Saburo Sakamoto

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.

Optical Study of Conduction and Breakdown in Dielectric Liquids

On the optical studies of electrical conduction and breakdown in dielectric liquids, recent works which have been studied in Japan are briefly reviewed first. Next, the results of study in our laboratory on the measurement of ion mobility and on the breakdown process in liquids are presented. When an electrode system such as razorblade emitter-grid-collector is used for the measurement of ion mobility by the time of flight method, the field uniformity between grid-collector is calculated, and the space charge effect related to the current density is investigated. Through these researches suitable condition for ion mobility measurement are revealed. On the breakdown process, the schlieren method and high-speed photography are used under a pulse voltage applied to a needle-plane gap. Various aspects of the breakdown process from streamer initiation to main stroke, the polarity effect of streamer propagation, the relation between the mean velocity of positive streamer Vm and molecular structure of liquid, the relation between vm and pre-breakdown current, Vm of mixed liquid etc. are described.

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.

Field emission into nonpolar organic liquids

Field emission into nonpolar liquids was studied for n‐hexane, 2,2,4‐trimethylpentane, and tetramethylsilane at 275–334 K. The current–voltage dependence was found to vary depending on the liquids and the temperature, and was reproduced qualitatively on the basis of the Fowler–Nordheim theory modified by taking account of the space charge effect. The quasifree electron energy (the electron energy in a liquid with reference to that in vacuum) was estimated from the current–voltage dependence in a space charge‐free region, i.e., from the apparent work functions of the metal emitter (tungsten tip) in the liquids, to be 0, −0.6, and −1.3 eV for n‐hexane (as a standard), trimethylpentane, and tetramethylsilane, at 293 K. These values lie in the same order as determined previously by the photoelectric emission from a metal surface in the liquids, though the dependence on the liquids is more pronounced in the present field emission study.

Measurement of the effective ionisation coefficient and the static breakdown voltage in SF6 and nitrogen mixtures

The effective ionisation coefficient alpha /p20 is measured for E/p20 from 70 to 160 V cm-1 Torr-1, and the static breakdown voltage Vs between parallel plates for p20d from 8 to 130 Torr cm, for SF6 and nitrogen mixtures. It is found that the alpha /p20 of the mixtures may be approximated by a linear function of the partial pressure of SF6 at fixed E/p20 and that Paschens law holds for the p20d may be estimated with good accuracy from the Vs of the respective pure gases using an experimental formula of Takuma et al. (1972) which has been used for much larger p20d.

Undervoltage breakdown between parallel plates in air

Transient discharges in air that are started at undervoltages by supplying a large number of initial electrons at the cathode of parallel plates are investigated at gas pressures of 20 and 200 Torr. The discharge current has been measured, and factors affecting the development of the current are investigated by comparing the experimental current with a computer-simulated current. The spatiotemporal development of the electron, positive-ion and negative-ion densities and of the electric field are also calculated to account for the mechanism of breakdown. It is found at 200 Torr with E/p between 43 and 46 V cm-1 Torr-1 that the electrons produced in the gas by electron detachment have a large effect on the process of the space charge accumulation, which eventually leads to breakdown, while the cathode photoemission ( gamma p) effect is the main secondary process contributing to the space charge accumulation and breakdown at 20 Torr with E/p between 76 and 84 V cm-1 Torr-1.

The variation of steady state electron mean energy between parallel plates in argon

The variation of the steady state electron mean energy between parallel plates in argon and, for comparison, helium gases has been calculated as a function of the distance from the cathode by a Monte Carlo simulation applying a new technique of sampling steady state localized swarm parameters. A brief preliminary survey for excitation and ionization collision cross sections for argon has been made. The results show that, in both gases, damped oscillatory fluctuations of the electron mean energy follow its initial linear increase from zero energy at the cathode and that a rapid rise above the equilibrium energy occurs in front of the anode. This behaviour has been explained qualitatively in terms of an electron energy balance equation. Position dependent, diffusion modified localized drift velocities are also obtained in connection with the explanation.

A Boltzmann equation analysis of electron swarm parameters in CO2 laser mixtures

Electron swarm parameters in CO2 laser gas mixtures have been analysed for a range of the ratio of electric field to gas number density E/N from 2.83 to 283 Td (E/p0=1-100 V cm-1 Torr-1 at 0 degrees C) by a Boltzmann equation method in which the effect of electron impact ionisation and attachment can be considered properly (Thomas 1969, Tagashira et al. 1977, Taniguchi et al. 1977). Gas mixtures of CO2, N2 and He with concentration ratios of the respective gases of 1:1:8, 1:7:30, 1:1:3 and 1:2:3 are studied. The dissociation and dissociative ionisation processes for CO2 have been taken into account in addition to the electron collision processes considered in a previous study of Lowke et al. (1973). The results show that the present values of the ionisation coefficient and of the ratio of electron diffusion coefficient to mobility for the steady-state Townsend experiment are by about a few tens of percent lower than the previous theoretical values of Lowke et al. at high E/N values, but are in good agreement with recent experimental values of Lakhsminarasimha et al. (1976) for the whole E/N range studied. The excitation coefficient for the vibrational levels, important for the CO2 laser action, and the electron attachment coefficient are also calculated and discussed.

Monte Carlo simulation of the reaction and transport of negative ions O- and O2- in oxygen

Properties of the O- and O2- ion swarm in oxygen are studied by a Monte Carlo simulation technique for E/N values from 5 to 200 Td (1 Td=0.354 V cm-1 Torr-1 at 0 degrees C). The generation and loss reactions of the ions, such as dissociative attachment, electron detachment and charge transfer, as encountered in electron avalanches in oxygen, are taken into account. The results show that the values of the ion mobility, diffusion coefficients and detachment and charge transfer coefficients are in reasonable agreement with experiments. With O- ions the drift velocity, defined as the centre-of-mass velocity of an ion swarm, is found to be smaller than that defined as the velocity averaged in the velocity space, as E/N>or approximately=80 Td.