LASER ACTION IN C IV, N V, AND O VI PLASMAS COOLED BY ADIABATIC EXPANSION

Millette,P.A.: 1980, M.Sc. thesis, Univ. Ottawa.

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The population densities of the discrete levels of the lithium-like ions C IV, N V, and O VI in optically thin plasmas cooled by adiabatic expansion have been calculated with the Collisional-Radiative (CR) plasma model, suitably modified to account for the small energy separation of the ground and the first excited states of these ions. The following elementary processes have been included in the CR model: electron impact ionization, excitation, and de-excitation, three-body and radiative recombination, and spontaneous transitions. The data and calculations available on these processes have been analysed and extended by various methods. The resulting rate coefficients are compared with the corresponding hydrogenic values, and a discussion of their accuracy is given. Population inversions have been found to occur in many of the transitions of these ions. We have concentrated our attention to such transitions of the ion C IV between levels with n < =6 which give rise to emission lines in the visible region of the spectrum. The gain of the inversely populated transitions is presented in the form of ne-Te diagrams. The C IV 4646, 4658 lines arising from the 6f-5d and 6g-5f transitions respectively, are found to be strongly inverted and should be excellent candidates for producing laser action in laboratory plasmas cooled by adiabatic expansion techniques. In addition, the behavior of the line C IV 4650 observed in the WC category of the Wolf-Rayet stars is found to be in agreement with that expected from the model calculations. The present investigation thus provides an understanding of the unusual strength of the C IV 4650 emission in Wolf-Rayet stars, and provides a strong basis for believing that laser action is responsible for it.


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