Low Frequency Optimization and Performance of Advanced Virgo Seismic Isolation System

The Gravitational Waves are perturbations of the metric of the space-time. Their effect on free falling masses is to alter their distance. The typical order of magnitude of the gravitational wave signal (for kilometer-scale distances), it is around 10−18m and for this reason it has been chosen to exploit the precision allowed by laser optics and symmetric interferometric detection for these devices. Since present interferometers are ground-based, the mirror suspension attenuation system, as well as its control strategy, is fundamental. Virgo detector was designed not to be limited by seismic noise starting from 10 Hz. To this purpose the INFN-Pisa Group developed and built a sophisticated system, the so-called Superattenuator, allowing to consider the mirror test masses as free-falling from few Hz. In order to be compatible with the amplitude of the gravitational signal to be detected, a hierarchical control scheme for the suspensions must be implemented. The main control implemented, applied to the Superattenuator top-stage is the active mode damping, the so-called Inertial Damping. The sophisticated mechanical design of the overall suspension and the performance of the Inertial Damping are among the key features of Virgo. The target of this thesis is the reduction of the impact of this control on the actual AdV sensitivity and the study of possible alternative developments, as Kalman filtering and Optimal control approach. In this document, modeling and simulation studies based upon actual data are presented in the context of AdV commissioning works.