The Virgo data acquisition system

International audienc

Last stage control and mechanical transfer function measurement of the VIRGO suspensions

The automatic control of the suspended mirrors is a major task in operating an interferometric gravitational wave antenna. To reach the extreme sensitivity required for this kind of detector, an accurate alignment and a stable locking of the interferometer on its working point are crucial. The solution of this problem is particularly complex in the case of a multistage pendulum, such as the suspension system for seismic isolation adopted in VIRGO. A precise knowledge of the suspension mechanical transfer functions (TFs) for different forces applied in the control servo-loops represents essential information to reach the goal. In this article, we describe the apparatus we developed to measure the VIRGO suspension TF and we report the results thus obtained on full-scale suspensions at the VIRGO site. Preliminary results for the implemented control system of the last suspension stage are also presented. (C) 2002 American Institute of Physics

The present status of the VIRGO Central Interferometer

The VIRGO Central Interferometer (CITF) is a short suspended interferometer operated with the central area elements of the VIRGO detector, The main motivation behind the CITF is to allow the integration and debugging of a large part of the subsystems of VIRGO while the construction of the long arms of the antenna is being completed. This will permit a faster commissioning of the full-size antenna. In fact, almost all the main components of the CITE with the exception of the large mirrors and a few other details, are the same as those to be used for the full-size detector. In this paper the present status of the VIRGO CITF is reported

The Inertial Damping of the VIRGO superattenuator and the residual motion of the mirror

The VIRGO superattenuator (SA) is effective in suppressing seismic noise below the expected thermal noise level above 4 Hz. However, the residual mirror motion associated with the SA normal modes can saturate the interferometer control system. This motion is reduced by implementing a wideband (DC–5 Hz) multidimensional active control (the so-called inertial damping) which makes use of both accelerometers and position sensors and of a digital signal processing (DSP) system. Feedback forces are exerted by coil–magnet actuators on the top of an inverted pendulum pre-isolator stage. The residual root mean square motion of the mirror in 10 s is less than 0.1 μm

The Virgo Suspensions

The VIRGO suspensions are chains of passive mechanical filters designed to isolate the interferometer mirrors from seismic noise starting from a few Hz. In order to reduce the low-frequency swing of the mirror along the beam, an active control system, acting at the level of the suspension point, damps the main resonant modes of the system (all below 2.5 Hz). Another control loop, at the level of the optical payload, makes use of a digital camera monitoring the mirror position in all six degrees of freedom. Its main goal is to decrease the rms angular displacements of the mirror, on a time scale of several minutes, down to less than 1 μrad. All the seven suspensions of the VIRGO central interferometer are presently in operation, while the assembly of the last two, for the terminal mirrors, is in progress. The design andperformance of the system are described in this paper

Data Analysis software tools used during Virgo engineering runs, review and future needs

During last years, data flow and data storage needs for large gravitational waves interferometric detectors have reached an order ofmagnitude similar to those of high energy physics experiments. Software tools have been developed to handle and analyze these large amounts ofdata, with the specificities associated to gravitational waves search. We will make a review ofthe experience acquired during engineering runs on the VIRGO detector with the currently used data analysis software tools, pointing out the peculiarities inherent to our type of experiments. We will also show what are the possible future needs for the offline analysis VIRGO data