Experimental results on beam halo

The increasing interest for high-intensity, high-energy linear accelerators has induced the scientific community to consider a phenomenon whose effects are worrying: the beam losses. Lost particles can, indeed produce enough radioactivation in the accelerator structures to considerably complicate operation and maintenance of the machine. They originate in a low-density halo which can extend far from the beam core. Theoretical and experimental studies have been undertaken in order to understand the physics of halo production and to develop methods to limit and control beam losses. The present work reviews efforts and advances in beam-halo and emittance-growth experiments. Experimental techniques, used in different laboratories, are described and measurement results are presented and commented. Experiments on the transport of space-charge dominated beams through periodic focusing channels are considered with particular emphasis.