P. Rapagnani

The Virgo interferometer

The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa to be commissioned in the year 2000. Virgo has been designed to achieve a strain sensitivity of a few times at 200 Hz. A large effort has gone into the conception of the mirror suspension system, which is expected to reduce noise to the level of at 10 Hz. The expected signals and main sources of noise are briefly discussed; the choices made are illustrated together with the present status of the experiment.

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.

The gravitational wave detector NAUTILUS operating at T = 0.1 K

Abstract We report on the ultralow-temperature resonant-mass gravitational-wave detector NAUTILUS, operating at the Frascati INFN Laboratories. The present aim of this detector is to achieve a sensitivity sufficient to detect bursts of gravitational radiation from sources located in our Galaxy and in the local group. Progress in transducer technology is likely to lead to sensitivities that will enable us to observe events from sources as far away as the Virgo cluster of galaxies. We describe the cryogenic apparatus, readout system, cosmic-ray veto system, and give first results obtained during one year of continuous operation at T = 0.1 K. In particular the Brownian noise of the detector at T = 0.1 K was measured. The measured strain sensitivity was h ≈ 6 · 10 −22 Hz − 1 2 at the frequencies of the two modes, 908 Hz and 924 Hz, with bandwidths of about 1 Hz.

Suspension last stages for the mirrors of the Virgo interferometric gravitational wave antenna

We describe the design of the last stage suspension for the mirrors of the Virgo gravitational wave detector and, in particular, its key mechanical elements: the marionette and the reaction mass. Since the whole suspension system is an electromechanical device, we present both its mechanical and electromagnetic components. The main features of the fully assembled prototype and those worked out for the Virgo final design are discussed.

The maraging-steel blades of the Virgo super attenuator

The blades are crucial components of the Virgo super attenuators. The material used for their construction is maraging steel, a low-carbon-content alloy with high ultimate tensile strength and low creep under stress. Youngs modulus, the shear modulus, the Poisson ratio and the corresponding elastic energy-loss coefficients have been measured. The measurements have been performed on specimens subjected to the same thermal treatments as those of elements for the Virgo interferometer realized with maraging steel. In addition, anelastic properties of the material subjected to different thermal treatments have been measured. It has been found that, for a maraging-steel structure (one free of plastic deformation), which undergoes an excitation with flexural vibrations, the elastic energy-loss coefficient can vary over a wide range as a function of the thermal treatment of the material and it is dominated by the thermo-elastic effect. The main reason for such a great alteration is supposed to be the dependence of the thermal conductivity on the average sizes of the precipitate particles and their relative separations.

The Virgo Project

The Virgo project is a Italian-French collaboration aiming at the construction of a long baseline interferometric antenna for the detection of gravitational radiation signals of cosmic origin. We describe the principles of the system, and high-light the technical challenges we need to overcome for reaching a sensitiity as low as 10−23Hz−1/2.The gravitational clustering of collisionless particles in an expanding universe is modelled using some simple physical ideas. I show that it is possible to understand the nonlinear clustering in terms of three well defined regimes: (1) linear regime; (2) quasilinear regime which is dominated by scale-invariant radial infall and (3) nonlinear regime dominated by nonradial motions and mergers. Modelling each of these regimes separately I show how the nonlinear two point correlation function can be related to the linear correlation function in hierarchical models. This analysis leads to results which are in good agreement with numerical simulations thereby providing an explanation for numerical results. Using this model and some simple extensions, it is possible to understand the transfer of power from large to small scales and the behaviour of higher order correlation functions. The ideas presented here will also serve as a powerful analytical tool to investigate nonlinear clustering in different models.

Vibration-free cryostat for low-noise applications of a pulse tube cryocooler

A new generation of gravitational wave interferometers is under study with the main goal to significantly improve the sensitivity of the detectors presently taking data. Two of the dominant noises which limit the present sensitivity of the interferometers are the thermal noise of the suspended optics and the thermal lensing process. At low temperature it is possible to reduce both these effects. Pulse tube cryocooler technology is a quite promising in this field, but it vibrates and it implies a mechanical link between the cooling element (cold finger) and the thermal load. We developed a vibration-free cryostat equipped with soft mechanical links to attenuate the transmission of cold finger vibration whose amplitude is attenuated by more than two orders of magnitude by means of an active system.

Upper limit for a gravitational-wave stochastic background with the EXPLORER and NAUTILUS resonant detectors

We discuss the sensitivity of resonant-mass gravitational-wave detectors to a cosmic stochastic background of gravitational waves. We report the experimental upper limits given by the gravitational wave detectors EXPLORER and NAUTILUS.

The VIRGO interferometer for gravitational wave detection.

The Virgo gravitational wave detector is an interferometer with 3 km long arms in construction near Pisa in Italy. The accessible sources at the design sensitivity and main noises are reviewed. Virgo has devoted a significant effort to extend sensitivity to low frequency reaching the strain level h = 10−21 Hz−1/2 at 10 Hz while at 200 Hz h = 3 · 10−23 Hz−1/2. Design choices and status of construction are presented.

Monitoring the acoustic emission of the blades of the mirror suspension for a gravitational wave interferometer

We monitored the acoustic emission activity of the steel blades to be used for the mirror suspension system of a gravitational wave interferometer. We have collected several sets of events getting evidence of a material memory effect (Kaiser effect) associated to the dislocation motion in the steel. This result is more evident when we apply a standard fractal analysis procedure (box counting method) to the timing series of acoustic emission bursts. We conclude that a significant reduction of the emission rate is obtained by applying a few stress cycles to the elastic blades.