The goal of LIGO is to detect and analyze the gravitational waves produced by astrophysical events. LIGO consists of 4 km scale interferometric detectors located in Washington (LIGO Hanford Observatory, LHO) and Louisiana (LIGO Livingston Observatory, LLO). Gravitational waves appear as differential length changes in the perpendicular arms. To reach the required level of sensitivity (to motions of 10"18 m or less), the interferometer test masses must be isolated from several noise sources, including seismic motion, which is dominant at low frequencies. Initial LIGO has searched for signals, but none have yet been identified. A significant improvement in sensitivity is underway with the pending installation of Advanced LIGO (or aLIGO). Once running at full sensitivity, sources of gravitational are predicted to be regularly detected. A key element in this upgrade is the installation of extremely effective broadband seismic isolation and positioning systems. It will include, for every optic, three levels of isolation in series: an external (in air) active isolation and alignment stage with quiet-hydraulic actuators called HEPI, an internal (in vacuum) passive/active isolator called ISI (Internal Seismic Isolator) presented in this paper, and predominantly passive multiple pendulums to hold the optics. A multi-stage Internal Seismic Isolation platform will be used to support the beam splitter and each of the test mass optics. The platform should provide an isolation factor of 10 at 0.1 Hz and up to 2000 at 10 Hz. This active/passive system, called BSC-ISI, under development for several years, is made of two stages in series, each controlled in six degrees of freedom (DOF). This paper presents the prototyping, testing and current performance of the BSC-ISI system supporting a 920 kg total payload (370 kg of optical payload with the remaining 550 kg configured as ballast). In the next section, the BSC-ISI architecture and the prototype are presented. The active control strategy is presented in section 3. Initial performance results and related structural investigation are presented in section 4, followed by improved performance in section 5.
展开▼
