Seismic Isolation of Einstein Telescope

Project Information

SILENT

Grant agreement ID: 866259

DOI

10.3030/866259

Project closed

EC signature date 18 February 2020

Start date 1 September 2020

End date 31 August 2025

Funded under

• EXCELLENT SCIENCE - European Research Council (ERC)

Total cost

€ 1 930 200,00

EU contribution

€ 1 930 200,00

Coordinated by

UNIVERSITE DE LIEGE
Belgium

Periodic Reporting for period 3 - SILENT (Seismic Isolation of Einstein Telescope)

Reporting period: 2023-09-01 to 2025-02-28

Summary of the context and overall objectives of the project

With the ever-developing new technologies and high-precision scientific instruments, the performance requirements of vibration isolation are more and more stringent. Gravitational- Wave Detectors (GWDs) are probably some of the most advanced of those high-precision scientific instruments. GWDs are basically kilometer-scale interferometers: devices capable of measuring the relative motion between two mirrors with high accuracy. The best state-of-the-art technologies, from many different domains of engineering, are used to isolate these mirrors from any external disturbances. The only remaining external factor that could cause a relative motion of the test mass is a distortion of space itself.

These ’’ripples’’ in the fabric of space-time, known as Gravitational-Waves (GWs), results from the theory of general relativity proposed by Albert Einstein in 1915. During decades, many instruments have been set up and built in order to detect those GWs. But they have remained undetected because of their extremely low strain level on Earth, estimated to be around 10-21 [1]. To emphasize, this strain would cause two masses placed 1 km apart to show a relative displacement less than the size of a proton [1]. This is only about one hundred years later, in September 2015, that for the very first time in history GWs have been detected by the American 2nd-generation GWD, named advanced LIGO [2].

In addition to LIGO [3] in the US, many other instruments, namely Virgo [4] and GEO600 [5] in EU, have been built all around the world, allowing to detect and study those GWs generated by strong astrophysical events, such as black-hole merging or supernovae. These new waves offer a completely fresh and new view of the universe, providing information that complements those obtained from ‘’traditional’’ telescopes, sensitive to electromagnetic waves. However, their sensitivity is limited to a certain frequency band and amplitude, meaning that many sources of GWs are still being undetected. This is why a 3rd-generation GWD, the Einstein Telescope, is being developed. By extending the detection bandwidth from a few 10’s of Hz, for the current detectors, down to 3 Hz and improving the sensitivity by a factor of 10, the ET telescope is expected to have a detection rate by a factor of 1000 larger than the initial design. In addition, it would allow to detect, and therefore study, GWs coming from different, and still unknown, astronomical events [7].

Reaching the desired sensitivity for the ET is a huge challenge. Among the limitations, seismic vibrations and Newtonian noise are the biggest threats to the sensitivity at low frequencies (1 - 10 Hz) [6]. This is why the SILENT (Seismic IsoLation of EinsteiN Telescope) have been launched. This project aims to develop a platform to isolate GWDs from ground motion [8]. Equipped with inertial sensors, liquid inclinometers and a gravimeter, the table will allow to decouple whatever is put onto it from the Earth, making it virtually float in inertial space [8]. The main goal of this project is to extend the sensitivity of advanced Gravitational Wave Detectors (GWD) towards lower frequencies. In order to reach this goal, we propose to develop a completely novel. Optomechatronic Active stabilization SYStem (OASYS). It will be controlled by optical seismometers, precise liquid inclinometers and a gravimeter, and will use robust control laws. Although this project is closely related to the ET telescope, it is also very applicable to many other fields of engineering.

References:

[1] ACCADIA T., et al. The seismic superattenuators of the VIRGO gravitational waves interferometer. Journal of Low Frequency Noise, Vibration and Active Control, 30(1):63-79, 2011.

[2] ABBOTT B. P., et al. Observation of Gravitational Waves from a Binary Black Hole Merger. Physical review letters vol.116 061102, 2016.

[3] ABBOT B.P. BP et al. LIGO: the laser interferometer gravitational-wave observatory. Rep. Prog.Phys. 72 076901, 2009.

[4] ACCADIA T., SWINKELS B. L. and the VIRGO Collaboration. Commissioning status of the Virgo interferometer. Class. Quantum Grav. 27 084002, 2010.

[5] GROTE H., and the LIGO Scientific Collaboration. The GEO 600 status Class. Quantum Grav. 27 084003, 2010.

[6] PUNTURO M., et al. The Einstein Telescope: a third-generation gravitational wave observatory. Class Quantum Grav. 27 194002, 2010.

[8] CERN. CERN’s new Einstein observatory to explore black holes, big
Bang. May. 2011. Online, accessed: January 25, 2021.

[9] ULiège. Christophe Collette obtains an ERC Consolidator Grant for his project SILENT. March 2020. Online, accessed: January 25, 2021.

Work performed from the beginning of the project to the end of the period covered by the report and main results achieved so far

The optical measurement system has been developed and produced. The design originates from an optical displacement sensor that has been designed for previous iterations of seismometers. The manufacturing of the device according to our custom optical scheme has been outsourced to a company specialized in optical assemblies. This allowed the design to be compact and aligned to a professional level of accuracy. The characterization test bench has been designed. The sensors will be tested in a very quiet Ultra-High Vacuum (UHV, 10-6 mbar) environment. To this end, a vacuum chamber has been purchased from the Pfeiffer Vacuum company. A horizontal inertial sensor has been designed and using a very well-known mechanics named Watt’s Linkage. The mechanics has been produced and is readily available. The interferometer is now being integrating in order to complete the assembly of the design.
In addition to the inertial sensor, the active platform is built. It is a hexagonal-shaped platform, placed on three vertical and three horizontal springs that are mounted on top of a passively isolated rectangular stage. On top of the active stage, three modules are added, each one of them constitutes a vacuum chamber, a horizontal inertial sensor, and a vertical inertial sensor. Six voice coil actuators are mounted between the passive and the active platform to allow actuation on the bottom face of the hexagonal platform. They are placed in a quasi-collocated manner with the sensors in order to facilitate control. Furthermore, a multi-body model of this system was implemented on SimscapeT M. The gravimeter was placed on the active platform and isolated to reduce seismic noise. This allowed to reach the intrinsic noise of the absolute gravimeter and thus obtain a better resolution. Several gravity measurements were performed in different locations in the laboratory to gain insights on the gravity signal obtained. Moreover, a comparison was done in Membach with the absolute quantum gravimeter from Exail and the FG5 from microG Solutions. This measurement had several purposes: calibrate the gravimeter with a reference, compare its resolution experimentally and analyse the data in a seismically calm environment.
The merging of the accelerometer with the gravimeter is an ongoing task. The accelerometer works with the atomic interferometer but does not subtract ground efficiently due to induced noise. We are currently designing a robust casing to isolate the sensor from external perturbations.

Progress beyond the state of the art and expected potential impact (including the socio-economic impact and the wider societal implications of the project so far)

By the end of the project, we expect to push further down the amount of ground isolation by means of advanced control strategies and very high resolution sensors. The use of the absolute quantum gravimeter in the control loop is also expected to further improve the isolation performances.
The gravimeter is expected to correct the low frequency drift from the sensors on the platform, as it is a drift-free instrument. Consequently, the isolation of the platform would improve with these corrected drift sensors. The isolation of the platform would provide a calm environment to the gravimeter, allowing it to work with a better resolution. As a result, the gravity signal can be fed to the platform as control input to mitigate gravity variation on the table.

A SILENT platform.