@@ -484,29 +484,47 @@ What we expect is a faster reach of locking and a longer state of lock of the in
\noindent
This work has been performed on LIGO during the commissioning break between O3a and O3b observing runs, in October 2019. The reason of this choice is that we needed the interferometer to \textit{not} be observing, since we were going to modify some software structure of the instrument.\\
\noindent
Through CPSs locking, we reduced the differential motion of HAM2 and HAM3 chambers and made them to move in sync. So they can be considered as a whole block. The IMC is entirely lying on HAM2 and HAM3, and it is straightforward to use it as a witness: to make this real, we need to feed the HAM2-HAM3 block with IMCL. This will lock the cavity signal to the HAM2-HAM3 block. The same feeding will be performed with PRCL, SRCL, DARM and MICH cavities, which optics lie on the other chambers, in and out the corner station. Fig. \ref{chamb} illustrates the chambers and the locations of the cavities.
Through CPSs locking, we reduced the differential motion of HAM2 and HAM3 chambers and made them to move in sync. So they can be considered as a whole block. The IMC is entirely lying on HAM2 and HAM3, and it is straightforward to use it as a witness: to make this real, we need to feed the HAM2-HAM3 block with IMCL. This will lock the cavity signal to the HAM2-HAM3 block. The same feeding will be performed with PRCL, SRCL, DARM and MICH cavities, which optics lie on the other chambers, in and out the corner station. Fig. \ref{chamb} illustrates the chambers and the locations of the cavities of interest.
\begin{figure}[h!]
\centering
\includegraphics[scale=0.5]{images/chambs.jpg}
\caption{Sketch of the blocks and the locations of the cavities.}
\includegraphics[scale=0.3]{images/chambs.pdf}
\caption[Sketch of the blocks and the locations of the cavities]{Sketch of the blocks and the locations of the PRC and IMC cavities (not in scale). the HSTS suspensions of the mode cleaner and the power recycling cavoty lie all on HAM2 and HAM3 chambers. the signal for PRCL come form the Corner Station, which can be grouped as a BSC block.}
\label{chamb}
\end{figure}
\noindent
To lock the LSC signals to ISIs, we need to do something similar to what we did with the HAM chambers: we need to connect via software two different setups which do not talk to each other. We made a quick computation (given the stretched timing) and we decided to start from the Power Recycling Cavity Length (PRCL) because we locked HAM2 and HAM3 chambers, so it was natural to start to lock the cavities on the x axis.\\
To lock the LSC signals to ISIs, we need to do something similar to what we did with the HAM chambers: we need to connect via software two different setups which do not talk to each other. We decided to start from the Power Recycling Cavity Length (PRCL) because we locked HAM2 and HAM3 chambers, so it was natural to start to lock the cavities on the x axis.\\
The same work is foreseen to be done for the other cavities: the very short period of time available during the commissioning break allowed us to modify only the software for PRCL, since the job involved the request of permissions to modify the structure of the interferometer and the synchronization with the job of other people working on different parts of LIGO. Moreover, during commissioning break, time is also used to work on the chambers, profiting of the out-of-lock mode. This means that, for every attempt of software modification, a locking trial was needed, to see if the new configuration of the instrument was giving better performances and, also, if it was affecting negatively other sides of the instrument. To try to lock LIGO, we needed people not to work besides the chambers. This was a huge and collaborative work, which involved many people on site, and their time.
\paragraph{The Power Recycling Cavity Length (PRCL)}
We need to connect the ISI to the cavity and to do it we need to know how the PR cavity is going to communicate with the ISI. The block diagram in Fig. \ref{prcl} illustrate the simplified concept of the PR cavity connected to the ISIs of the block of HAM2 and HAM3 chambers.\\
Te work done in this case is similar to the one done for the HAM chambers, except from the fact that a new filter need now to be built in order to control how the ISI affect the motion of the PRC optics.
The work done in this case is similar to the one done for the HAM chambers, except from the fact that a new filter need snow to be built in order to control how the ISI affect the motion of the PRC optics.
\begin{figure}[h!]
\centering
\includegraphics[scale=0.7]{images/PRCLfeed.png}
\caption{Block diagram of PRCL locked to ISI.}
\caption[Block diagram of PRCL locked to the ISI]{Block diagram of PRCL locked to the ISI. This drawing highlights the details of the PRCL cavity sections involved in active control. In the standard diagram, only the PRCL sections would be involved, while now the cavity is connected via software to the ISI. The LSCfilter block is the crossover filter between the cavity and the ISI (and the connection between them is enabled by a switcher) while the ISItoM3 block represents the plant block of the suspension point of M3 after the connection.}
\label{prcl}
\end{figure}
\noindent
This block diagram has been solved with Mathematica in order to find the correct crossoover filters to add. The system was simulated via Matlab and includes information from calibration filter modules, PRM control filters, and HSTS models via the calibration filters. This is needed to simulate the addition of the ISI as a PRCL actuator. The aim was to offload low-frequencies to the ISI and hence we needed to decide the best configuration of gains and offsets of the crossover filter.\\
After every simulation which could possibly work for the system, we locked the interferometer and took a measurement of the PRM suspension point. The plot in Fig. \ref{prcltest} shows a comparison between the simulation and the actual measured PRCL signal: the test is positive because the two traces differ by only a factor of 2. This result has been obtained implementing the filter in Fig. \ref{prclfilter}. The test shows that the offloading works as expected and that the PRCL signal can be driven (and hence controlled) by the ISI. Further studies of the crossover filters might improve the results.
\begin{figure}[h!]
\centering
\includegraphics[scale=0.4]{images/PRM.png}
\caption[PRCL-ISI offloading test]{Best measurement of the PRCL signal with respect to the expected signal from the simulations: the two traces differ by a factor of 2.}
\caption[PRCL-ISI crossover filter]{Open loop gain (OLG) crossover filter implemented at LHO for a measurement of PRCL signal in offloading conditions.}
\label{prclfilter}
\end{figure}
\section*{Conclusions}
Due to lack of time, it was not possible to take further measurements of ISI motion and LSC signals, especially with an accurate study of the blending filters. The study is however promising to be of great help in the improvement of the LSC signals of LIGO and its stabilization when in observing mode.\\
Due to the stretched time, it was not possible to take further measurements of ISI motion and LSC signals, especially with an accurate study of the blending filters. The study is however promising to be of great help in the improvement of the LSC signals of LIGO and its stabilization when in observing mode.\\
LIGO Livingston site has also actuated a similar process, following the progression at LHO during 2019 collaboration and since the software skeleton of the new configuration has been built and installed on LIGO CDS, further studies and tests were due in 2020 to complete the last steps and test it fully on the interferometer. However the pandemic and the correlated travel restrictions have moved forward in the future the schedule for these tests.
@@ -30,3 +30,4 @@ Currently, the ground-based observatories are tuned to detect binary systems sou
The first detection of gravitational waves happened on the 14th September 2015 and confirmed the Theory General Relativity, opening a new window on the Universe: the signal from a merger of two black holes have been observed thanks to the emission of gravitational waves, confirming the existence of these objects, still mostly unknown \cite{first}. The detector responsible of the new discovery is based in the USA and it is one of the terrestrial interferometers currently in use for gravitational waves detection.
\section{Hidden GW sources}
PER QUESTA SEZIONE, FARE RIFERIMENTO A TUTTI I VARI WORKSHOP.
