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The drift velocity of electrons in mixtures of gaseous water and helium is measured over the range of reduced electric fields 0.1-300 Td using a pulsed-Townsend technique. Admixtures of 1% and 2% water to helium are found to produce negative differential conductivity (NDC), despite NDC being absent from the pure gases. The measured drift velocities are used as a further discriminative assessment on the accuracy and completeness of a recently proposed set of electron-water vapour cross-sections [K. F. Ness, R. E. Robson, M. J. Brunger, and R. D. White, J. Chem. Phys. 136, 024318 (2012)]. A refinement of the momentum transfer cross-section for electron-water vapour scattering is presented, which ensures self-consistency with the measured drift velocities in mixtures with helium to within approximately 5% over the range of reduced fields considered.

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.

Photo-double ionization of gaseous water has been investigated in the 30-53 eV energy range using synchrotron radiation and the threshold photoelectrons coincidence (TPEsCO) method. The TPEsCO spectrum of H 2 O, encompassing the formation of theX 3 B

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A numerical code which calculating the spatiotemporal development of the discharge current in the Townsend regime has been developed. This code was written in Fortran 77. The individual contribution of electrons, positive, and negative ions to the total measurable current in the pulsed discharge is calculated by solving the corresponding set of coupled continuity equations for each species, for which boundary and initial conditions are set adequately. This simulation code is also capable of considering a more realistic scheme of photoelectron emission, such as that produced using a pulsed ultraviolet laser. In addition, the processes of ion and electron drift and longitudinal diffusion, electron detachment, negative-ion stabilization, and ion conversion have been calculated successfully. Some of these effects are extremely difficult to model analytically, and the simultaneous inclusion of two or more of them proves practically impossible. The power of this method in simulating complex discharge situations is discussed in this paper by means of examples considering the processes of electron and ion drift, diffusion, electron impact ionization and attachment, electron detachment, and negative-ion stabilization.Index Terms-Attachment, continuity equations, diffusion, drift velocity and diffusion, electron and ion swarms, electron detachment, finite difference methods, ionization, ionization and attachment, low temperature plasmas, negative ions, numerical simulation codes, pulsed Townsend experiment.