Fluid Description of a Plasma
In its fluid description, a plasma is characterized by a few local parameters such as the particle density, temperature and flow velocity. These quantities are advanced in time by means of fluid equations that are analogous to, but usually more complicated than, the equations of ordinary hydrodynamics. Thus the goal of plasma fluid theory is to construct and solve a plasma version of the Navier-Stokes equation. An approach to closure at long mean-free path is possible when the plasma is magnetized, since the ratio of gyroradius to scale size can play a role similar to the Chapman-Enskog parameter. The distribution of a confined plasma will become nearly Maxwellian after a few collision times, even when the mean-free path is longer than the system size; all that is required is that the collision time be short compared to the confinement time.