The longitudinal control for the Advanced Virgo Plus gravitational wave detector

Ground-based gravitational wave detectors are evolving at a rapid pace. In the five years that followed the first direct detection of gravitational waves, the Advanced LIGO and Advanced Virgo experiments have been subject to substantial upgrades, increasing their sensitivities by many times and allowing them to detect dozens of other gravitational wave signals. Third-generation ground-based interferometers (Einstein Telescope and Cosmic Explorer) and space-based detectors (such as LISA) are being researched and planned to enter into function in the second half of the next decade. If successful, these experiments will allow the detection of thousands of signals coming from an ever-increasing range of cosmological sources. In the meantime, second-generation interferometers are approaching the conclusion of ambitious upgrades started with the end of the third observing run “O3” in march 2020. The work of this thesis revolves around the planning and the commissioning of the “Advanced Virgo plus” upgrade project, which aims to increase the detector’s sensitivity by a factor of two, allowing a ten times higher detection rate than the previous configuration. In particular, the main topic is the update of the interferometer longitudinal sensing and control scheme required by the upgrade in the detector’s optical configuration. The design and simulation of the new control scheme carried out in constant collaboration with the “Interferometer Sensing and Control” team, started years before the actual implementation of the upgrades. Following that, I participated in the full-time commissioning of the upgraded configuration, which started in January 2021 and is currently ongoing. We will first explain the new interferometer configuration, then go into the details of the lock-acquisition procedure, presenting the results of the related simulation studies and the commissioning. A particular focus will also be given to the simulations of the interferometer’s state at the end of the lock acquisition, called “steady-state”. In addition to the study and implementation of the current lock-acquisition procedure, the thesis will present simulation activities to study an alternative lock-acquisition technique that has not yet been implemented. Chapter 1 of the thesis introduces gravitational wave detection and the detectors’ operating principles. Chapter 2 will dive deeper into topics related to the control of the main suspended optics of the interferometer. Chapter 3 explains the newly implemented lock-acquisition procedure for the lock of Advanced Virgo Plus, showing the most relevant studies carried out during the design phase and the results obtained during the commissioning. An in-depth study regarding the impact of misalignments during a particularly crucial part of the lock acquisition is reported in appendix A.