Advanced instrumentation for gravitational-wave research

The first part of chapter 2 briefly introduces a few relevant aspects of Eintein’s theory of general relativity and the generation of gravitational waves. The second part deals with the interferometric detection principles, in particular focusing on the sensitivity limitations, and ends with an outlook on the future landscape of this field of research. Chapter 3 contains a detailed description of a laboratory-scale test setup of a laser interferometer that includes suspended optics. This small size instrument mimicks the fundamental building blocks of a laser interferometer, like those of the kilometerscale gravitational-wave detectors LIGO and Virgo. This test setup has been used to develop and demonstrate novel methods for fast recovery from large misalignments. In particular the last section is dedicated to a proof of concept of using beam camera images for automated pre-alignment of optical cavities, even when they are far outside their linear response region. Chapter 4 presents an overview of Nikhef’s contribution to the Advanced Virgo Plus filter cavity realization and commissioning. This is part of the detector upgrade in preparation of the O4 observation run. It focuses on the activities by the author on the preparation and installation of the filter cavity mirror suspension systems and the optimization of the local controls. Chapter 5 is dedicated to one of the most demanding elements in the seismic attenuation systems: the maraging steel blade springs. These mechanical components, designed to operate at a high level of stress while maintaining low creep, are subject to sporadic failures that have a large impact on the duty cycle of a gravitational-wave detector. The most common cause of failure is hydrogen embrittlement of the blade material. This chapter starts with an introduction about the properties of the material, followed by theoretical considerations of hydrogen embrittlement. An overview of recorded blade failure events in gravitational-wave detectors is presented. Results of the study on the hydrogen contamination levels and on the binding energies of the trapping sites inside the material are discussed. Finally, the results of a comparative study, in which the resistance of different materials to stress corrosion cracking was investigated, are presented. Chapter 6 covers the characterization of a novel instrument to measure gradients of the gravitational field. The chapter starts with theoretical considerations of geophysical aspects. Then the concept of a ribbon gravity gradiometer is illustrated, followed by a description and the characterization of the individual subsystems. A dedicated gravity-gradient source, a spinning rotor, has been developed to test the instrument and its capabilities to resolve fluctuations in the gravitational field for various rotor spinning frequencies and stand-off distances. The results reveal the potential of the instrument for future application in both third generation gravitational-wave observatories and in geophysical applications. The thesis ends with a summary and outlook.