The Barkas Effect in Plasma Transport

Molecular dynamics simulations reveal that a fundamental symmetry of plasma kinetic theory is broken at moderate to strong Coulomb coupling: the collision rate depends on the signs of the colliding charges. This breaking of charge-sign symmetry is analogous to the ``Barkas effect'' observed in charged-particle stopping experiments. It gives rise to significantly enhanced electron-ion collision rates and is expected to affect any neutral plasma with moderate to strong Coulomb coupling such as ultracold neutral plasmas (UNP) and the dense plasmas of ICF and laser-matter interaction experiments. The physical mechanism responsible for the Barkas effect is screening of binary collisions by the correlated plasma medium. By including screening directly in the interaction potential governing collisions, it is shown that the Barkas effect arises in the close interactions that lead to large-angle scattering. Because the effect hinges on how screening affects close -- not distant -- interactions, it is a phenomenon beyond what is predicted by traditional transport models based on cut-off Coulomb collisions or mean-field dielectric response. A model for the effective screened interaction potential is presented that is suitable for the coupling strengths achieved in UNP experiments. Transport calculations using this potential agree with simulated relaxation rates and predict that the Barkas effect can cause up to a 70\% increase in the electron-ion collision rate at the conditions of present UNP experiments, where the electron coupling parameter can range from $\Gamma_e\approx0.1$ to $0.5$. The influence of the Barkas effect in other transport processes is also considered.