High performance vibration isolation design for a suspended 72m Fabry-Perot cavity

Jean-Charles Dumas

    Research output: ThesisDoctoral Thesis

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    [Truncated abstract] Interferometric gravitational wave detectors require test masses to be isolated from seismic noise to satisfy sensitivity requirements at frequencies as low as a few Hertz. In this thesis we report on research undertaken for the development of a high performance compact vibration isolator and the implementation of two such prototypes in an 72m Fabry-P erot laser cavity. One of the goals of this research is to test novel methods of vibration isolation designs and to con rm their feasibility in a real interferometer. This thesis also focuses largely on the development of a digital control system to ful ll the control scheme necessary to govern the alignment, drift and normal modes of the isolation stack. In order to test the performance of the isolation and control system, an optical cavity was formed between the two isolators, and was fringe locked to a Nd:YAG slab laser. The culmination of this research was to develop the East arm of the High Optical Power Facility (HOPF) in Western Australia, a milestone towards a fully functional suspended cavity. The AIGO seismic isolator is a passive design, relatively compact 3m stack, combining new passive isolation techniques. It consists of three cascaded passive three-dimensional (3D) isolator stages suspended from an Ultra Low Frequency (ULF) horizontal Robert linkage stage which itself is suspended from a ULF 3D pre-isolator. The 3D isolators use self-damping pendulums and Euler springs for the horizontal and vertical stages respectively, while the 3D pre-isolator is the combination of an inverse pendulum which provides the low frequency horizontal pre-isolation, and a LaCoste linkage for low frequency vertical pre-isolation. In this thesis, we discuss the design and concepts behind each stage of the isolator, additionally we present some individual performance measurements of the 3D isolators, as well as modelling techniques to tune the Roberts Linkage. We present the implementation of a local control system using a digital control scheme. The test mass is suspended from a control mass which can be actuated in pitch, yaw, rotation and horizontal translation. The control mass is itself suspended from the last 3D isolation stage. All three ULF stages can be position monitored and actuated upon with a large dynamic range (6mm) through shadow sensors and coil-magnet actuators respectively, as can the control mass. There is no direct actuation on the test mass, and all alignment corrections are done indirectly through the control mass. An optical lever is used to read the angular position of the test masses. A local control system hosted on a DSP (digital signal processor) maintains alignment and position against drifts, and damps ULF and low frequency torsional modes without exciting the test mass normal modes...
    Original languageEnglish
    QualificationDoctor of Philosophy
    Publication statusUnpublished - 2009

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