Seismic vibrations mechanical filters for the gravitational waves detector VIRGO

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Abstract
VIRGO is a detector of gravitational waves based on a laser interferometer with 3 km long arms, now under construction. In order to detect gravitational waves down to 10 Hz, the seismic noise has to be reduced by several orders of magnitude. New mechanical filters using cantilever springs as elastic elements have been developed for the VIRGO seismic isolation system. The spring total stiffness is reduced by more than one order of magnitude using magnetic antispring. A control system allows one to keep the spring length constant. The new filters provide an attenuation ≥40 dB each for frequencies above 10 Hz and, compared to the previously adopted solution (gas springs), are much less sensitive to temperature changes.