Measured and calculated SF-6 collision and swarm ion transport data in SF6 -Ar and SF6 -Xe mixtures.

The measurement of the mobility of SF-6 in the mixtures SF6 -Ar and SF6 -Xe is reported over the density-reduced electric field strength E/N 1-180 Td (1 Townsend = 10(-17) V cm(2)), from a time-resolved pulsed Townsend technique. Simultaneously, the mobility of SF-6 in the same binary mixtures has been calculated from a set of collision cross sections for SF-6 -Ar, SF-6 -Xe, and SF-6 - SF6 using a Monte Carlo simulation procedure for ion transport. The good agreement between measured and calculated mobilities in these gas mixtures has led us to conclude that the validation of our cross section sets is confirmed. The elastic collision cross section, a predominant process for ion energies lower than about 10 eV, was determined from a semiclassical JWKB approximation using a rigid core potential model for the ion-neutral systems under consideration. This elastic cross section was then added to several other inelastic collision cross sections found in the literature for ion conversion, electron detachment of SF-6 and charge transfer. Moreover, the calculations of the mobility and the ratios of the transverse and longitudinal diffusion coefficients to the mobility were extended into a much wider E/N range from 1 to 4000 Td. Additionally, we have also calculated the energy distribution functions and the reaction coefficients for ion conversion and electron detachment. Finally, we have shown that the range of validity for the calculation of the mobility in gas mixtures from Blanc's law is only valid for the low E/N region, where the interaction is dominated by elastic collisions and the ion distribution function remains essentially Maxwellian.

Electron drift velocities in mixtures of helium and xenon and experimental verification of corrections to Blanc's law.

Measurements of electron drift velocities were performed in pure Xe and He and in a number of mixtures ranging up to 70% of Xe. The data were obtained by using a pulsed Townsend technique over the density-normalized electric field strength E/N between 1 and 100 Td . Even for pure gases there are no data in the entire range covered here, and these data represent an extension of accurate drift velocities to higher E/N. A selection of well-established cross sections for low energies, which was extended to higher energies, led to a reasonably good agreement of the calculated transport coefficients with the available data. At the same time we have applied the standard (common E/N) Blanc's law and two forms of common mean energy (CME, due to Chiflykian) procedures. Blanc's law fails for most mixtures at low and moderate E/N, while the CME procedure is capable of following the experimental data for the mixtures much more closely, and even predicting the negative differential conductivity region when such effect does not exist for pure gases. Thus the present paper also represents an experimental test of procedures to correct the standard Blanc's law. Finally, we have used the data for two mixtures to obtain results for the third mixture and in all cases this procedure gave excellent results even though only the standard Blanc's law was used in the process.