Many aspects of the properties and behavior of matter at extreme conditions are adequately described by simple fluid theory. Indeed, most of the rest of this text is devoted to developing mathematical models for the properties and behavior of matter at extreme conditions, models that are based on treating the matter as a fluid of interacting ions, electrons, and radiation. The fluid description is adequate largely because the spatial scales and time scales over which the interacting particles, via collisions, establish an equilibrium distribution of particle energies is – as we shall see in this chapter – very short compared to typical time scales over which equilibrium conditions change as a result of the fluid motion, for example. Thus, we do not, in general, have to follow the motion of each individual particle of which the plasma is comprised, but only the bulk motion of larger-scale volume or mass elements of the plasma. This is what is done in a radiation-hydrodynamics simulation code. In subsequent chapters we will learn the mathematical models from which radiation-hydrodynamics simulation codes are constructed, how the codes are constructed, and how to do simulations with a radiation-hydrodynamics code.

There are some phenomena, however, for which the fluid treatment is inadequate, and the macroscopic fluid picture must be replaced by a microscopic kinetic theory. Such situations arise, for example, when and where the three fluids are not in equilibrium with each other, or when multiple ion species are present in the plasma. In those situations where it is necessary to follow the motions and interactions of the individual particles on the very short interaction spatial scales and time scales, we need to use a simulation code that is based on kinetic models rather than fluid models. There are a number of such codes, called “particle-in-cell” (PIC) codes, but we will not discuss these codes in this text. There are also a number of hybrid codes available, that treat the electrons as a fluid and the ions kinetically. We also do not discuss hybrid codes in this text.