Waves in Plasmas

Abstract

Waves are basic manifestation of collective effects in plasmas. Wave types occurring in the plasma state are introduced and discussed with experimental applications, e.g. for the diagnostics of plasmas. This Chapter is fundamental to Part II and covers many aspect introductory to Chapters on applications (see Part III), e.g. on plasma diagnostics. 2.1 Introduction Wave phenomena are ubiquitous in nature and immediately affect human life. Sound waves in air let us hear, light waves give us vision, and vibrations of solids can form music. To the physicist, the study of wave phenomena gives an immediate insight into the elastic properties of matter, which define the wave speed. Sound waves in air and light waves in vacuum have the property that perturbations of different frequencies propagate with a unique velocity, the sound speed or the speed of light, respectively. This is quite different from surface waves on a pond, where sinusoidal waves of different frequency have different propagation speed and are therefore called dispersive. They resemble light in a glass prism, which is dispersed into its various colors. Light waves and sound waves differ in their way of oscillation, which is transverse to the direction of propagation in light and along the propaga-tion for sound. In solid matter, transverse " shear " waves and longitudinal " compressional " waves can even coexist at the same frequency. However, the different propagation speed of these two " modes " gives the two waves dif-ferent wavelength at the same frequency. Ordinary gases can only support longitudinal compressional waves, because there is no restoring shear force. Plasmas have a much higher variety of wave modes than ordinary matter because plasmas combine the aspects of a gas with electromagnetic forces. Moreover, the entanglement of particle motion with magnetic fields leads to wave types unknown in other fields of physics. This chapter attempts to give a survey of the various wave types in unmagnetized and magnetized plasmas. The mathematical apparatus is kept as simple as possible. The chosen topics of this tutorial, which emphasize diagnostic applications (see also Chap. 13 in Part III), reflect the preferences of the author, who is a devoted experimentalist. Important aspects, like Landau damping and Alfvén