LHC Injection Beam Quality During LHC Run I

The LHC at CERN was designed to accelerate proton beams from 450 GeV to 7 TeV and collide them in four large experiments. The 450 GeV beam is extracted from the last pre-accelerator, the SPS, and injected into the LHC via two 3 km long transfer lines, TI 2 and TI 8. The injection process is critical in terms of preservation of beam quality and machine protection. During LHC Run I (2009-2013) the LHC was filled with twelve high intensity injections per ring, in batches of up to 144 bunches of 1.7*10^11 protons per bunch. The stored beam energy of such a batch is already an order of magnitude above the damage level of accelerator equipment. Strict quality and machine protection requirements at injection have a significant impact on operational efficiency. During the first years of LHC operation, the injection phase was identified as one of the limiting factors for fast LHC turnaround time. The LHC Injection Quality Check (IQC) software framework was developed as a part of this thesis to monitor the beam quality and steer the injection process. Equipment in the SPS-to-LHC transfer lines and in the LHC injection regions, such as beam position monitors and beam loss monitors, are analysed in the IQC. The evolution of LHC injection quality over the years of LHC Run I has been studied and the results are discussed in this thesis. Beam loss at injection caused by large SPS-to-LHC transfer line trajectory variations was a big concern during LHC Run I. The sources of slow trajectory drifts, shot-by-shot and bunch-by-bunch trajectory variations have been identified after an in-depth study. Several mitigations could be put in place to improve the trajectory stability. Measurements during a beam test confirm the reduction of the trajectory variations. The other dominant source of beam losses at injection was large non-Gaussian tails in the transverse particle distribution. Systematic tail scraping in the SPS was found to be an efficient way to keep the losses under control. A study was carried out to determine the optimum scraping depth with respect to loss reduction and luminosity performance. Many efficient mitigations have been put in place to improve injection quality and reduce the time spent at injection. A large number of these were identified from data collected and analysed in this thesis.