Kinetic theory of photons in a plasma

A kinetic equation for the photon gas in a relativistic plasma, neglecting the ions motion is derived. From this equation the fluid equations for the photon gas are obtained and their main properties are discussed. In order to illustrate the potential use of these equations to the study of new physical phenomena, which eventually can occur in relativistic plasmas, they are applied to three distinct situations. First, it is shown that a photon beam propagating in a plasma can excite a beam–plasma instability, similar to that occurring for the well-known case of electron beams. Second, it is shown that a bunch of photons, moving with constant velocity across a homogeneous plasma, can emit radiation similar to the Cherenkov effect associated with charged particles. Finally, the filamentation instability of a photon beam is discussed.