Collimation with Hollow Electron Beams

Conventional collimation schemes are based on scatterers and absorbers, possibly incorporating several stages. The primary collimators (or targets) are the devices closest to the beam. They generate random transverse kicks mainly via multiple Coulomb scattering. In the Tevatron, the primary collimators are 5-mm tungsten plates positioned about 5 standard deviations (s ) away from the beam axis. The random multiple-scattering kick has a root mean square (r.m.s.) of 17 mrad for 980-GeV protons. The betatron oscillation amplitude of the affected particles increases, and a large fraction of them is captured by the secondary collimators (or absorbers), suitably placed around the ring. In the Tevatron, the absorbers are 1.5-m steel blocks at 6s . The conventional two-stage system offers robust shielding of sensitive components and it is very efficient in reducing beam-related backgrounds at the experiments. However, it has limitations. In high-power accelerators, the minimum distance between the collimator and the beam axis is limited by instantaneous loss rates, radiation damage, and by the electromagnetic impedance of the device. Moreover, beam jitter, caused by ground motion and other vibrations and partly mitigated by active orbit feedback, can cause periodic bursts of losses at aperture restrictions.