ABSTRACT
In the preceding chapters we described the three most important aspects of the behaviour of charged particles in accelerators and the devices associated with them: the effects of electric fields upon charges; the effects of magnetic fields upon charges; how the charges produce electromagnetic radiation. In those chapters each charge was considered to be moving independently of the other charges that may accompany it; the effect of having many charges moving together in a bunch being merely additive. For example, the beam loading in a cavity is proportional to the amount of passing charge, as is the intensity of synchrotron radiation. Moreover, we ignored that the moving charges might influence each other. This is true in situations where the bunch charge is sufficiently low. However, there are many circumstances where we must take account of the beam intensity – expressed either in terms of the bunch charge (or current) or in terms of the bunch density (which depends upon the bunch volume). There are a variety of phenomena that can manifest themselves – too many for the scope of this book – so in this chapter we describe the principles underlying them, and give a few of the most important examples that the reader may encounter. We divide our discussion of these self-fields in terms of i) the effect of moving charges upon each other (intra-beam forces, space charge and scattering), ii) The effect of bunches upon the vacuum system and the consequent effect onto the bunches (wakefields and instabilities) and iii) the enhancement of radiation by coherent effects.
