Development of Suspensions for the GEO 600 Gravitational Wave Detector

Torrie, Calum Iain Eachan (1999) Development of Suspensions for the GEO 600 Gravitational Wave Detector. PhD thesis, University of Glasgow.

Abstract

Einstein, in his 1916 Theory of General Relativity, predicted the existence of gravitational waves. These can be thought of as ripples or waves m the curvature of space-time. So far there has only been indirect evidence of the existence of gravitational waves. Scientists around the world are working on a number of gravitational wave detectors. The direct detection of gravitational waves will provide information about astrophysical processes and sources that produce them. Gravitational waves are quadrupole in nature and produce a tidal strain in space. However their interaction with matter is very weak, making them difficult to detect. Gravitational waves emitted by violent astrophysical events are predicted to produce strains at the Earth of the order of 10e-21 to 10e-22 at frequencies accessible to ground based detectors (~ 10 Hz to a few kHz). One method for detecting these strains in space is based on the Michelson interferometer. The gravitational waves group at the University of Glasgow led by Prof J. Hough is working with collaborators from the Max-Planck-Institut fiir Quantenoptik at Hannover and Garching, the University of Hannover, the University of Cardiff and the Albert-Einstein-Institut at Potsdam on a project called GEO 600 to build a laser interferometer with 600 m arm length. The GEO 600 gravitational wave detector is currently in an advanced stage of construction in Germany. The GEO 600 (German-British) detector is designed to operate down to 50 Hz. The sensitivity limit at this frequency is set by the thermal noise from the internal modes of the fused silica test masses. The strain sensitivity limit from thermal noise is expected to be 2x10e-22 /√Hz at a frequency of 50 Hz. The design goal for the seismic isolation system is to achieve a noise level a factor of 10 lower than this. To ensure that the detector sensitivity is not limited by seismic noise above 50 Hz a significant degree of isolation has to be provided for each test mass. It is expected that this level of isolation can be obtained with a combination of several elements in series: a two-layer isolation stack consisting of one active and one passive stage, and a triple pendulum, the final stage of which is the test mass which will be made from fused silica (mass ~ 6 kg). The triple pendulum will incorporate two stages of cantilever springs in order to enhance the vertical isolation and will use fused silica fibres in the lower pendulum stage in order to minimise thermal noise from the pendulum modes. The work contained in this thesis covers the design, modelling, construction and testing of various aspects of suspension systems for isolating optical components in ground based interferometric gravitational wave detectors, and in particular for GEO 600. The first suspension system considered was for the subsidiary mirrors that form the mode-cleaner cavities. These take the form of high finesse Fabry-Perot cavities used to reduce the geometry perturbations of the laser beam which is used to illuminate the interferometer. The mode-cleaner optics have less stringent requirements for seismic isolation; they therefore do not require the extra vertical isolation provided by the cantilever spring stages. Further work in this thesis involved the design and testing of the various stages of this suspension system. This includes the modelling of a suitable platform from which to suspend the various optics, the construction and testing of a double pendulum, and the testing of a two-layer passive isolation stack. The various isolation stages, for the mode-cleaner, were installed at the GEO 600 site in the summer of 1999. As stated above, for the main test mirrors a triple pendulum is required. In order to understand the mechanics of such a pendulum it was necessary firstly to model a single pendulum by calculating the equations of motion for six degrees of freedom. A theoretical model of a triple pendulum was obtained, again by writing down the equations of motion. MATLAB was used to predict, for example, the resonant mode frequencies, and the response of the triple pendulum to the application of control for active damping. Using this analysis the design of a well-damped triple pendulum with good coupling between the various stages can be achieved. A prototype triple pendulum was set-up in Glasgow in order to verify the predictions from the model. Further experiments on the individual stages of the overall suspension system in Glasgow, including the testing of the cantilever blades, indicated that a seismic noise level which is a factor of ~ 4 lower than the thermal noise level at 50 Hz should be achievable with the current design. At the time of writing the main suspension systems were beginning to be installed in the GEO 600 detector.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Additional Information: Adviser: Norma Robertson
• Keywords: Astronomy
• Date of Award: 1999
• Depositing User: Enlighten Team
• Unique ID: glathesis:1999-76201
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Dec 2019 09:15
• Last Modified: 19 Dec 2019 09:15
• URI: https://theses.gla.ac.uk/id/eprint/76201