TOWARDS LOW FREQUENCY SEISMIC ISOLATION OF LARGE CRYOGENIC MIRROR

Gravitational waves (GWs) are ripples in space-time that travel at close to the speed of light and are detected on Earth using highly precise laser interferometers. Observing these waves is essential for our understanding of the universe. To further improve detection accuracy and reduce noises, it is crucial to upgrade the sensitivity of second-generation GW detectors to the more advanced third-generation systems. Seismic noise, particularly at frequencies below 10 Hz, is one of the primary sources of interference, significantly affecting the sensitivity of existing GW detectors. This study introduces a novel concept of compact low-frequency isolation system for the seismic noise reduction. The isolation system uses a passive inverted pendulum and cascaded pendulums mounted on an active inertial platform. The new isolation system serves as a proof-of-concept, demonstrating potential to reduce the overall height of the Einstein Telescope (ET) isolator (currently 17.5 m) and to enhance seismic noise reduction at low frequencies (below 10 Hz). Such an isolator could compensate for drifts in the test mirror or the entire isolation system, simplifies the underground infrastructure, and reduces associated costs if constructed underground. Moreover, better seismic noise reduction at low frequency could decrease the Root Mean Square (RMS) motion of the mirror, thereby improving the locking capability of the interferometer.

To experimentally validate the new isolation concept, this study presents the design of a full-scale prototype. It provides detailed information on modal analysis, control system design, and performance evaluation. Various considerations are also discussed, including the criteria for selecting the isolator structure, the CAD design and FEM analysis of its components, the placement of sensors and actuators, the tilt-horizontal coupling, and the performance limitations of the isolator. The study shows five stages of the isolator assembly: an active inertial platform, an inverted pendulum, a marionette, and a payload (cold platform and mirror).

The simulation results demonstrate the outstanding performance of the isolator, achieving approximately two orders of magnitude of seismic isolation around 1 Hz in the vertical and horizontal directions when the control system is applied. This level of isolation is attained with a compact isolator measuring 4.2 m in height and 2.5 m in diameter.

The study revealed that the closed-loop control system could not be implemented during the first experimental run of the entire isolator inside the vacuum chamber due to friction between some components of the prototype. However, preliminary experimental results from Haidar Lakkis, who continued this work, demonstrated that applying the closed-loop control system to the AP system alone achieves approximately two orders of magnitude of isolation at 1 Hz. This performance aligns with the predictions of the simulation model presented in this study.

Overall, the study successfully achieved designing a compact isolation system with low-frequency seismic suppression. The additional reduction in ground motion transmission below 10Hz has the potential to break the seismic noise barrier for the new generation of GW observatories. This advancement will help achieve the goal of the third-generation GW observatories, extending the detection frequency range below 10 Hz.