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PubMed2002

Diffusion in two-temperature thermal plasmas.

Combined diffusion coefficients, which allow species to be grouped into their parent gases, are derived in a thermal plasma in which the kinetic temperature of the electrons is different from that of the heavy species. As in previous calculations of combined diffusion coefficients for a one-temperature plasma, chemical equilibrium is assumed, and a plasma in a mixture of two homonuclear nonreacting gases is treated. Expressions for ambipolar diffusion in a two-temperature plasma are derived. Coupling between the electrons and the heavy species is retained in the calculations. It is confirmed that the values of the diffusion coefficients are consistent with mass conservation, in contrast to previous calculations that were based on a theory in which the electrons and heavy species were decoupled. Illustrative results are presented for a thermal plasma in different mixtures of argon and hydrogen at 1 bar, for temperatures up to 25 000 K and different ratios of electron to heavy species temperature.

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PubMed2001

Transport properties in a two-temperature plasma: theory and application.

An alternate derivation of transport properties in a two-temperature plasma has been performed. Indeed, recent works have shown that the simplified theory of transport properties out of thermal equilibrium introduced by Devoto and then Bonnefoi, very often used in two-temperature modeling, is questionable and particularly does not work when calculating the combined diffusion coefficients of Murphy. Thus, in this paper, transport properties are derived without Bonnefoi's assumptions in a nonreactive two-temperature plasma, assuming chemical equilibrium is achieved. The electron kinetic temperature T(e) is supposed to be different from that of heavy species T(h). Only elastic processes are considered in a collision-dominated plasma. The resolution of Boltzmann's equation, thanks to the Chapman-Enskog method, is used to calculate transport coefficients from sets of linear equations. The solution of these systems allows transport coefficients to be written as linear combinations of collision integrals, which take into account the interaction potential for a collision between two particles. These linear combinations are derived by extending the definition and the calculation of bracket integrals introduced by Chapman et al. to the thermal nonequilibrium case. The obtained results are rigorously the same as those of Hirschfelder et al. at thermal equilibrium. The derivation of diffusion velocity and heat flux shows the contribution of a new gradient, that of the temperature ratio straight theta=T(e)/T(h). An application is presented for a two-temperature argon plasma. First, it is shown that the two-temperature linear combinations of collision integrals are drastically modified with respect to equilibrium. Secondly, the two-temperature simplified theory of transport coefficients of Devoto and Bonnefoi underestimates the electron thermal conductivity with respect to the accurate value at T(e)=20 000 K. Lastly, contrary to the simplified theory of transport coefficients, the diffusion coefficients satisfy the symmetry conditions. An example is given at T(e)=6000 K for different values of straight theta for the diffusion coefficient between electrons and heavy species D(e-Ar) as well as for that between argon atoms and argon ions D(Ar-Ar+).

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