• Einstein Telescope
• gravitational waves
• Pendulum Inverted Pendulum
• seismic noise
• seismic noise attenuation

Since the first direct detection of a Gravitational Wave (GW) in September 2015, the current second generation of GW observatories (comprising Advanced Virgo, Advanced LIGO and KAGRA) has yielded numerous groundbreaking results in both astrophysics and fundamental physics, giving GW physics a leading role in these research fields.
A third generation of detectors is planned to be operational in the middle of the next decade, paving the way for a new era in GW science. The Einstein Telescope (ET) is the European proposal for the next generation of detectors. This gravitational antenna, which will be located underground, aims at enhancing by at least an order of magnitude the sensitivity of second-generation detectors and to lower the frequency threshold down to 2Hz. In this context, the attenuation of the seismic noise constitutes a challenging problem. Indeed, an attenuation system that fulfills the ET requirements, based on the Advanced Virgo Superattenuator, would result in a 17m high chain, with a consequent significant increase in costs, time and workforce necessary for the infrastructure construction. It is therefore necessary to reconsider the design of the ET seismic attenuation chain, with the goal of developing suspensions that fit in ~10m (compact suspensions).
This thesis work represents the first study of a prototype of a novel and innovative filter for the ET Superattenuator: the Pendulum Inverted Pendulum (PIP).
This filter has been designed to provide an overall attenuation proportional to 1/f^4, which is only achievable with two filters of the standard Superattenuator, in the space occupied by a single filter. Thus, a cascade of PIP filters reduces the total length of the attenuation system.
The filter is made up of a base ring, that acts as a pendulum, and three inverted pendulum legs installed on it, interconnected by a vertical filter similar to the Filter 7 used in the Virgo Superattenuator. Inertia provides the attenuation of horizontal motion, while the Virgo-like filter is responsible for the vertical damping.
In this work, we present the tests conducted on a full-scale prototype of the PIP. Particular attention was paid to the characterization of the experimental setup (i.e. sensors, actuators). This is of fundamental importance for the correct interpretation of the obtained results. The tests performed on the filter were divided into two phases. A first set of tests was carried out on the single inverted pendulums: these tests aimed at estimating the stiffness of their elastic joint, an important parameter in the design of the filter, and to study the effects of the center of percussion position on the attenuation performance. The second phase concerned the filter prototype resting on the ground. In particular, we focused on the analysis of the horizontal normal modes and the corresponding resonance frequencies.
Finally, the PIP filter has been suspended and the experimental activities dedicated to its characterization could start. Preliminary results obtained with the PIP and perspectives conclude this work.