The seismic isolation system of Advanced Virgo Plus, Phase II

In 2025, the Advanced Virgo Plus (AdV+) gravitational-wave detector will enter Phase II, a thermal noise reduction upgrade involving the increase of the terminal mirror masses from 42 kg to 104 kg. This requires a revision of the SuperAttenuator (SA), the main attenuation system in AdV+, currently capable of reducing seismic noise by more than ten orders of magnitude in all six degrees of freedom above a few Hz. We briefly report the multiple updates of SA parts that have been planned to keep performance unchanged. As part of the study, simulations and/or design of upgraded elastic (maraging blades) and counter-elastic (magnetic antisprings) components were carried out, leading to a revision plan of all the filters in terminal SAs and notably of Filter 7 (F7), the lowest one. Prototyping and tests of the upgraded F7 and blades are underway at this time to validate the outputs of the study.