charged particles to neutral molecules, we present calculations of this force based on a two-dimensional fluid model of the surface discharge. We show that this force is of the same nature as the electric wind in a corona discharge. However, the force in a DBD is localized in the cathode sheath region of the discharge expanding along the dielectric surface. While its intensity is much larger than the analogous force in a direct-current corona discharge, it is active during less than one hundred nanoseconds for each discharge pulse and the time-averaged forces in the two cases a...

Two-dimensional simulation of an alternating current matrix plasma display cell: Cross-talk and other geometric effects

A two-dimensional, user-friendly model of the discharge occurring in a plasma display panel cell was developed. This model was used to study the transient discharges in an alternating current plasma display with a matrix electrode configuration. The space and time variations of the charge particle densities, excitation rates, electric potential, and surface charge densities are described. The model is also used to study the conditions of existence of electrical interaction between adjacent cells and the effects of electrode misalignment.

Cathode sheath formation in a discharge‐sustained XeCl laser

A one‐dimensional longitudinal model of a XeCl discharge including the cathode region, the plasma, and the external circuit has been used for conditions close to experiments for 50–100 ns laser pulse durations and electron power deposition in the MW/cm3 range in a 300 cm3 chamber. This model provides the space and time variations of the electric field, electron and positive‐ion densities in the cathode region, as well as the time variations of the charged‐particle densities and excited species concentrations in the plasma obtained with a simplified kinetic model. Results show that under normal conditions of operation the cathode electric field can reach values as high as several 106 V/cm. The influence of photoemission and secondary emission due to ion impact on the cathode is discussed. A transition from capacitive to resistive behavior of the sheath is seen to occur rapidly due to the multiplication of cathode‐emitted electrons in the large sheath electric field, leading to a fast increase in the ion co...

Current pulses in dc glow discharges in electronegative gas mixtures

Regular oscillations in the current are predicted in low‐pressure, planar discharges under certain conditions in electronegative gas mixtures in which the attachment rate coefficient is large at low values of E/P, the ratio of the electric‐field strength to the gas pressure. The frequency of the oscillations is about 10 kHz, and depending on the conditions of pressure, gap spacing, and applied voltage, the current wave form varies from a near‐sinusoidal shape to regularly repeating and well‐separated spikes with a peak current density on the order of or less than 1 mA/cm2. The instability which gives rise to these oscillations is due to attachment, and the oscillations result from alternate phases of space charge buildup and decay. Thus, the current oscillations predicted here in planar discharges are analogous to Trichel pulses, periodic current spikes which are observed in negative point‐plane corona discharges in electronegative gases.

Electron‐beam generation during the hollow cathode phase of pseudospark discharges

From calculations using a two‐dimensional, hybrid fluid‐particle model, the mechanism for the production of the high‐current electron beam observed during the hollow cathode phase of pseudospark discharges is identified. The predicted beam properties are in good qualitative agreement with experiments; the onset of the beam current is abrupt and the current in the beam is some 10%–30% of the discharge current. Two distinct components appear in the energy spectrum of the beam. A sizable fraction of the current is carried by electrons with an energy corresponding to the voltage applied to the discharge. A distinct low‐energy beam is also predicted with a spread in energy of several 100s of eV.

Electrical characteristics of a RF-GD-OES cell

The purpose of this work is to describe, using a self-consistent two dimensional hybrid fluid-particle model, the electrical characteristics of a discharge operating in a RF-GD-OES cell. We show that, for typical operating conditions at 13.56xa0MHz, the discharge has a capacitive electrical behavior and that the main current component at the powered electrode is the displacement current. These results are not in agreement with previously published calculations and this point is discussed.

THE BREAKDOWN AND GLOW PHASES DURING THE INITIATION OF DISCHARGES FOR LAMPS

High intensity discharge (HID) lamps are often initiated by the application of one or more short, high-voltage, breakdown pulses superimposed on a 50 or 60 Hz generator voltage. A successful transition from the breakdown event to steady-state operating conditions in HID lamps requires that the lamp-circuit system be adequate to sustain the plasma created during breakdown until the electrodes are heated to thermionic temperatures. In this article, we use a one-dimensional (in the axial direction) transient discharge model to study the conditions needed to sustain the cold-cathode discharge after a breakdown event has occurred. While the application of our one-dimensional model to real lamps is approximate, we find that the model predictions are consistent with experimental results in HID lamps, a few of which are presented here. The main conclusion from this work is that, after breakdown, the voltage necessary to sustain a glow discharge is dependent on the source impedance, the gas composition, and on the...

Ion and neutral energy flux distributions to the cathode in glow discharges in Ar/Ne and Xe/Ne mixtures

The work presented in the article is a study of the heavy particle (ion and neutral) energy flux distributions to the cathode in conditions typical of discharges used for luminous signs for advertising (“neon” signs). The purpose of this work is to evaluate the effect of the gas mixture on the sputtering of the cathode. We have combined two models for this study: a hybrid model of the electrical properties of the cathode region of a glow discharge and a Monte Carlo simulation of the heavy particle trajectories. Using known sputtering yields for Ne, Ar, and Xe on iron cathodes, we estimate the sputtered atom flux for mixtures of Ar/Ne and Xe/Ne as a function of the percent neon in the mixture.

Effect of anisotropy in the elastic scattering cross sections on the ionization source terms in glow discharges in argon

The main purpose of this work is to evaluate, using a Monte Carlo simulation, the extent to which anisotropy in the elastic electron-neutral scattering cross sections influences the spatial distribution of the electron-impact ionization profile in glow discharges in argon. We also briefly discuss the effect of anisotropic elastic scattering on the slowing down of an electron beam and on the transmission factor, the fraction of electrons emitted from the cathode that are not scattered back to the cathode. Monte Carlo simulations were performed using different functional forms for the anisotropy. We show that, for a given momentum transfer cross section and the assumption of isotropic scattering, the ionization profiles over a range of conditions in argon glow discharges in parallel plane electrode geometries are not significantly affected by the inclusion of higher order anisotropies in the elastic cross sections. The anisotropy in the elastic scattering cross section influences only slightly the electron ...

Modelling non-thermal plasmas generated in glow discharges

Over the past decade, tremendous progress has been made in the modeling of nonthermal plasmas, and models have now reached the point that they are being used to help guide the experimental optimization of plasma based devices. In this communication we provide an overview of models used to describe nonthermal plasma generated in glow discharges. We show results from two applications, plasma display panels and glow discharge mass spectrometry for materials analysis.