LAL, Orsay, France
UA9 was operated in the CERN-SPS for more than two years in view of investigating the feasibility of the halo collimation with bent crystals. Silicon crystals 2 mm long with bending angles of about 150 urad were used as primary collimators. The crystal collimation process was steadily achieved through channeling with high efficiency. The crystal orientation was easily set and optimized with the installed goniometer which has an angular reproducibility of about ± 10 μrad. In channeling orientation, the loss rate of the halo particles interacting with the crystal is reduced by a factor of ten, whilst the residual off-momentum halo escaping from the crystal-collimator area is reduced by a factor five. The crystal channeling efficiency of about 75 % is reasonably consistent with simulations and with single pass data collected in the North Area of the SPS. The accumulated observations, shown in this paper, support our expectation that the coherent deflection of the beam halo by a bent crystal should considerably help in enhancing the collimation efficiency in LHC.
