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The High-Luminosity upgrade of the Large Hadron Collider (HL-LHC) will doubleits beam intensity for the needs of High Energy Physics frontier. In order toensure coherent stability until the beams are put in collision, the transverseimpedance has to be reduced. As the major portion of the ring impedance issupplied by its collimation system, several low resistivity jaw materials havebeen proposed to lower the collimator impedance and a special collimator hasbeen built and installed in the machine to study their effect. The results showa significant reduction of the resistive wall tune shift with novel materials,in agreement with the impedance model and the bench impedance and resistivitymeasurements. The largest improvement is obtained with a 5 {\mu}m Molybdenumcoating of a Molybdenum-Graphite jaw. This coating can lower the machineimpedance by up to 30% and the stabilizing Landau octupole threshold by up to120 A. The collimators to be upgraded have been chosen based on the improvementof the octupole threshold, as well as the tolerance to steady state losses andfailure scenarios. A half of the overall improvement can be obtained by coatingthe jaws of a subset of 4 out of 11 collimators identified as the highestcontributors to machine impedance. This subset of low-impedance collimators isbeing installed during the Long Shutdown 2 in 2019-2020.

Transverse Beam Stability with Low-Impedance Collimators in the High Luminosity Large Hadron Collider: Status and Challenges