  Generalising the work of Lenard and Bernstein, we introduce a new, fully
relativistic model to describe collisional plasmas. Like the Fokker-Planck
operator, this equation represents velocity diffusion and conserves particle
number. However, unlike the Fokker-Planck operator it is linear in the
distribution function, and so more amenable to a fluid treatment. By taking
moments, we derive a new fluid model, and demonstrate the damping effects of
collisions on Langmuir waves.
