Measurements of Superattenuator seismic isolation by Virgo interferometer

Abstract

Each mirror of the interferometric gravitational wave antenna Virgo is attached to a Superattenuator, a chain of mechanical filters designed to suppress seismic vibrations, starting from a few Hz The filter chain attenuation has been measured by exciting its suspension point with sinuisodal forces and using the interferometer as sensor. The attenuation, measured at different frequencies, is compliant with the requirements of the next generation antenna Advanced Virgo In the third generation detector Einstein Telescope, the attenuation is sufficient above 3 Hz, independently of the underground site choice. (C) 2010 Elsevier B.V. All rights reserved

Calibration and sensitivity of the Virgo detector during its second science run

“…The Virgo mirrors are suspended to a complex seismic isolation system [3]. The last stage is a double-stage system with the so-called marionette [4] as the first pendulum.…”

Toward a third generation of gravitational wave observatories

“…The so-called Super-Attenuator pushes the residual seismic noise below the thermal noise of a first generation detector like Virgo starting from about 4 Hz [14]. Recent evaluations [15], supported by direct measurements in the Virgo detector, demonstrated the full validity of the passive filtering design also considering the stringent requirements of a second generation gravitational wave detector like advanced Virgo, where the suspension thermal noise is reduced by an order of magnitude. Passive filtering is not the only way to fight the seismic noise in a gravitational wave detector; in the advanced LIGO design, an active philosophy [16] is being implemented: in a chain of three sub-systems the displacement and the accelerations caused by the seismic noise are read through position and acceleration sensors and are actively and hierarchically suppressed through hydraulic and electro-magnetic actuators.…”

“…A preliminary investigation [15] on the compliance of the Virgo Super-Attenuator with the requirements of a third generation detector has shown a satisfactory behavior above 4 Hz, meanwhile to access the 1-4 Hz frequency range major technical upgrades of the suspension system must be realised. These upgrades are currently objects of the design activity within the Einstein Telescope (ET) project, and some ideas are currently under discussion, like a very long ( 50 m) suspension chain, with each element being about 7-10 m long (in order to push down the pendulum frequencies) and a mixed active-passive design.…”

Status of the Advanced Virgo Gravitational Wave Detector

“…The isolation of the mirror from the ground motion is ensured by the super attenuator 10 . The core of the system is a long series of cascaded pendula (8 m high, in vacuum) installed for the Initial Virgo interferometer and was already compliant with the specifications for Advanced Virgo.…”