Introduction

Plasma physics deals, in its most general form, with studies of the dynamics of charged particles. In principle this includes the motions of single charged particles in a priori given electric and magnetic fields, but the most interesting problems are concerned with collective interactions between many charged particles. More recently, it has been realized that studies of the plasma state include topics which have important industrial applications. The dynamics of hot plasmas are in many respects similar to those of neutral gases, and in many cases it might be argued that no particular new physical insight is gained by including elements from plasma physics in the analysis. One major stimulus for the development of plasma sciences in the time following the mid-1950s can be identified as its importance for controlled fusion. Sound-like waves are found in plasmas and they are in many ways similar to sound waves in other media.