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VIRGO Perugia

The VIRGO Perugia group is part of the Noise in Physical Systems (NiPS) Laboratory – at the Physics Department of the University of Perugia, Italy. In the VIRGO interferometer, the mirrors are suspended masses. In particular steel wires were used for Virgo, while fused silica fibers are mounted for AdVirgo. The thermal noise of the whole suspension, limits the detector sensitivity in the low frequency region, where it is more expected to detect the gravitational waves emitted by various sources. The Perugia group is specialized in high sensitivity measurements for evaluating the mechanical dissipations linked to the thermal noise through the Fluctuation/Dissipation Theorem. In order to reduce the thermal noise in the low frequency region, the Perugia group studied the dissipations of the mirrors induced by each component of the last stage of the suspension, characterizing their dissipations, developing new bonding techniques and designing a new anchoring system. In particular low dissipation suspensions were designed and realized over time, such as the monolithic suspensions. In this case only fused silica (FS) components are used and each FS part is joined to the other by a chemical gluing process which doesn’t introduce any extra material reconstructing the silica chains between the glued parts, thus reducing the dissipations. Furthermore in Perugia were designed and realized the lateral supports of the mirrors (called ears, a FS element to join the the fiber and the mirror), the supports for the magnets (as a mechanical mismatch between the mirror and the control system) and a low dissipation technique to join the monolithic suspension to the upper steel stage (the marionetta). To improve the performance of the future interferometers, new materials and their combinations are under test, i.e. the coating of the mirrors. Here the main goal is to reduce the thermal noise without increasing optical absorption. NiPS approaches both theoretical and experimental aspects studying the behavior of thin and ultra-thin coated systems in order to find the main properties linked to the coating dissipations.

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