Impact of cross-sectional changes in a beam tube on beam dynamics

Based on the experimental data, it is concluded that there are two possible consequences when adopting a nonuniform beam tube. The first is the single bunch instability due to large transverse impedances of these transitions. This is the case of the APS. The second is the probable transverse emittance dilution due to the multibunch effects, which is demonstrated using the SSC parameters. The best solution is to use a uniform beam tube. When this is impossible, one has to adjust the beam parameters carefully to reach the desired value of the bunch intensity or to apply a feedback system to stabilize the beam. If there is concern about the trapped RF modes, dampers may be installed.<>