Some Aspects of Vibration Isolation and Feedback Control for Interferometric Gravitational Radiation Detectors

Cantley, Caroline Agnes (1991) Some Aspects of Vibration Isolation and Feedback Control for Interferometric Gravitational Radiation Detectors. PhD thesis, University of Glasgow.

Abstract

Gravitational radiation, first predicted by Einstein in his General Theory of Relativity (1916), remains undetected despite considerable effort by researchers over the past few decades. Direct observation of gravitational radiation would not only provide a test of Einstein's theory, but would yield information on the astrophysical sources and processes involved in its production. Gravitational radiation is quadrupole in nature and it gives rise to a tidal strain in space. However its interaction with matter is very weak making it extremely difficult to detect. The prototype detector developed at Glasgow University is designed in such a way that it exploits the quadrupole nature of gravitational radiation by attempting to measure the differential change in length between two resonant cavities making up the orthogonal arms of an interferometer. Similar laser interferometric detectors are currently under development at various sites worldwide. Development of the 10m prototype detector at Glasgow was initiated about 13 years ago and plans are now well under way for the development of a more sensitive detector of 3km arm length (the GEO project) in collaboration with various other research groups including a group at the University of Wales (College of Cardiff) and one at the Max-Planck-Institute for Quantum Optics in Garching, Germany. Chapter 1 is a general introduction to the nature of gravitational radiation and the astrophysical sources likely to produce detectable levels of this radiation at the Earth. The main features of the two most promising types of detector currently being developed -resonant bar detectors and laser interferometric detectors - are described. The ultimate sensitivity of any gravitational wave detector is limited by various sources of noise and the dominant noise sources which degrade the performance of interferometric detectors are discussed in some detail. It is shown here that the sensitivity of such detectors is likely to be severely limited by the effects of seismic noise at low frequencies (below about 100Hz). The test masses forming the arms of an interferometric gravitational wave detector must be isolated from all external influences, particularly the seismic background, and must be 'free' to move under the influence of a gravitational wave. Furthermore, in order to operate an interferometric detector efficiently the position and orientation of the test masses must be controlled to a very high degree of accuracy. In Chapter 2 the level of seismic noise expected at a typical detector site is discussed. Some of the methods commonly used to seismically isolate the test masses in interferometric detectors are then described. A brief introduction to the concept of feedback control and the methods of analysis available for designing feedback systems is then presented. Finally, preliminary experimental investigations into the position control of a test mass suspended as a simple pendulum are described. In order to achieve the required level of seismic isolation of the test masses in the planned 3km detector (GEO) (an isolation factor of ~10e10 at ~100Hz) it is proposed to use double pendulum suspensions in conjunction with five-layer vibration isolation stacks and air mounts. Chapter 3 gives an account of various theoretical investigations carried out into feedback control and damping of a test mass suspended as a double pendulum. Experimental investigations into feedback control and damping of various double pendulum systems were also conducted and the results from these are presented. On applying feedback control and damping signals to an isolated test mass it is important to avoid re-introducing displacement noise. Chapter 4 describes a novel design of double pendulum suspension system with frequency selective (split) feedback control. This system was designed specifically to attempt to reduce the level of displacement noise occurring at the test mass due to the application of the feedback signals. Two different split feedback control systems were designed and their perfomance was tested experimentally. A finite element model was generated to predict the levels of isolation in the horizontal and vertical directions achievable with the double pendulum system described in Chapter 4. The horizontal isolation was measured experimentally to compare it with the theoretically predicted isolation. The theoretical and experimental results are presented in Chapter 5. (Abstract shortened by ProQuest.).

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Astronomy
• Date of Award: 1991
• Depositing User: Enlighten Team
• Unique ID: glathesis:1991-78270
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 30 Jan 2020 15:34
• Last Modified: 30 Jan 2020 15:34
• URI: https://theses.gla.ac.uk/id/eprint/78270