SF6 Electron Research Articles

Generation of runaway electrons beams during the breakdown of high-pressure gases

Generation of run-away electrons in SF6, CO2, argon and nitrogen at high and super high pressures is studied. Super-short avalanches electron beams (SAEB) was obtained and measured with a collector at pressures up to 0.3, 0.7, 1.0 and 1.2 MPa in SF6, CO2, argon and nitrogen, respectively. The SAEB duration was shown to be ∼60 ps (FWHM) and gas composition has only minor effect on the duration. It was found that in a gap of 4 mm in SF6, CO2, argon and nitrogen at pressure up to 0.3, 0.7, 1.0 и 1.2 MPa the voltage pulse duration (FWHM) and amplitude increase with pressure.

Breakdown temperature of electrons in SF6 gas

The breakdown temperature Tb of electrons in SF6 gas is obtained by expressing the ionization and attachment coefficients of the gas in terms of the electron temperature Te assuming that the electron energy in the gas is in a Maxwellian distribution. The sparking criterion of the SF6 gas was obtained in terms of the gas pressure p and the anode-cathode distance d. It was found that the breakdown temperature Tb of electrons in SF6 in a high-pressure gas characterized by pd⪢0.001 is 4.9 eV, which is independent of the parameter pd. Here, pd is in units of atm cm.

Low-lying states of SF6 and SF6?: Electron affinity of SF6 and electron detachment of SF6?

Calculations have been performed on low-lying states of SF6 and SF at the complete active space multiconfiguration self-consistent field (CAS–MCSCF) level. These calculations involved optimization of basis function exponents, particularly the f-functions on sulfur and their effect on the potential energies and the SF6 electron affinity. The CAS–MCSCF calculations for the anion and neutral included 4,863 and 2,115 configuration functions, respectively. Octahedral and selected distorted geometries were considered. The ground-state equilibrium geometries correspond to the octahedral configuration of SF6 (1A1g), R(SF) = 1.56 A, ωe = 776 cm−1, and of the anion SF(2A1g), R(SF) = 1.70 A, ωe = 558 cm−1. The adiabatic and vertical electron affinities are found to be -0.63 and 1.8 eV, respectively. Electron autodetachment phenomena associated with the anion are also discussed and a possible mechanism for this process is presented. © 1993 John Wiley & Sons, Inc.

Monte Carlo simulation of the motion of electrons in SF6 in uniform electric fields

The motion of electrons in SF6 in uniform electric fields is simulated using a Monte Carlo technique for the range of reduced electric fields 100≤E/p≤180 V cm−1 Torr−1. The swarm parameters evaluated are compared with the experimental results of drift velocity, ratio of diffusion coefficient to mobility, ionization, and attachment coefficients. The electron-molecule collision cross sections adopted in the simulation result in a good agreement with the experimental values over the range of E/p investigated.

Breakdown field strength of SF6, N2O, SF6+N2, and SF6+N2O

The breakdown voltages of some electron attaching gases and gas mixtures have been calculated from their fundmental properties such as ionization and attachment cross sections, the momentum transfer cross section, the drift velocity or the mean energy of electrons. The calculated breakdown voltages in SF6, N2O, SF6+N2, and SF6+N2O gas mixtures, on the basis of Maxwellian electron energy distribution, agree very well with the measured values. The good agreement between the calculated and measured values shows that the mean energies of electrons in SF6+N2 corresponds to that of SF6 up to 90% concentration of N2 in the mixture, whereas the mean energies of electrons in SF6+N2O mixtures correspond to that of the majority gas.

Diffusion and drift of electrons in SF6

Measurements have been made of the drift velocities and the ratios of the diffusion coefficient to mobility (D/μ) of electrons in SF6 over the range 115<E/p<210 V cm−1 Torr−1 normalized to 20°C with gas pressures in the range 0·6 to 2 Torr and for diffusion gap distances of 3, 5 and 7 cm. In the derivation of the D/μ values, Townsend's primary ionization and electron attachment coefficients reported by Bhalla and Craggs (for the range 110 <E/p<160) and Moruzzi (for the range 160 <E/p<210) were used. The close agreement obtained between the D/μ values at various pressures and gap lengths supports the data of these workers. Anomalous diffusion of electrons was observed particularly at high pressures for E/p<130.

Drift velocities of electrons in sulphur hexafluoride

Values of the drift velocity of electrons in SF6, obtained from measurements of the back diffusion of photoelectrons from the cathode of a uniform field electrode system, are reported for values of E/N (E the electric field and N the gas number density) in the range 15*10-17 <or=E/N <or=150*10-17 V cm2.