Low-Impedance Collimators for HL-LHC

The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) will double its beam intensity for the needs of High Energy Physics frontier. This increase requires a reduction of the machine's impedance to ensure the coherent stability of the beams until they are put in collision. A major part of the impedance is the resistive wall contribution of the collimators. To reduce this contribution several coating options have been proposed. We have studied numerically the effect of the novel coatings on the beam stability. The results show that a decrease of up to 30% of the machine impedance and a reduction of up to 120 A in the stabilizing octupole current threshold can be achieved by coating the secondary collimators with Molybdenum. Half of that improvement can be obtained by coating the jaws of a subset of four collimators identified as the highest contributors to machine impedance. The installation of this subset of low-impedance collimators is planned for the Long Shutdown 2 in 2019-2020.