Beam observation and the nature of instabilities

The point of view presented in this paper will be that of an experimental accelerator physicist who had to struggle for many years with all sorts of collective beam instabilities occurring in different accelerators and storage rings and this, most often, with great enjoyment. Detailed and rigorous mathematical descriptions, as well as quantitative formulae to evaluate thresholds and growth rates will be found elsewhere. Here the author will dwell on the various means which have been invented to observe the beams, to track and record their most intimate behavior in order to assess their margin of stability, and finally through a better understanding of their properties to devise ways of increasing the machine performance. Although in this exercise good theories are a necessary ingredient, more emphasis will be put on the almost intuitive understanding of the nature of the fundamental phenomena at work. The first chapter will describe the observable manifestations of an ensemble of particles with no interactions between them. The second and third chapters will study the effect of wake fields on the collective behavior of the beam particles. The basic nature of beam instabilities will be shown with the help of simple examples. 49 references, 39 figures