Laurence E. Kline

Dielectric properties for SF6 and SF6 mixtures predicted from basic data

We have calculated α and η, the ionization and attachment coefficients, and (E/N) *, the limiting breakdown electric‐field–to–gas‐density ratio, in SF6 and SF6 mixtures by numerically solving the Boltzmann equation for the electron energy distribution. The calculations require a knowledge of several electron collision cross sections. Published momentum transfer and ionization cross sections for SF6 were used. We measured various attachment cross sections for SF6 using electron‐beam techniques with mass spectrometric ion detection. We determined a total cross section for electronic excitation of SF6 by comparing the predicted values of α, η, and (E/N) * with our measured values obtained from spatial current growth experiments in SF6 in uniform fields over an extended range of E/N. With this self‐consistent set of SF6 cross sections, together with published He and N2 cross sections, it was then possible to predict the dielectric properties of SF6‐He and SF6‐N2 mixtures. Published experimental values of α fo...

Dielectric properties for SF/sub 6/ and SF/sub 6/ mixtures predicted from basic data

We have calculated α and η, the ionization and attachment coefficients, and (E/N) *, the limiting breakdown electric‐field–to–gas‐density ratio, in SF6 and SF6 mixtures by numerically solving the Boltzmann equation for the electron energy distribution. The calculations require a knowledge of several electron collision cross sections. Published momentum transfer and ionization cross sections for SF6 were used. We measured various attachment cross sections for SF6 using electron‐beam techniques with mass spectrometric ion detection. We determined a total cross section for electronic excitation of SF6 by comparing the predicted values of α, η, and (E/N) * with our measured values obtained from spatial current growth experiments in SF6 in uniform fields over an extended range of E/N. With this self‐consistent set of SF6 cross sections, together with published He and N2 cross sections, it was then possible to predict the dielectric properties of SF6‐He and SF6‐N2 mixtures. Published experimental values of α fo...

Electron and chemical kinetics in methane rf glow‐discharge deposition plasmas

Experimental measurements and theoretical modeling of methane deposition plasmas have led to the identification of the most likely homogeneous and heterogeneous reaction paths leading to the deposition of amorphous carbon thin films. Experimental measurements of the voltage, current waveforms, mass flow rates, and pressure are used as inputs to the model. The magnitude and flow‐rate dependence of the discharge luminosity, film deposition rates, and downstream mass spectra are compared with the model predictions and used to identify the dominant reaction paths. The model uses Monte Carlo simulation of the electron kinetics to predict the electron impact dissociation and ionization rates. These rates provide input for a plug flow chemical kinetics model.

Measurements of swarm parameters and derived electron collision cross sections in methane

A pulsed drift tube has been used to measure the electron drift velocity in methane over the range of E/N from 10 to 1000 Td. In addition, measurements of the positive ion mobility and ionization coefficient have been made over the range of E/N from 80 to 1000 Td. Within the experimental sensitivity, no evidence of attachment has been observed in this range. A set of electron collision cross sections has been assembled and used in Monte Carlo simulations to predict values of swarm parameters. The cross‐section set includes a momentum transfer cross section which is based primarily on the present and previous drift velocity measurements, cross sections for vibrational excitation and ionization based on published experimental cross‐section measurements, and a cross section for dissociation into neutral products obtained by subtracting a measured dissociative ionization cross section from a measured total dissociation cross section. Isotropic scattering is assumed for all types of collisions in the Monte Car...

Longitudinal electron diffusion coefficients in gases: Noble gases

Values of the ratio of the longitudinal diffusion coefficient to mobility DL/μ for electrons in He, Ar, Kr, and Xe are derived from current waveforms obtained during earlier measurements of electron mobility. The electric field to gas density ratios E/N cover the wide range of 10−3 to 20 Td, thereby bridging previous experiments at low E/N to recent experiments at high E/N. Here 1 Td=1×10−21 V m2. The corresponding DL/μ values range from 0.0066 eV for thermal electrons at 77 K to 10 eV. In addition to the well‐known peak in DL/μ for Ar at E/N between 0.01 and 0.1 Td caused by the Ramsauer minimum in the momentum transfer cross section, we find previously unreported low‐energy peaks in DL/μ vs E/N in Kr and Xe and previously unreported pronounced leveling‐off in DL/μ at E/N≳8 Td in Ar, Kr, and Xe. Calculations of transport coefficients using numerical solutions of the Boltzmann equation and cross section sets in the literature give good agreement with experiment from E/N producing thermal electrons up to a...

Calculations of discharge initiation in overvolted parallel‐plane gaps

The spatiotemporal development of electron and positive‐ion densities, electric field, and luminosity are calculated for electron‐pulse experiments in parallel‐plane gaps by numerically solving continuity equations together with Poissons equation. Experimental coefficients for electron transport, cathode photoemission, and gas photoionization are used. The results show that anode‐ and cathode‐directed ionizing waves, or streamers, develop from the electron‐pulse‐initiated avalanche when it reaches midgap, and produce a weakly ionized plasma. Both filamentary plasmas and uniform plasmas that fill the interelectrode volume have been experimentally observed. Calculated and experimental streak photographs agree well in both cases. In filamentary discharges, a return ionizing wave also propagates from the anode when the anode‐directed wave arrives. The results show that electrons must be present ahead of the wave for cathode‐directed wave propagation. These electrons are supplied by photoionization, photoemis...

Investigations of glow discharge formation with volume preionization

The discharge formation process has been studied experimentally for CO2 planar TEA laser discharges. Theoretical models are presented which predict the preionization electron and ion densities, the spatiotemporal development of the discharge plasma, discharge voltage, and current waveforms, and the quasisteady operating characteristics of the discharge. The preionization is provided in the experiments by a pulse of ultraviolet radiation. The discharge formation model accounts for cathode photoemission and anode collection of electrons, discharge−circuit interactions, and gaseous ionization processes. The model predicts that photoemission and anode collection can be neglected when strong preionization and moderate overvoltages are used. When photoemission and anode collection are neglected, the discharge formative time is independent of the discharge volume. Calculated and experimental voltage and current waveforms are in very good agreement. The results of the calculations show that the discharge formativ...

Diagnostics and modeling of RF discharge dissociation in N/sub 2/O

Measurements were made of the RF discharge dissociation of N/sub 2/O in a parallel-plate reactor by downstream mass spectrometry using a wide range of gas flows and powers at 10 kHz and 13.56 MHz. The results show that the mass 44 signal (N/sub 2/O/sup +/), which is a measure of the amount of undissociated N/sub 2/O, is a function of the discharge input energy per N/sub 2/O molecule (eV/N/sub 2/O). A plug flow, rate equation model of the discharge was used to predict the experimental dissociation rates. A DC Monte Carlo simulation was used to calculate rate coefficients for electron-impact neutral dissociation, ionization, and dissociative ionization. The rate equation model also includes reactions among the dissociation products of N/sub 2/O and species which are synthesized in the discharge, as well as neural and electron-ion recombination at the electrodes. The model predictions identify the major reaction pathways and the sensitivities of the results to the rate coefficient values used. >

Arc suppression in CO2 laser discharges

We have parameterized the times to arc formation in CO2 : N2 : He mixtures using a TEA configuration which produces homogeneous 10‐liter laser discharges. The onset of arcing is suppressed by increasing the helium or decreasing the CO2 concentration. Arcs are also suppressed by adding a low‐ionization‐potential seedant such as tripropylamine. Direct variation of the uv spark intensity which establishes the initial preionization has minimal impact on the arc formation process.

Effect of negative ions on current growth and ionizing wave propagation in air

The spatiotemporal development of electron and ion densities, electric fields, and luminosity are calculated for electron pulse experiments in overvolted parallel‐plane gaps by numerically solving continuity equations together with Poisson’s equation. Experimental coefficients for primary ionization, cathode photoemission, photoionization, and luminosity are used. Unambiguous determination of the coefficients for attachment, detachment, and charge transfer is not possible from available experimental results. Therefore, the calculations are repeated for three sets of coefficients for these processes, corresponding to the following assumptions: unstable negative ions, stable negative ions, and no negative ions. The results of the calculations show, in each case, that the electron pulse initiates an avalanche which grows exponentially until the onset of space‐charge effects. The calculated growth rate is strongly affected by the assumed attachment, detachment, and charge‐transfer coefficients. When the total...