@@ -121,6 +121,7 @@ Technical noises arise from electronics, control loops, charging noise and other
This thesis focuses on the improvement of the seismic isolation system, which noises affect the inertial sensors placed on the suspension benches and the stabilization of the resonant cavities, which in turn limit the sensitivity of the detector in the low frequency bandwidth. The goal is to provide solutions to reduce seismic motion and improve the detector sensitivity.
\section{LIGO seismic isolation system}
\label{ligosei}
Every optic needs to be stable with respect to seismic motion, because movements in the mirrors will cause unwanted displacement of the laser beam on the optical surface, resulting in noise during the laser travel into the cavities and then at the output. The main mirrors (test masses and beam splitter) are suspended from a stabilized bench and every suspension chain is placed in vacuum chambers called \textit{Basic Symmetric Chamber} (BSC). The auxiliary optics are placed on optical benches enclosed in the \textit{Horizontal Access Module} (HAM) chambers.
\begin{figure}[h!]
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@@ -149,6 +150,16 @@ The BSCs have a similar design as the HAMs, but they have two stages of ISI to s
\label{bsc}
\end{figure}
\paragraph{The sensors on the chambers}
The devices dedicated to monitoring the seismic motion are inertial and displacement sensors, which are horizontal and vertical, according to the different motion they need to sense. Currently, no sensors for tilt motion are installed on the platforms. Actuators are paired to each sensors, for active isolation of the sensed noise. The vertical displacement sensors are called Capacitive Position Sensors and are placed between every stages of every chamber: they measure the relative motion between the platforms. These are the sensors we will use in Chapter \ref{CPSdiff}. The vertical and horizontal inertial sensors with the dedicated actuators are placed on the platforms, underneath the optical tables, measuring the seismic motion in the horizontal and vertical directions. The position and the use of these sensors are different for HAM and BSC chambers, depending on the number of stages and the presence of the suspensions. The calibration and the specific role of each sensor into the seismic isolation system can be found in \cite{kisselthesis}, with references to the covered range of frequencies in \cite{kisseltalk3}.
\begin{figure}[h!]
\centering
\includegraphics[scale=1]{images/seifig.png}
\caption[Example of HAM-ISI scheme]{Example of ISI inertial sensor scheme for a HAM chamber (figure taken from \cite{kisselthesis}). All the main inertial and displacement sensors involved in the seismic isolation are shown in their locations. The CPSs are the displacement sensors located between stages to measure the relative position. For the BSC chambers, the setup is similar.}
\label{isi}
\end{figure}
\paragraph{Stabilizing the ISI}
Part of the work presented in this thesis focussed on the enhancement of the performances of the active isolation system of the ISIs for both BSC and HAM chambers.\\
Active isolation implies a sensing system of the noise to reduce and a control system to compensate the disturbance. Each platform includes relative position sensors, inertial sensors and actuators, working in all degrees of freedom.\\
@@ -219,8 +219,11 @@ Beginning of Gravitational Wave Astronomy}
\bibitem{mat} F. Matichard et al, \textit{Seismic isolation of Advanced LIGO: Review of strategy, instrumentation and performance}, Class. Quantum Grav. 32 185003, 2015
\bibitem{kisseltalk3} J. Kissel, \textit{On Seismic Isolation in 2nd Generation Detectors}, GWADW talk, 2012, dcc.ligo.org/LIGO-G1200556
\bibitem{lsc} K. Izumi, D. Sigg, \textit{Advanced LIGO: length sensing and control in a dual recycled interferometric gravitational wave antenna}, 2017 Class. Quantum Grav. 34 015001
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\bibitem{mca} C. Di Fronzo \textit{Optical sensors for improving low-frequency performance in GW detectors}, Mid-Course Assessment, University of Birmingham, 2018
