A multistage vibration isolation system for Advanced Virgo suspended optical benches

We present a compact, vacuum compatible seismic attenuation system designed to isolate five auxiliary optical benches for Advanced Virgo, a second generation gravitational wave detector. We report on the design of the device, coined MultiSAS (multistage seismic attenuation system) and on its measured vibration isolation performance. The latter can be summarized by quoting a payload isolation ratio at 10 Hz of 100 dB and 140 dB in vertical and horizontal, respectively. We also present the design and performance of the MultiSAS control system along the translation degrees of freedom, as well as a discussion of the possible coupling to the angular degrees of freedom. Over a time-scale of 100 s, 1 m magnitude RMS for translational degrees of freedom is achieved for seismic conditions observed in the past five years at the Virgo site and in Amsterdam. The spectral displacement levels are expected to be lower than 10−14 m from 10 Hz onwards in vertical and horizontal. In addition we discuss effects that could deteriorate the performance of the device such as thermal drifts of the mechanical filters, residual acoustic coupling and cradle effects on the inverted pendulum pre-isolation stage. Mitigation strategies or solutions were devised and installed in the five Advanced Virgo systems.