Beam-Based Error Identification and Correction Methods for Particle Accelerators

Doktorsavhandling, 2014

Modern particle accelerators have tight tolerances on the acceptable deviation from their desired machine parameters. The control of the parameters is of crucial importance for safe machine operation and performance. This thesis focuses on beam-based methods and algorithms to identify and correct errors in particle accelerators. The optics measurements and corrections of the Large Hadron Collider (LHC), which resulted in an unprecedented low β-beat for a hadron collider is described. The transverse coupling is another parameter which is of importance to control. Improvement in the reconstruction of the coupling from turn-by-turn data has resulted in a significant decrease of the measurement uncertainty. An automatic coupling correction method, which is based on the injected beam oscillations, has been successfully used in normal operation of the LHC. Furthermore, a new method to measure and correct chromatic coupling that was applied to the LHC, is described. It resulted in a decrease of the chromatic coupling by a factor ∼2 for both beams. The good control of the optics is a significant part of the success of the LHC and hence contributed to the discovery of the Higgs particle. Following the discovery of the Higgs particle there is a demand for precise measurements of its properties in a lepton collider. Compact Linear Collider (CLIC), an electron-positron collider aiming at collision energies up to 3 TeV, is one of the leading candidates. The acceleration in CLIC relies on a two-beam acceleration scheme where one of the beams, referred to as the Drive Beam, is decelerated while transferring its energy to the Main Beam. This scheme puts tight constraints on the parameters of the Drive Beam in terms of beam current, phase and bunch length. In CLIC Test Facility 3 (CTF3) the mechanisms behind the observed drifts of these parameters have been studied in detail. The findings have shown that these drifts are mainly linked to variations in the amplitude of the Radio Frequency (RF). A feedback to mitigate the RF-amplitude fluctuations has been implemented and is described in detail. In conjunction with a dedicated energy feedback it reduces the energy variation by a factor ∼3. Together with precise machine tuning this has resulted in a beam current stability very close to the CLIC requirement. The beam phase stability is improved through a feedback operating on the two first klystrons in the CTF3 injector. Two-beam acceleration at the nominal CLIC gradient of 100 MV/m and above has been demonstrated in CTF3. These results, and other recent achievements in CTF3, are presented in this thesis.