Topic number: 1
Electron distribution function and transport coefficients relevant for air plasmas in the upper atmosphere F. J. Gordillo-Vázquez (1) and Z. Donkó (2) (1) (2)
Instituto de Óptica, CSIC, Serrano 121, 28006 Madrid, Spain
Research Institute for Solid State Physics and Optics, H-1525 Budapest, P.O. Box, 49, Hungary [
[email protected]]
The generation of air plasmas in the mesosphere of the Earth due to the electrical activity in underlying thunderstorms is a subject of research and debate nowadays. One type of transient luminous events, the so called Sprites that are weakly ionized plasmas, has been recorded all over the planet since 1989 [1]. It is of particular interest to clarify how these plasmas are created, what mechanisms are responsible for breakdown, what is their chemical composition, and what are their important microscopic processes (non-equilibrium chemistry). In order to study both prebreakdown and breakdown phenomena in air under the action of an electric field and to investigate the non-equilibrium chemistry of air plasmas, it is important to have a good understanding of the behaviour of transport coefficients and of the electron distribution function (EDF) in dry and humid air plasmas. The air background composition considered in this work is: N2 (78.084 %), O2 (20.946 %), Ar (0.934 %) plus 35 ppm of CO2 and a variable concentration of H2O. The latter has been allowed to take the values of 0 ppm (dry air), 4 ppm and 10 ppm, of interest for air plasmas in the upper atmosphere. In addition, we have also evaluated the impact of high (> 50 %) relative humidity in air at room temperature (293 K). Thus, we also present results for the cases of 50 %, 75 % and 100 % relative humidity, which are of possible interest for different applications of air plasmas.
Figure. 1 Comparison between calculations, by Monte Carlo (solid lines) and ELENDIF (dashed lines), and available experimental data (symbols, see text) of transport coefficients in dry air (a-c) as a function of E/N. The EDFs are shown in (d) as obtained from Monte Carlo (solid lines) and ELENDIF (dashed lines).
The transport parameters and the EDF have been calculated by two methods: one is based on the solution of the Boltzmann equation in the two-term scheme using ELENDIF [2], the second is the Monte Carlo (MC) method. The MC simulation implemented here uses the null-collision method and isotropic scattering has been assumed for all collisional processes. Our Monte Carlo code has been tested with benchmark calculations to verify its accuracy [3]. We have used an updated set of cross sections for a total of 81 elastic and inelastic collisional processes. The sets of cross sections for H2O and CO2 include 22 and 12 processes, respectively. The chief results for dry air plasmas are shown in Figure 1; the general agreement between the results of our calculations and the available experimental data is quite reasonable, except perhaps in the case of the mean energy at medium values of E/N. The different experimental data are taken from Crompton et al. [4] (mean energy and DT/µ); Rees [5], Davies [6], Ryzko [7] and Frommhold [8] (drift velocity); Raja Rao and Govinda Raju [9] (α/N and DT/µ) and Moruzzi [10] (α/N); Rees and Jory [11] and Lakshminarasimha and Lucas [12] (DT/µ). It is important to note that in [4], [9], and [11] the authors report their measured data as D/µ, while from their experimental technique it is quite obvious that they have measured DT/µ. It is worth mentioning that no updates of experimental work on transport coefficients in dry air have been produced (to the best of the authors’ knowledge) in the last 25 years, since the compilation by Gallagher et al. [13] and the report by Davies [7]. The EDFs calculated with the Monte Carlo method and ELENDIF – shown in Figure 1(d) – are in quite good agreement, except in their high energy tails. This difference becomes more important as the value of the E/N increases, which might be a consequence of the breakdown of the validity of the two-term approximation used in ELENDIF. Measurements of transport coefficients in air at high relative humidity are very few [14,15]. Our calculations indicate that, at room temperature and relatively low field E/N = 5 Td, DT/µ, DL/µ and decrease from 0.76 eV, 0.40 eV and 0.86 eV in the dry cases to 0.30 eV (by about 60 %), 0.16 eV (by about 65 %) and 0.47 eV (by above 45 %), respectively, when the relative humidity grows from 0 % to 100 %. DTN also decreases (by 35 %) with humidity and the drift velocity grows (by around 75 %) following the trend measured by Milloy et al. [15] for 50 % relative humidity. Acknowledgments This work has been supported by the Hungarian Fund for Scientific Research (OTKA-T-48389) and by a bilateral grant CSIC (2006HU0002) – Hungarian Academy of Sciences (08). References [1] D. D. Sentman, E. M. Wescott, D. L. Orborne, D. L. Hampton and M. J. Heavner, 1995, Geophys. Res. Lett., 22 (10), 1205. [2] W. L. Morgan and B. M. Penetrante, 1990, Comput. Phys. Comm., 58 (1-2), 127. [3] Z. M. Raspopovic, S. Sakadzíc, S. A. Bzenic and Z. Lj. Petrovic, 1999, IEEE Trans. Plasma Science, 27 (5), 1241; A. N. Nolan, M. J. Brennan, K. F. Ness and A. B. Wedding, 1997, J. Phys. D 30, 2865. [4] R. W. Crompton, L. G. H. Huxley and D. J. Sutton, 1953, Proc. R. Soc. London. Ser. A 218, 507. [5] J. A. Rees, 1973, Aust. J. Phys. 26, 427. [6] D. K. Davies, 1983, Theoretical Notes, Note 346, Westinghouse R&D Center, Pittsburgh, Pa, USA. [7] H. Ryzko, 1965, Proc. Phys. Soc. London 85, 1283. [8] L. Frommhold, 1964, Fortschr. Phys. 12, 597. [9] C. Raja Rao and G. R. Govinda Raju, 1971, J. Phys. D: Appl. Phys 4, 769. [10] J. L. Moruzzi and D. A. Price, 1974, J. Phys. D: Appl. Phys 7, 1434. [11] J. A. Rees and R. L. Jory, 1964, Aust. J. Phys. 17, 307. [12] C. S. Lakshminarasimha and J. Lucas, 1977, J. Phys. D: Appl. Phys 10, 313. [13] J. W. Gallagher, E. C. Beaty, J. Dutton, L. C. Pitchford, 1983, J. Phys. Chem. Ref. Data, 12 (1), 109. [14] V. N. Maller and M. S. Naidu, 1976, Indian J. Pure Appl. Phys. 14, 733; A. N. Prasad and J. D. Craggs, 1960, Proc. Phys. Soc. London 76, 223; H. Ryzko, 1966, Proc. 7th ICPIG, vol 1, 97. [15] H. B. Milloy, I. D. Reid and R. W. Crompton, 1975, Aust. J. Phys. 28, 231.
