Some issues on beam breakup in linear accelerators

BBU is perhaps the most fundamental instability in a linear accelerator its excitation requires no more than the passage of a charged particle beam through a sequence of cavities. The resurgence of interest in this instability naturally accompanied the construction of high current accelerators. In fact, the authors interest in this instability was originally stimulated by the two-beam accelerator experiment that was recently conducted at the Naval Research Laboratory. As the authors gained more experience with BBU, they are confronted with various nagging issues. Some of these are well-known and are, in fact, generic to all stability theories. Others are of a more specific nature. The authors describe them in this paper