Y Sakai

The development of electron avalanches in argon at high E/N values. II. Boltzmann equation analysis

For pt.I see ibid., vol.10, no.7, p.1035 (1977). The electron swarm parameters for the steady-state Townsend, pulsed Townsend and time-of-flight experiments are calculated using a Boltzmann equation for argon at E/N values from 85 to 566 Td, when electron impact ionization is appreciable. The results suggest that the value of an electron swarm parameter depends on the type of the experiments involved, because of the presence of the ionization, in agreement with the Monte Carlo simulation work in the previous paper. The drift velocities and diffusion coefficients when ionization is present, are discussed.

Boltzmann equation analysis of the electron swarm development in SF6

The electron swarm development in SF6 gas is studied for E/p20 values from 15 approximately 200 V cm-1 Torr-1 (E/N=45.6-607 Td) by a Boltzmann equation method in which the effect of ionisation and electron attachment is considered properly. The momentum transfer, vibration, ionisation and attachment cross-sections are taken from experiments, but the electronic excitation cross-section is determined by fitting the calculated ionisation and attachment coefficients to previous measurements. The calculation is performed for the steady-state Townsend, pulsed Townsend and time-of-flight experiments. The results show that the deduced electron drift velocities and diffusion coefficients agree reasonably well with experiment and the value of a swarm parameter should depend on the type of experiment not only at high E/p20 values, at which ionisation is appreciable, but also at low E/p20 values, at which electron attachment is appreciable. The electron energy distribution, which is found to have depressions at low E/p20 values, the electron mean energy and the excitation coefficients are also calculated and discussed.

The development of electron avalanches in argon at high E/N values. I. Monte Carlo simulation

The behaviour of electron avalanches in argon when appreciable electron impact ionization occurs, is studied by a Monte Carlo simulation. The values of electron swarm parameters are obtained for steady-state Townsend, pulsed Townsend and time-of-flight experiments, by applying sampling techniques appropriate to the respective experiments. The results suggest that the value of an electron swarm parameter, such as the ionization frequency or the electron drift velocity, depends on the type of experiments for which there is appreciable electron impact ionization. The properties of electron avalanches in an electron energy non-equilibrium region, for example the variation of the electron energy distribution with distance from the cathode, and the effect of anisotropic scattering of electrons on the swarm parameters very close to the anode, are also studied.

Effects of additives on prebreakdown phenomena in n-hexane

Prebreakdown phenomena in n-hexane are observed in detail for positive and negative polarities by using simultaneously a high speed schlieren technique and an LED current measuring system, when an impulse voltage (1.1/225 /spl mu/s) is applied to a point-to-plane electrode gap. Furthermore, the effects of several additives on the streamer propagation are investigated. Especially the effects of electron-trapping additives on negative streamer propagation and of low ionization potential additives on the positive streamer propagation, are examined, as is a correlation between the shape and the propagation velocity of the streamers. >

Boltzmann equation analysis of the electron swarm development in nitrogen

The electron swarm development in nitrogen is studied for E/N values from 56.6 to 1131 Td by a Boltzmann equation method in which the effect of ionisation is considered properly. Alteration of the electron collision cross-sections from the literature is limited to a minimum, although minor amendments are found necessary for the ionisation coefficient to agree well with previous measurements. The cross-sections for vibrational excitation are considered separately for its quantum number v=1-10. The electron swarm parameters are calculated for the time-of-flight, pulsed Townsend and steady-state Townsend experiments. The results show that the deduced electron drift velocity, diffusion coefficient and mean energy are generally in good agreement with previous experimental and theoretical values.

Boltzmann equation analyses of electron swarm parameters in Ar/Ne, Kr/Ne, Xe/Ne, Hg/Ar and Hg/Kr mixtures and derived effective excitation cross sections for metastable states of rare atoms

The steady-state electron swarm parameters in a series of Penning gases, Ar/Ne, Kr/Ne, Xe/Ne, Hg/Ar and Hg/Kr mixtures, were analysed using a Boltzmann equation in which generation of secondary electrons through Penning ionization and ionization collision between two metastable Hg atoms, as well as electron-impact ionization, are property considered. The ratios of the Townsend first ionization coefficient to the field strength, alpha /E, calculated using the present equation in Ne gas with admixtures of Ar, Kr and Xe, were strongly influenced by Penning electrons. On the other hand, in Ar and Kr gases with Hg vapour, the ionization between two metastable Hg atoms was found to be the dominant ionization process at low E/N. The present alpha /E in every gas agrees well with experimental values over a wide range of alpha /E. The effective cross section of excitation for the metastable states of the rare atoms was derived by comparing the present alpha /E with the experimental values.

Development of electron avalanches in argon-an exact Boltzmann equation analysis

Development of the electron avalanche in argon has been studied at E/N=141, 283 and 566 Td by two exact Boltzmann equation methods, a Fourier expansion (FE) of the distribution function of electrons by Tagashira et al. (1978), and a direct estimation of moments (DEM) of the electron density distribution in the real space by starting from a Boltzmann equation. Elastic momentum transfer, total electronic excitation and ionisation collisions are considered. The electron swarm parameters obtained by FE agree exactly with those by DEM, justifying the expansion used in FE, and the swarm parameters obtained by two-term expansion (TE) are in good agreement with those obtained by the exact FE and DEM except the longitudinal diffusion coefficient DL and a higher order coefficient D3 at E/N=566 Td and the transverse diffusion coefficient DT at all the E/N studied, suggesting that the disagreement with DL and D3 is due simply to breakdown of TE at high E/N but the disagreement with DT is due to more essential insufficiency in TE. The electron velocity distributions are also obtained and discussed.

Boltzmann equation analyses of electron swarm parameters in Hg/Ar gas mixtures: effect of metastable Hg and Ar atoms

The steady-state electron swarm parameters in Hg/Ar mixtures are analysed using a Boltzmann equation in which the collision terms providing generation of secondary electrons via metastable Hg and Ar atoms presented in the preceding paper (see Y. Sakai et al., ibid., vol.22, p.276, 1989) are taken into consideration. The ratio of the Townsend first ionisation coefficient to the electric field strength, alpha /SiE, calculated using the present equation for E/N values from 5 to 1412 Td in Ar with various admixtures of Hg vapour (K=ratio of Hg to total gas atom concentrations from 2*10-4 to 2*10-2) is in good agreement with the experimental values of Burgmans and Smeets (1983). For E/N below 100 Td and K above 6*10-3 ionisation collision between two metastable Hg atoms is found to be the dominant process for electron multiplication. The influence of electron concentration on the swarm behaviour is discussed quantitatively.

Influence of insulating barrier on the creepage discharge in transformer oil

The propagation aspects of a creepage discharge in transformer oil are observed simultaneously and in detail under a positive impulse voltage condition, by means of a high-speed Schlieren optical system, an LED current measurement system and a charge measurement system using a capacitor. In the present paper, the influence of the material type and the thickness of insulating barriers on the propagation of the impulse creepage discharge are discussed for a point to plane electrode geometry. In addition, the influence of a barrier surface to the electric field direction on the propagation of the impulse creepage discharge is discussed for a parallel plane electrode gap with a protruding point.

Relaxation process of the electron velocity distribution in neon

The relaxation process from an initial velocity distribution to the equilibrium distribution for electrons in neon is calculated by a finite difference method for the ratios of electric field to gas number density E/N between 56.6 and 566 Td (E/p0=20 and 200 V cm-1 Torr-1 at 0 degrees C) without using the usual two-term spherical harmonics expansion of the velocity distribution. The pulsed Townsend condition, in which the evolution of all the electrons involved in an avalanche is observed as a function of time only, is assumed. The results suggest that the electron velocity distribution reaches through randomisation the equilibrium distribution which has a structure with a minimum near the origin in the velocity space.