Introduction

The attractive features of the ‘beam’ model for galactic and extragalactic radio sources are predicated on the contention that astrophysical processes permit the formation of high-power collimated flows, and that such flows survive their passage from the generating engine to the outer parts of the source without losing most of their energy. In other words, astrophysical plasma beams must be capable of exceptional stability, although there are sources for which less stability is necessary. This may be seen in the ‘P-D’ diagram of Baldwin (1982) (cf. Chapter 2) – for a given radio power (say 1027 W Hz −1 sr −1), radio sources spanning a wide range of linear sizes (about 1 to 1000 kpc) are found, and hence the beams driving these sources must be stable over distances exceeding 1 Mpc in the largest objects, but need be stable for only 1 kpc in the smallest. The purpose of this chapter is to discuss the physical mechanisms that are effective in stabilising and destabilising beam flows, and to calculate the stability properties of some beams that might be components of extragalactic radio sources. Much of the physics discussed here can be applied to Galactic jets (Chapter 10), with some modification for the difference in physical parameters.