Bump-On-Tail Instability Across Coupling And Interaction-Range Regimes

The impact of collisionality and the range of the interparticle interaction on the bump-on-tail instability is examined both computationally and theoretically. Using three-dimensional, nonequilibrium molecular dynamics with a force law that varies continuously from long range (pure 1/r Coulomb limit) to short range and across coupling regimes from weakly to strongly collisional, we examine properties of the instability; unlike other computational methods, molecular dynamics includes large-angle scattering, including the extreme limit of caging, and dynamical nonlinear screening across length scales as small as the interparticle spacing. A simpler theoretical model is also developed for comparison to the molecular dynamics results; we find that the two methods agree well in the appropriate limit, revealing that molecular dynamics is an accurate tool for the exploration of plasma instabilities. Our results reveal that the bump-on-tail instability exists only for long-range interactions.