Aspects in the Design of Spaceborne and Terrestrial Gravitational Wave Detectors

The aim behind much of the work outlined in this thesis has been mainly towards the development of test mass pendulum suspension systems suitable for laser gravitational wave detectors. These suspension systems have a dual purpose, namely providing seismic isolation and thermal noise reduction at the test mass. The techniques utilised throughout this work are comprised of a passive seismic isolation unit constructed from alternate layers of heavy metal and rubber (or spring) units, followed by the suspension of the test mass as a pendulum hung by fused silica fibres from the top of the isolation stage. Reviews of the theoretical work underpinning these systems have also been given, where the importance of low resonant mode frequencies of the isolation system, and high quality factors of the pendulum suspension fibres are highlighted. In chapter 2, details of the design, testing and current performance of a passive test mass seismic isolation system, constructed from layers of heavy metal and RTV-615 silicone rubber, are presented. The results of an experimental study of the anomalous compression characteristics of RTV (which lead to vertical stack resonant modes which are higher in frequency than one would normally expect) are also reported. Further to this it is shown that the compression modulus is subject to considerable stiffening when such units are placed under load. The construction and testing of a suspension system for the measurement of high pendulum and material quality factors are described, focusing on reducing recoil damping of the test pendulum. Results to date are reported for both measurement of pendulum and material quality factor of fused silica fibres. A theoretical analysis of the level of cross-coupling of seismic noise expected in the GEO 600 suspension system is presented, where the main mechanisms considered are cross coupling due to the radius of curvature of the Earth, and stiffness inequality of the test mass suspension wires. A theoretical study of attitude stability and phase-front distortion is carried out for LISA, a space-borne laser interferometric system. By considering the Fraunhofer phase variation in the far field resulting from distortions on a wavefront transmitted between two craft, the tolerable limit to the level of distortion on transmission is established