F. Raffaelli

Measurement of the VIRGO superattenuator performance for seismic noise suppression

Below a few tens of hertz interferometric detection of gravitational waves is masked by seismic vibrations of the optical components. In order to isolate the mirrors of the VIRGO interferometer, a sophisticated suspension system, called superattenuator, has been developed. Its working principle is based on a multistage pendulum acting on seismic vibrations as a chain of second order mechanical low-pass filters. A complete superattenuator has been built and tested. This apparatus allows extending the VIRGO detection band down to a few Hz. A detailed description of the attenuation system and its performance are presented in this article.

Extending the VIRGO gravitational wave detection band down to a few Hz: metal blade springs and magnetic antisprings

Abstract The detection band of the interferometric gravitational wave detector VIRGO can be extended down to a few Hz by suspending each optical component of the interferometer from a chain of mechanical filters designed to suppress the transmission of seismic vibrations. Each mechanical filter supports the weight of the stages below it through a set of cantilevered blade springs. A system of permanent magnets, providing an “antispring” force, helps to reduce the highest vertical resonance of the chain from 7 Hz to about 2 Hz. This improvement allows VIRGO to reduce the frequency detection threshold from 10 Hz to about 4 Hz. A characterization of the mechanical filters is provided in this paper.

Relevance of Newtonian seismic noise for the VIRGO interferometer sensitivity

In this paper we analyse the noise level induced by changes in the mass density distribution around the Virgo interferometric antenna. These stochastic mass density fluctuations generate a gravitational field which couples directly to the mirrors of the optical apparatus, and it could be relevant if the planned final sensitivity of the Virgo interferometer is to be reached.

THE CREEP PROBLEM IN THE VIRGO SUSPENSIONS : A POSSIBLE SOLUTION USING MARAGING STEEL

Abstract Each optical component of the interferometric gravitational wave detector VIRGO is suspended from a cascade of mechanical filters designed to suppress the transmission of seismic vibrations. Each mechanical filter supports the weight of the filters below it by means of a set of steel cantilever blade springs. The stress from the load acting on the blades was found to induce a drooping of the blade tips of several microns per day due to a series of microscopic yielding events (micro-creep). This process induces a mechanical displacement shot-noise on the optical component which can dominate the small displacements produced by gravitational waves. The use of a special precipitation hardened steel (Maraging C250), instead of common spring steel, allows the construction of blades that show an acceptable stability under stress.

Air bake-out to reduce hydrogen outgassing from stainless steel

Hydrogen outgassing is the most significant factor limiting the attainment of outgassing rates below 10−12 mbar l s−1 cm−2 in stainless steel vacuum systems. This limit turns out to be crucial in very large vacuum systems, like the VIRGO vacuum tubes (2 tubes 1.2 m diam, 3000 m length). Heating the raw material at 400 °C in air was suggested as a money saving alternative to the classical vacuum heating at 950 °C. We report the results of hydrogen content analysis performed on stainless steel samples submitted to different treatments, and also the measurement performed on a prototype tube (1.2-m-diam, 48-m-long). We concluded that air bake-out drives out most of the hydrogen absorbed in the bulk stainless steel, while the presence of an oxide layer does not reduce the hydrogen outgassing.

Ground tilt seismic spectrum measured with a new high sensitivity rotational accelerometer

We describe a new rotational accelerometer exhibiting excellent rotational acceleration sensitivity of 2–3×10−9 rad s−2/√Hz for frequencies below 2 Hz and increasing as the square of frequency for frequencies higher than 2 Hz. The sensitivity to horizontal linear accelerations and static tilts is 60 dB less than its rotational sensitivity. The accelerometer has been used to characterize the seismic ground tilt spectrum. Measurements of the performance of the device and of the seismic rotational spectrum are presented.

Experimental upper limit on the estimated thermal noise at low frequencies in a gravitational wave detector

A. Di Virgilio, L. Barsotti, S. Braccini, C. Bradaschia, G. Cella, V. Dattilo , M. Del Prete, I. Ferrante, F. Fidecaro, I. Fiori, F. Frasconi, A. Gennai, A. Giazotto, P. La Penna, G.Losurdo , E. Majorana, M. Mantovani , F. Paoletti, R. Passaquieti, D. Passuello, F. Piergiovanni, A. Porzio, P. Puppo, F. Raffaelli, P. Rapagnani, F. Ricci, S. Solimeno, G. Vajente, F. Vetrano INFN, Sez. di Pisa, Pisa, Italy 2 EGO, European Gravitational Observatory, Cascina (Pi) 3 Universita’ di Pisa, Italy 4 INFN Sezione di Firenze, Sesto Fiorentino, Italy 5 Università di Roma1, and INFN-Roma1, Roma Italy 6 Universita’ di Siena, Italy 7 Università di Urbino, Urbino, Italy 8 Coherentia, CNR-INFM, and CNISM Unitá di Napoli 9 INFN, Sez. di Napoli, Università di Napoli 10 Scuola Normale Superiore, Pisa

First results of the low frequency facility experiment

The first low frequency facility (LFF) data are presented; the main purpose of the LFF is to study the pendulum thermal noise around 10 Hz. The displacement noise floor is at the level of 10−13 m Hz−1x/2 at 10 Hz, and at about 11 Hz enters the 10−14 m Hz−1/2 sensitivity region; this level is compatible with the electronic noise of the drivers of the actuators used to lock the apparatus. The calibration method and the seismic noise contribution are discussed.

The low frequency facility Fabry-Perot cavity used as a speed-meter

Fabry–Perot cavities have many different applications as scientific instruments. In the gravitational waves research field they are extensively used to frequency stabilize lasers and to measure very small distance variations. In the present Letter a method to evaluate from the transmitted power only the relative speed and position of the mirrors of a cavity, having finesse F > 40, is described. A displacement spectral sensitivity of the order of about 3 × 10 −10 m/Hz −1/2 at 10 Hz is obtained with the cavity of the low frequency facility.  2003 Elsevier B.V. All rights reserved.

Considerations on collected data with the Low Frequency Facility experiment

The Low Frequency Facility consists of a 1 cm Fabry-Perot cavity suspended to a single SuperAttenuator, which is the mechanical system adopted to isolate the test masses of the Virgo interferometer. In this paper we present the preliminary results of measurements performed with a cavity of finesse 4000 and lasting 1-2 hours in different working conditions. The analysis presented here is focused mainly on the region below 100 Hz, and uses data collected with longitudinal control bandwidth below 150 Hz. A calibration test confirmed that the collected data are in good agreement with the model of the longitudinal control loop based on the open loop measurements. In addition to this, above 2 Hz the power spectrum of the two mirrors relative displacement shows a stationary noise floor and few peaks with high mechanical quality factor. Studying these peaks in the time domain, it has been observed that the energy associated with a single peak is Boltzman distributed, whether the oscillations are not excited. The measured upper limit of the seismic noise contamination at 10 Hz is around 2 × 10−14 m/√Hz.