The diffusion tensor in an isolated electron or light ion swarm subject only to elastic collisions with molecules has been formulated from the multiterm expansion of the Boltzmann equation in the hydrodynamic description. Derived diffusion coefficients for the transverse and longitudinal directions include the additional terms representative of the curvature effect under the action of an electric field besides the usual two-term expressions. Discussion is, particularly, focused on the relation between the transverse diffusion coefficient DT and its conventional expression DV. Numerical calculations have been made for the electron swarm in a model gas having a momentum transfer cross-section, Qm( epsilon )=Q0 epsilon beta ( beta =0, 1/2, 1), with isotropic scattering. It will be recognised that an appreciable degree of discrepancy occurs between DT and DV with the increase of the power beta , i.e. of the cross-sectional dependence on energy.
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