Interaction Between Beams and Vacuum System Walls.

In modern high-intensity accelerators, the circulating beam interacts in many ways with the vacuum beam pipe, causing a variety of different phenomena. Most of them have been discussed at length in other contributions to this CAS report. I concentrate here on the effects associated with the presence of electrons in the accelerator beam pipes. Low-energy electrons in accelerators are known to interact with the circulating beam, giving raise to the formation of a so-called e$^-$ cloud. e$^-$ cloud effects can be detrimental to beam quality and stability, especially for positively charged beams. I will first describe the origin and the basic features causing e$^-$ cloud formation in accelerators, which depend not only on the beam properties but mainly on the vacuum vessel surface properties. Such material properties and the way one can study them will be here briefly presented. Finally, some of the e$^-$ cloud mitigation strategies adopted and proposed so far will be described and discussed.