The first Fused Silica monolithic suspension for Advanced Virgo

The first fused Silica monolithic suspension for Advanced Virgo

by Helios Vocca (University of Perugia)

 

Thermal noise in mirror suspension  is the most severe limit to the low-frequency sensitivity of first generation interferometric gravitational wave detectors. In order to minimize the pendulum thermal noise, a monolithic suspension, using fused silica as a low dissipation material, has been developed for Advanced Virgo. Now the first monolithic suspension has been realized last Thursday June 11st, thanks to a strong collaboration between the INFN groups of Perugia, Firenze and Roma1, that also collaborate in the development of such key components of the mirror suspensions.

 

Silica Suspension 1

The first fused Silica monolithic suspension for Advanced Virgo (Credits: Helios Vocca/EGO and The Virgo Collaboration)

The monolithic design for this suspension stage presents very high mechanical quality factor (around 107), meaning that the dissipations are so low that the pendulum oscillations in vacuum can last for more than two months before stopping. This suspension also has high breaking strengths, about twice that of the steel. Such strenght is very important, since the suspension must be highly resistant to the mechanical stress of the oscillations of the mirror itself and to the possible mechanical shocks induced by the shots of the mirror with the surrounding materials.

Silica Suspension 4

Details of first fused Silica monolithic suspension for Advanced Virgo (Credits: Helios Vocca/EGO and The Virgo Collaboration)

The material of the suspension has to be well controlled, well produced and well manipulated. For this reason the fused silica fibres (diameter of about 400 micron) are directly produced in our laboratories in Cascina using a laser machine, and tested with very high accuracy and reproducibility.

Silica Suspension 2

The first fused Silica monolithic suspension for Advanced Virgo (Credits: Helios Vocca/EGO and The Virgo Collaboration)

To create a monolithic suspension, a low dissipation and high strength bonding technique, using potassium silicate bonding, has been developed. To perform this kind of bonding, the surfaces need to be extremely flat and clean. The glue is constituted of very pure water plus a small amount of catalyser (potassium hydroxide). This bonding technique reconstructs mainly the silicate chains between the two surfaces making it possible to transform the two parts in a unique material. This technique has been used to bond the upper fused silica fibre heads to the steel stage, the lateral flat surfaces of the interferometer mirror to the so-called fused silica ears and the lower fused silica fibre heads to the lower surfaces of the ears themselves.

Silica Suspension 3

Helios Vocca (University of Perugia) with the first fused Silica monolithic suspension for Advanced Virgo (Credits: Helios Vocca/EGO and The Virgo Collaboration)

The interferometer mirror with a weight of 42 kg is suspended in this case with 4 fibres with a diameter of 400 microns each and a length of about 70 cm. This kind of suspension is apparently very fragile, but instead it is highly resistant. The only caution is to prevent any possible lateral damage to the fibres itself: anything touching the fibres surface seriously affects their structure opening cracks that immediately bring to the explosion of the suspension itself.

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