LIGO Document P0900076-v1

Astrophysics issues and low frequency mechanical noise for third generation gravitational waves detectors

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The major challenge in GW astronomy, at present, is the realization of the conceptual design of a third generation, lower frequency interferometric detector, which should be able to reach a good sensitivity in the frequency range 1 to 10 Hz, thus opening a new GW era.
In the fi rst two chapters I will talk about the theory of GW detection and I
will expose the necessity to build an underground facility to cover the sensitivity range 1 to 10 Hz.
The Einstein Telescope (ET), the European design study project for the
construction of an underground GW interferometer, will be presented in the
third chapter. I will discuss its features, expected sensitivity and astrophysics
potentialities. I will show, moreover, that the emission of continuous gravitational waves from a few hundred pulsars is detectable by the ET antenna, using its proposed sensitivity. One of the major challenges, in the realization of a third generation GW detector, will be the reduction of the noise at low frequency, which is dominated by the seismic and the gravity gradient one: the seismic isolation thus plays a fundamental role in the improvement of the low frequency band sensitivity of these interferometers.
All seismic isolation systems developed for gravitational waves interferometric detectors, such as LIGO, VIRGO and TAMA, make use of Maraging steel blades, a precipitation hardened alloy that allows the production of creep-free blade springs. Although these springs provide exceptional attenuation performance at high frequency (>1 Hz), anomalies were observed at lower frequency.
The main subject of my thesis will be presented in chapter four, which is
the result of my experimental work at the California Institute of Technology,
LIGO project. It concerns the dissipation properties of the above mentioned blades, studied at low frequencies using a Geometric Anti Spring (GAS) filter, which, together with the Electro Magnetic Anti Spring (EMAS) mechanism, allowed the exploration of resonant frequencies below 100 mHz. At this frequency an anomalous transfer function has been observed in the GAS fi lter: this is one of the several motivations for starting this research.
I will discuss the theoretical model which best explains our experimental
fi ndings, the mechanical filter that we used, the experimental procedure, the
data analysis and the results obtained.
Finally, in chapter fi ve, I will conclude giving an overview about the future
plans in this fi eld.
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