R. J. Van Brunt

The Gaseous Electronics Conference radio‐frequency reference cell: A defined parallel‐plate radio‐frequency system for experimental and theoretical studies of plasma‐processing discharges

A “reference cell” for generating radio-frequency (rf) glow discharges in gases at a frequency of 13.56 MHz is described. The reference cell provides an experimental platform for comparing plasma measurements carried out in a common reactor geometry by different experimental groups, thereby enhancing the transfer of knowledge and insight gained in rf discharge studies. The results of performing ostensibly identical measurements on six of these cells in five different laboratories are analyzed and discussed. Measurements were made of plasma voltage and current characteristics for discharges in pure argon at specified values of applied voltages, gas pressures, and gas flow rates. Data are presented on relevant electrical quantities derived from Fourier analysis of the voltage and current wave forms. Amplitudes, phase shifts, self-bias voltages, and power dissipation were measured. Each of the cells was characterized in terms of its measured internal reactive components. Comparing results from different cells provides an indication of the degree of precision needed to define the electrical configuration and operating parameters in order to achieve identical performance at various laboratories. The results show, for example, that the external circuit, including the reactive components of the rf power source, can significantly influence the discharge. Results obtained in reference cells with identical rf power sources demonstrate that considerable progress has been made in developing a phenomenological understanding of the conditions needed to obtain reproducible discharge conditions in independent reference cells.

Stochastic properties of partial-discharge phenomena

The author presents a bibliography and survey of the literature concerned with theory and measurement of the stochastic behavior of pulsating partial-discharge (PD) phenomena that can occur when insulation is subjected to electrical stress. The types of PD phenomena considered include AC and DC generated electron avalanches, pulsating positive and negative corona in gases, and PD that occur in liquid media and in the presence of solid dielectric surfaces. The basic physical mechanisms of discharge initiation, growth, and memory propagation that determine the probability distributions for pulse occurrence times and pulse amplitudes are discussed. Consideration is also given to special problems associated with the measurement and interpretation of data on the various statistical properties of PD phenomena. >

Polarization of Atomic Fluorescence Excited by Molecular Dissociation

Fluorescence from excited atomic fragments which arises from molecular dissociative processes can be shown in general to be polarized. The degree of polarization is related to the form of the anistropy in the angular distribution of dissociation products.

Fundamental processes of SF/sub 6/ decomposition and oxidation in glow and corona discharges

It is known that sulfurhexafluoride (SF/sub 6/), used as an insulating gas in HV apparatus, will oxide in electrical discharges in the presence of oxygen or water vapor to form various reactive and stable by-products. In order to interpret experimental data on rates of oxidation and by product formation in discharges, meaningfully, it is necessary to apply theoretical chemical kinetics models that utilize rates for numerous gas-phase processes as functions of gas temperature and/or electric-field-to-gas-density ratio. Current knowledge about the fundamental collision processes involving electrons, ions, free radicals, and molecules needed to understand the gas-phase discharge chemistry in SF/sub 6/ is reviewed. Implications of the fundamental rate data reviewed here for recently proposed chemical-kinetics models of corona and glow-type discharges in SF/sub 6/ are discussed. >

Electron-transport, ionization, attachment, and dissociation coefficients in SF6 and its mixtures

An improved set of electron‐collision cross sections is derived for SF6 and used to calculate transport, ionization, attachment, and dissociation coefficients for pure SF6 and mixtures of SF6 with N2, O2, and Ne. The SF6 cross sections differ from previously published sets primarily at very low and high electron energies. At energies below 0.03 eV the attachment cross section is adjusted to fit recent electron swarm experiments, while the elastic momentum transfer cross section is increased to the theoretical limit. At high energies an allowance is made for the excitation of highly excited levels as observed in electron beam experiments. The cross‐section sets used for the admixed gases have previously been published. Electron kinetic energy distributions computed from numerical solutions of the electron‐transport (Boltzmann) equation using the two‐term, spherical harmonic expansion approximation were used to obtain electron‐transport and reaction coefficients as functions of E/N and the fractional concen...

Plasma chemical model for decomposition of SF6 in a negative glow corona discharge

A zonal plasma chemical model is proposed to account for the observed oxidation and decomposition of sulfur hexafluoride (SF6) by a negative, point-to-plane glow-type corona discharge in pressurized SF6/O2/H2O gas mixtures. The model yields dependencies of stable neutral oxidation by-products such as SOF2, SO2F2, SOF4, S2F10, and SO2 on time, discharge current, and O2 and H2O concentrations which are consistent with measured results. Electron-impact-induced dissociation of SF6 in the glow region of the discharge is the decomposition rate-controlling process. The relative roles played by different reactions involving neutral free radicals and ions in different zones of the discharge are examined, and in some cases, reaction rate coefficients have been adjusted within reasonable limits to give best fits to observed production rates of various by-products. Problems of uniqueness that arise because of gaps in our knowledge about important processes that should be included in the model are also discussed.

Kinetic‐energy distributions of ions sampled from argon plasmas in a parallel‐plate, radio‐frequency reference cell

Kinetic‐energy distributions are presented for ions sampled from 13.56‐MHz discharges in argon in a capacitively‐coupled, parallel‐plate, Gaseous Electronics Conference (GEC) radio‐frequency reference cell. The cell was modified to allow sampling of ions through an orifice in the grounded electrode. Kinetic‐energy distributions are presented for Ar+, Ar++, Ar+2, ArH+, and several trace ions for plasma pressures ranging from 1.3 Pa, where ion‐atom collisions in the plasma sheath are not important, to 33.3 Pa, where collisions are important. Applied peak‐to‐peak radio‐frequency (rf) voltages of 50, 100, and 200 V were used, and the current and voltage waveforms at the powered electrode were measured. Dependences of the ion fluxes, mean energies, and kinetic‐energy distributions on gas pressure and applied rf voltage are interpreted in terms of possible ion‐collision processes. The results agree with previously measured kinetic‐energy distributions of ions sampled from the side of the plasma through a ground...

Importance of unraveling memory propagation effects in interpreting data on partial discharge statistics

The significance of memory propagation in controlling the stochastic behavior of partial-discharge phenomena is demonstrated by determination of various conditional amplitude and phase-of-occurrence distributions for both measured and simulated discharge pulses. A system that can be used to measure directly a set of both conditional and unconditional pulse amplitude and phase distributions needed to reveal memory effects and quantify the phase-resolved stochastic properties of partial-discharge pulses, is briefly described. It is argued that not only is an unraveling of memory effects essential in any attempt to understand the physical basis for the observed stochastic behavior of partial-discharge phenomena, but also that the data on conditional distributions provide additional statistical information that may be needed to optimize the reliability of partial-discharge pattern recognition schemes now being considered for use in insulation testing. >

Gas‐phase hydrolysis of SOF2 and SOF4

The rates for gas‐phase hydrolysis of SOF2 (thionylfluoride) and SOF4 (thionyl tetrafluoride) have been measured at a temperature of 298 K. The second order rate constant for SOF2 hydrolysis in SF6 buffer gas was found to have the value (1.2±0.3)×10−23 cm3/s which agrees with previous estimates of Sauers et al., but is three orders of magnitude lower than the value obtained by Ruegsegger et al. at 340 K. The rate constant for SOF4 hydrolysis has not previously been measured and its value in both SF6 and N2 buffer gases was found here to be (1.0±0.3)×10−21 cm3/s.

Ion kinetic‐energy distributions and Balmer‐alpha (Hα) excitation in Ar‐H2 radio‐frequency discharges

Excited neutrals and fast ions produced in a 13.56 MHz radio‐frequency discharge in a 90% argon −10% hydrogen gas mixture were investigated, respectively, by spatially and temporally resolved optical emission spectroscopy, and by mass‐resolved measurements of ion kinetic energy distributions at the grounded electrode. The electrical characteristics of the discharge were also measured and comparisons are made with results obtained for discharges in pure H2 under comparable conditions. Measurements of Balmer‐alpha (Hα) emission show Doppler‐broadened emission that is due to the excitation of fast atomic hydrogen neutrals formed from ion neutralization processes in the discharge. Temporally and spatially resolved emission profiles of the Hα radiation from the Ar‐H2 mixture are presented for the ‘‘slow’’ component produced predominately by electron‐impact dissociative excitation of H2, and for the ‘‘fast’’ component corresponding to energies much greater than can be derived from dissociative excitation. For t...