The pulsed Townsend technique has been used to measure the electron drift velocity, the density-normalized effective ionization coefficient, the density-normalized longitudinal diffusion coefficient NDL, and the `characteristic energy' of electrons DL/K, in CO2 and its mixtures with SF6 over a wide range of the density-reduced field strength E/N, from 100 to 700 Td (1 Townsend = 10−17 V cm2). The SF6 content in the mixture was varied between 2% and 70%. It was observed that for small concentrations (2–5%) of SF6 in the mixtures, the electron drift velocity is relatively close to that for pure CO2. A similar behaviour was observed for the longitudinal diffusion coefficients. In contrast, the influence of SF6 in the mixture is strongly apparent in the values for the effective ionization coefficients. From the latter parameter, the critical field strength E/Ncrit for each SF6 concentration could be derived, and it was found that its value is smaller than that measured for the SF6–N2 mixtures.

This paper reports on the measurement of the electron drift velocity, the density normalized effective ionization coefficients (α-η)/N, and the ratio η/α (η and α are the attachment and ionization coefficients, respectively), over a wide range of the density-reduced field strength, E/N, from 40 to 560 Td (1 Td = 10-17 V cm2). From the measured values of (α-η)/N and η/α, the density normalized ionization and attachment coefficients were obtained. Further analysis of avalanche development past the first electron transit allowed us to derive the average mobility of both positive and negative ions. The mobility of negative ions was found to agree very well with Blanc's law at small fields, thereby indicating that the majority negative ion in the avalanche is SF6-. The critical field strength was found to vary almost linearly with the SF6 content in the mixture, comparing well with previous measurements.

This paper reports the measurement of the electron drift velocity ve, the longitudinal diffusion coefficient NDL and the density-normalized effective ionization coefficient (α − η)/N in pure CF3I and in the CF3I–N2 mixtures, where α and η are the electron impact ionization and attachment coefficients, respectively, and N is the gas density. The E/N range covered was 100–850 Td (1 Td = 10−17 V cm2). The present results were derived from a pulsed Townsend experiment. For pure CF3I, the values of ve and (α − η)/N were found to increase linearly with E/N. Moreover, the E/N value at which ionization equals attachment, commonly referred to as the limiting field strength, was found to be E/Nlim = 437 Td, which is greater than that of SF6 (360 Td), a widely used insulating gas. For the CF3I–N2 mixture with 70% CF3I, this E/Nlim value was found to be essentially the same as that for pure SF6.

A standard swarm analysis of electron scattering cross sections in nitrous oxide (N 2 O) is presented. The experimental results for drift velocities and effective ionization coefficients (differences between the ionization and attachment coefficients), obtained over an extended range of E/N (electric field normalized to the gas number density) by the pulsed-Townsend technique, are compared with the numerical solution of the Boltzmann equation. Our analysis shows that commonly used sets of cross sections have to be modified in order to fit the new experimental data, in particular the dissociative cross sections for attachment and electronic excitation (with a threshold energy of around 4.0 eV). Using a single set of cross sections it was possible to fit both the data for pure N 2 O and those for the N 2 O/N 2 mixtures with 20%, 40%, 60% and 80% N 2 O.

The pulsed Townsend technique has been used to measure the electron drift velocity, the density-normalized effective ionization coefficient (α − η)/N (α and η are the ionization and attachment coefficients, respectively), the density-normalized longitudinal diffusion coefficient NDL, and the ratio between the longitudinal diffusion coefficient and the electron mobility DL/K, in CHF3 mixed with Ar and N2, over a wide range of the density-normalized electric field strength E/N, from 0.2 to 400 Td (1 Td = 10−17 V cm2). The CHF3 content in the mixtures was varied between 1% and 50%. Regions of negative differential conductivity (NDC) appear in the plots of the electron drift velocity as a function of E/N. This effect is more pronounced for the CHF3–Ar mixtures than for the CHF3–N2 ones, since it results from the presence of a Ramsauer–Townsend minimum in the momentum transfer cross section for Ar, which is absent in N2. For the CHF3–N2 case, a shallow region of NDC is observed, and it is thought to be due to various inelastic collision processes between the electrons and the buffer gas, and also to the steep fall of the scattering cross sections for CHF3 at low electron energies. Additionally, the dependence of NDL on E/N displays well-defined minima at low E/N, which are a result of the strong inelastic energy loss of the electrons. The effective ionization coefficients were found to be weakly dependent on the concentration mixture for CHF3–N2, while a strong dependence on this parameter was found for the CHF3–Ar mixture. In all cases, the attaching character of the mixtures was found to be very small. For low E/N (< 20%) in the CHF3–N2 mixture it was observed that the values of (α − η)/N are higher than those expected due to electron impact. We believe that Penning ionization may be responsible for this effect.