# The Barkas Effect in Plasma Transport

## TL;DR

This study uncovers a charge-sign dependent asymmetry in plasma collision rates at strong coupling, driven by screening effects, which significantly impacts plasma transport models and experimental interpretations.

## Contribution

It introduces the first evidence of the Barkas effect in plasma transport, showing how screening causes charge-sign asymmetry in collision rates at moderate to strong Coulomb coupling.

## Key findings

- Collision rates depend on charge sign at strong coupling.
- The Barkas effect can increase electron-ion collision rates by up to 70%.
- Traditional models do not predict this asymmetry.

## Abstract

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.