NSF Award
1404269
Einstein's Theory of General Relativity predicts that the motion of massive astronomical bodies such as black holes or supernovae will generate tiny changes in the curvature of space called gravitational waves. Experiments such as the Laser Interferometer Gravitational-Wave Observatory (LIGO) are designed to detect these waves by measuring the length difference between two paths with laser light. The optics in these experiments are suspended in vacuum to isolate them from all forms of vibration. With no air to conduct it away, excess surface charge can build up on the optics over time. The motion of this charge changes the surrounding electric field, vibrating the optic and degrading the sensitivity of the experiment. The group at Trinity University has been measuring the rate at which surface charge builds up and a method of removing the charge that does not harm the delicate reflective coatings on the optics. Funding from this grant supports the completion of a charge removal system for delivery to the LIGO observatories and the study of the effect of different optical cleaning and handling techniques on the mobility of surface charge, and even the individual charge motions themselves. Pursuing this research will help improve the sensitivity of Advanced LIGO and future gravitational-wave detectors, for which surface charge has been a known limiting noise source since 2006. The funding supports undergraduate research at Trinity University, where students can gain valuable experience working locally on a scaled-down vacuum system prototype while contributing to the overall LIGO research effort.<br/><br/>The Laser Interferometer Gravitational-Wave Observatory (LIGO) is most sensitive to gravitational waves at signal frequencies near 100 Hz. A limiting noise source at these frequencies is surface charge on test masses, the motion of which generates fluctuating electric fields that vibrate the masses. The noise contribution depends on the charge magnitude and the correlation time for charge motion. This project tackles three issues related to charging noise. One is to finalize a deliverable discharging system and assist with its installation at the LIGO observatories. Prototype systems using ionized nitrogen gas have been demonstrated at both MIT and Trinity University, but it still must be verified that the process does not damage the test mass reflective coating. This is done by acquiring a test mass of known absorption, exposing it to an ionized nitrogen system, and then having its absorption retested (this process has been used in cooperation with Stanford University for previous discharging techniques). The second is to characterize the relaxation time constant and charging/discharging rate for Advanced LIGO test masses using different cleaning and handling techniques, to determine which methods will reduce charging noise. This requires obtaining several LIGO test mass witness samples, applying different cleaning techniques (methanol, liquinox, First Contact, etc.) to each, moving them into a vacuum system, charging with a high-voltage electrode pattern, and then using a Kelvin probe to measure the rate at which the deposited charge dissipates across the face of the test mass. This methodology will also be applied to proposed optical coatings and substrates for third-generation interferometers in cooperation with the Optics Working Group of the LIGO Scientific Collaboration. The third is to modify an existing atomic force microscope at Trinity University so that it can perform Kelvin probe microscopy. This will make it possible tovisualize individual charge "hopping" across an insulating surface, with the goal of refining the Markov process model of low frequency noise contributions.
