A Fokker-Planck study of the eigenmodes in an unmagnetized pair plasma

Abstract

Linearized Fokker-Planck equations for unmagnetized pair plasmas are solved as an eigenvalue problem to investigate the sound waves and Langmuir waves. The frequencies and damping rates of sound waves and Langmuir waves as a function of kλ and kλD are presented, where k is the wave number, λ is the mean-free path, and λD is the Debye length. It is found that no electrostatic field is evolved in the process of sound wave. As a consequence, Landau damping is not relevant to the sound waves in a pair plasma. The damping mechanics of sound waves is fully governed by the Coulomb collisions. The valid regimes of fluid descriptions for the waves are also discussed by comparing with the computational results.