E. Majorana

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.

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.

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.

Evaluation of heat extraction through sapphire fibers for the GW observatory KAGRA

Currently, the Japanese gravitational wave laser interferometer KAGRA is under construction in the Kamioka mine. As one main feature, it will employ sapphire mirrors operated at a temperature of 20 K to reduce the impact from thermal noise. To reduce seismic noise, the mirrors will also be suspended from multi-stage pendulums. Thus the heat load deposited in the mirrors by absorption of the circulating laser light as well as heat load from thermal radiation will need to be extracted through the last suspension stage. This stage will consist of four thin sapphire fibers with larger heads necessary to connect the fibers to both the mirror and the upper stage. In this paper, we discuss heat conductivity measurements on different fiber candidates. While all fibers had a diameter of 1.6 mm, different surface treatments and approaches to attach the heads were analyzed. Our measurements show that fibers fulfilling the basic KAGRA heat conductivity requirement of κ ≥ 5000 W m−1 K−1 at 20 K are technologically feasible.

Vacuum-compatible vibration isolation stack for an interferometric gravitational wave detector TAMA300

Interferometric gravitational wave detectors require a large degree of vibration isolation. For this purpose, a multilayer stack constructed of rubber and metal blocks is suitable, because it provides isolation in all degrees of freedom at once. In TAMA300, a 300 m interferometer in Japan, long-term dimensional stability and compatibility with an ultrahigh vacuum environment of about 10−6 Pa are also required. To keep the interferometer at its operating point despite ground strain and thermal drift of the isolation system, a thermal actuator was introduced. To prevent the high outgassing rate of the rubber from spoiling the vacuum, the rubber blocks were enclosed by gas-tight bellows. Using these techniques, we have successfully developed a three-layer stack which has a vibration isolation ratio of more than 103 at 300 Hz with control of drift and enough vacuum compatibility.

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.

Low-frequency terrestrial tensor gravitational-wave detector

Terrestrial gravitational-wave (GW) detectors are mostly based on Michelson-type laser interferometers with arm lengths of a few km and signal bandwidths of tens of Hz to a few kHz. Many conceivable sources would emit GWs below 10 Hz. A low-frequency tensor GW detector can be constructed by combining six magnetically levitated superconducting test masses. Seismic noise and Newtonian gravity noise are serious obstacles in constructing terrestrial GW detectors at such low frequencies. By using the transverse nature of GWs, a full tensor detector, which can in principle distinguish GWs from near-field Newtonian gravity, can be constructed. Such a tensor detector is sensitive to GWs coming from any direction with any polarization; thus a single antenna is capable of resolving the source direction and polarization. We present a design concept of a tensor GW detector that could reach a strain sensitivity of 10−19–10−20 Hz−1/2 at 0.2–10 Hz, compute its intrinsic detector noise, and discuss procedures of mitigating the seismic and Newtonian noise.

Test facility for resonance transducers of cryogenic gravitational wave antennas

In order to reach the maximum sensitivity of the new ultra-low temperature gravitational wave antennas, a further improvement in transducer technology is necessary. The authors present the test system developed for the laboratory study of the resonant capacitive transducer and discuss some of the experimental results obtained at low and room temperatures.

CHARACTERIZATION OF MECHANICAL DISSIPATION SPECTRAL BEHAVIOR USING A GRAVITOMAGNETIC PENDULUM

Abstract We have developed an experimental method based on the reaction of a magnetic mass with respect to a dc biased coil to change the frequency of a pendulum. In a preliminary experiment this method has allowed us to investigate the mechanical dissipation of nylon pendulum wires over the 0.1-1.4 Hz frequency range. According to the procedure shown in this paper, it is possible to deduce from the measurements whether the dissipation process is viscous or not. The method is generally applicable and adaptable to be used to characterise the behaviour of the suspension systems developed for gravitational waves detectors.