MD1447 –β∗–Reach: 2016 IR7 Collimation Hierarchy Limit and Impedance

This report summarizes the results of MD1447 about the hierarchy limit of the LHC collimation system in IR7 at 6.5 TeV and its impedance, performed during MD block 1 of 2016. The hierarchy breakage seen in a similar activity in 2015 when deploying 1σ-retractions between TCSG and TCP collimators was confirmed, and a viable solution demonstrated with beam. In addition, measurements of tune–shifts induced by changes in collimator gaps were performed when moving the IR7 TCSG collimators all together, or when moving only those with the largest contribution to impedance. The MD activity included a re-alignment of all the IR7 collimatorsto the beam, prior to the investigations on the hierarchy breakage, and an instability threshold measurement, at the end. The MD activity is important for understanding the limitations from impedance; these limitations have direct impact on the choice of the machine configuration kept during operation (e.g. for deploying TCP-TCSG retractions in IR7 smaller than the present ones), or for optimising the design of future devices (e.g. as presently foreseen by the HL-LHC project, since outcomes are important ingredients for finalizing the baseline configuration of the upgraded collimation system).