J. Phys. II France
Volume 2, Numéro 3, March 1992
Page(s) 295 - 314
DOI: 10.1051/jp2:1992135
J. Phys. II France 2 (1992) 295-314

Coherent anti-Stokes Raman scattering study of the dynamics of a multipolar plasma generator

M. Lefebvre1, M. Péalat1, J.-P. Taran1, M. Bacal2, P. Berlemont2, D.A. Skinner2, J. Bretagne3 and R.J. Hutcheon4

1  Office National d'Etudes et de Recherches Aérospatiales, 29 Avenue de la Division Leclerc, 92320 Châtillon, France
2  Laboratoire de Physique des Milieux Ionisés, Laboratoire du CNRS, Ecole Polytechnique, 91128 Palaiseau, France
3  Laboratoire de Physique des Gaz et des Plasmas, Université Paris-Sud, Bâtiment 212, 91405 Orsay Cedex, France
4  Department of Physique, Unviersity of Cape Town, 7700 Rondebosch, South Africa

(Received 16 July 1991, revised 12 November 1991, accepted 19 November 1991)

A Coherent Anti-Stokes Raman Spectroscopy (CARS) study of the hydrogen plasma generated by a discharge with magnetic multipolar confinement has been conducted at pressures in the range 0.5-5 Pa. The steady-state radial distribution of the rovibrational populations has been measured. The vibrational temperature is always uniformly distributed and so is the rotational temperature at the lower pressures, while a strong gradient is seen at 5 Pa for the rotation. Time-resolved measurements with the discharge operated in a square-pulse mode give additional insight into the dynamics of the discharge. Some results are compared with the predictions of two computer models of the plasma kinetics. We observe H 2 vibrational excitation by the Joule-heated filament alone (in the absence of the discharge) and show it to be caused primarily by the confined discharge between the filament and its cold positive copper connector. Another interpretation of the presence of vibrationally excited H 2 by recombinative desorption (Hall R.I. et al., Phys. Rev. Lett. 60 (1988) 337) is not comforted by our results, within instrumental sensitivity. The densities of the first rotational levels that the ortho and para forms of H 2 have different electron collisional cross-sections. Under pulsed excitation, the vibrational temperature rises on a time scale of 1-2 ms in agreement with numerical predictions. At switch-off, we show by matching the experimental and theoretical decays that vibrational state v=1 survives $16\pm 5$ wall collisions; meanwhile, the rotation cools very rapidly, probably because of superelastic electronic collisions.

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