Session H20 - Gravitational Radiation - Experiment.
MIXED session, Sunday morning, April 30
102A, LBCC
The advanced Seismic Attenuation System (SAS) is developed to isolate the LIGO II test masses from all external mechanical disturbances. The SAS performance depresses the external noise levels well below the mirror internal thermal noise, for all frequencies above 6-8 Hz. SAS is conceptually similar to the Virgo superattenuator chains but employs more advanced attenuation units. It is mainly a passive system, with the exception of inertial damping to neutralise internal resonances. The first elements of SAS are, an Ultra Low Frequency Inverted Pendulum and a Geometrical Anti Spring Filter (GASF). These oscillators absorb the microseismic peak perturbations. They are instrumented with an inertial damping system that couples the signal of higly sensitive position and acceleration sensors to actuators in the frequency range between 10 mHz and 4 Hz. This pre-attenuator is followed by a chain of 3 or 4 passive GASF filters, operating above 0.4 Hz, which isolates a multiple pendulum mirror suspension from the seismic noise. We expect to achieve even smaller residual, integrated r.m.s. payload motion than the 50 nm already achieved by Virgo. The measured performance and the passive nature of the GASF units ensure that the required attenuation is reliably achieved, free of external coupling and electronics excess noise and with a large safety margin. The extreme softness of the pre-attenuator allows for precision positioning with negligible external forces. Multiple independent SAS chains may be mounted in each LIGO vacuum chamber to handle multiple optical elements.