Beam Halo Formation in Linacs, Theory and Experiment

A large number of high-intensity linac studies within the last 20 years gave rise to a substantial effort to understand the formation of beam halo in hadron linacs. Even though most experts agree on the fundamental mechanisms which expel particles to large radii, there is only little experimental evidence on the process. This paper reviews the current understanding of halo formation, including recent findings on the role of statistically distributed errors, and outlines the difficulties of experimental validation. Past and future experimental efforts are summarized.