Doctor of Philosophy (PhD)


Physics and Astronomy

Document Type



LIGO hosts the world’s most sensitive GW detectors capable of detecting strain of h=10-22 at 100-200Hz. To maximize the amount of useful scientific results LIGO generates, it is necessary to operate the instrument uninterruptedly. However, for ground-based detectors, this is not possible without isolating it from the environmental disturbances, which in most cases is predominated by seismic noise. We examined the recent seismic noise records at LLO and LHO, and found the seismic noise at both sites had decreased from Oct. 1, 2003 to Oct. 1, 2006. No long term trend can be derived from the there-year data studied, however it is clear that the LLO seismic environment has been consistently noisier than that of LHO, from 0.1 to 3Hz, by a factor of 2.4 to 9.0. To cope with noisier ground conditions at LLO, an additional stage of seismic isolation system external to the LLO vacuum tanks was developed. The system is called the Hydraulic External Pre-Isolator (HEPI), which uses position and geophone sensors, ground seismometers, and hydraulic actuators to suppress payload motion. HEPI prototype was designed and tested in 2003 and installed on site in 2004. The first stage of HEPI commissioning was finished by the LLO’s fifth science run (S5). The design, control strategy, performance, as well as the problems of the HEPI system at LLO are described in Chapter 3. Based on the isolation HEPI provided, we were able to achieve an operational duty factor of 66.7% and maximum detection range of 15.8Mpc for 1.4 binary neutron star inspiral at LLO in S5. While the current HEPI configuration is stable, it is by no means finalized. Several possible improvements to HEPI are introduced in Chapter 5. These enhancements require minimal to moderate addition and iteration to existing hardware and software, and should improve the isolation performance and stability of the HEPI system.



Document Availability at the Time of Submission

Release the entire work immediately for access worldwide.

Committee Chair

Joseph A. Giaime