Investigations of charging noise in future gravitational wave detectors

Gravitational waves are a result of Einstein's theory of general relativity which postulates that these waves, created by asymmetrically accelerating masses, propagate through the Universe as ripples in the curvature of space-time at the speed of light. Gravitational waves have yet to be directly detected, however, there is sufficient indirect evidence of their existence through observations of inspiraling pulsars and white dwarfs. There is a worldwide network of interferometric detectors that are currently being, or have already been, upgraded to an advanced detector status. In order to get these detectors to a sensitivity high enough to stand a chance of detecting gravitational waves, researchers around the world have been developing innovative and novel technologies, and investigating high quality, low mechanical loss materials, to reduce the limiting noise sources of these instruments. One noise source that may become more conspicuous as gravitational wave detectors become more sensitive is noise due to the movement of electrical charges on the detector optics. Charge can be transferred to the detector optics through various processes such as; dust abrasion on the surface of the optics as the vacuum chamber is being pumped out, contact with nearby structures such as earthquake stops, cleaning of the optics and cosmic rays. In some instances the charge density transferred can be as high as 10^(-7) C/m^2 which would cause the sensitivity of advanced detectors to be significantly reduced at frequencies less than 100Hz. The work contained within this thesis investigates different methods of mitigating the effects of charging noise through the use of glow and corona discharges. It is shown that both of these methods successfully reduce surface charge on silica with an advanced LIGO style optical coating and that these methods do not cause damage to the optical coating within the sensitivity of the equipment. Investigations were carried out to see if the charge deposited on the detector optics during cleaning could be reduced by mixing carbon nano-tubes into the polymer based cleaning solution used to clean the advanced LIGO optics. It is shown that mixing carbon nano-tubes into the polymer cleaning solution does not reduce the amount of charge deposited on the optics. This thesis also presents experimental verification of the frequency dependence of charging noise. Using a torsion balance it was possible to measure charging noise directly and confirm that it does follow the Weiss theory of charging noise