  Protons and heavy-ion beams at unprecedented energies are brought into
collisions in the CERN Large Hadron Collider for high-energy experiments. The
LHC multi-stage collimation system is designed to provide protection against
regular and abnormal losses in order to reduce the risk of quenches of the
superconducting magnets as well as keeping background in the experiments under
control. Compared to protons, beam collimation in the heavy-ion runs is more
challenging despite the lower stored beam energies, because the efficiency of
cleaning with heavy ions has been observed to be two orders of magnitude worse.
This is due to the differences in the interaction mechanisms between the beams
and the collimators. Ion beams experience fragmentation and electromagnetic
dissociation at the collimators that result in a substantial flux of
off-rigidity particles that escape the collimation system. These out-scattered
nuclei might be lost around the ring, eventually imposing a limit on the
maximum achievable stored beam energy. Accurate simulation tools are crucial in
order to understand and control these losses. A new simulation framework has
been developed for heavy-ion collimation based on the coupling of the Sixtrack
tracking code and the FLUKA Monte Carlo code that models the electromagnetic
and nuclear interactions of the heavy-ions with the nuclei of the collimator
material. In this paper, the new simulation tool is described. Furthermore,
Sixtrack-FLUKA coupling simulations are presented and compared with
measurements done with Pb ions in the LHC. The agreement between simulations
and measurements is discussed and the results are used to understand and
optimise losses. The simulation tool is also applied to predict the performance
of the collimation system for the High-Luminosity LHC.
