Paper detail

Revision of the Coulomb logarithm in the ideal plasma

The standard picture of the Coulomb logarithm in the ideal plasma is controversial, the arguments for the lower cut off need revision. The two cases of far subthermal and of far superthermal electron drift motions are accessible to a rigorous analytical treatment. We show that the lower cut off $b_{\min}$ is a function of symmetry and shape of the shielding cloud, it is not universal. In the subthermal case shielding is spherical and $b_{\min}$ is to be identified with the de Broglie wavelength; at superthermal drift the shielding cloud exhibits cylindrical (axial) symmetry and $b_{\min}$ is the classical parameter of perpendicular deflection. In both situations the cut offs are determined by the electron-ion encounters at large collision parameters. This is in net contrast to the governing standard meaning that attributes $b_{\min}$ to the Coulomb singularity at vanishing collision parameters $b$ and, consequently, assigns it universal validity. The origin of the contradictions in the traditional picture is analyzed.