Preamble

Plasmas can support a great variety of wave motions. For many purposes it suffices to have knowledge of three classes of waves, two of which are discussed here. These are waves in isotropic thermal plasmas, and waves in cold magnetized plasmas. The third class of waves are the MHD waves (MHD is short for magnetohydrodynamics), which are derived within the framework of a fluid model for the plasma. The MHD waves are not discussed in detail here.

Waves in Isotropic Thermal Plasmas

An isotropic plasma is defined to be a plasma (a) with no ambient magnetic field (it is unmagnetized), and (b) in which all species of particles have a Maxwellian distribution of velocities (or its relativistic generalization if relativistic effects are included). In any isotropic medium the waves are either longitudinal or transverse (§12.1). The longitudinal waves satisfy the longitudinal dispersion equations (12.6), viz., KL(ω, k) = 0, and the transverse waves satisfy the transverse dispersion equations (12.7), viz., n2 = KT(ω, k). The longitudinal and transverse parts of the dielectric tensor for an isotropic thermal plasma are given by (10.23) and (10.24), respectively.

Langmuir Waves

There are two solutions of the longitudinal dispersion equation that are important in practice. These are for Langmuir waves, which involve only the motion of the electrons, and ion sound waves (also called ion acoustic waves) that are associated with motion of the ions. As mentioned in §10.3, both these wave modes were identified by Tonks and Langmuir in 1929 in what is now recognized as the first major article in the development of modern plasma theory.