  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 symmetry breaking is
analogous to the Barkas effect observed in charged-particle stopping
experiments and gives rise to significantly enhanced electron-ion collision
rates. It 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 is screening of binary collisions by the correlated plasma medium,
which causes an asymmetry in the dynamics of large-angle scattering. Because
the effect pertains only to close interactions, it is not predicted by
traditional transport models based on cut-off Coulomb collisions or linear
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 based on this potential and the effective
potential kinetic theory 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. The influence of the Barkas
effect in other transport processes is also considered.
