Session H20 - Gravitational Radiation - Experiment.
MIXED session, Sunday morning, April 30
102A, LBCC
We have developed an advanced seismic attenuation system for the future LIGO II detector. Our design consists of an Inverted Pendulum (IP) holding stages of Geometrical Anti Springs Filters (GASF), which isolate the test mass suspension from ground noise. The ultra-low frequency IP suppresses the horizontal microseismic peak. The three legs of the IP flex at Maraging steel joints, which have structural damping. Tunable counterweights allow for precise center of percussion tuning to optimize good attenuation up to the first leg internal resonance (\sim60Hz). The IP can be tuned to very low frequencies, by carefully adjusting its load. We achieved \sim12mHz pendulum frequency for one of the radial pendulum modes. The quality factor (Q) of the IP is compatible with structural damping. Q is proportional to the square of the pendulum frequency. Q was measured from \sim2500 (at 0.6Hz) to \sim2 (at 12mHz). All measured functions are in very good agreement with our models. We therefore expect excellent attenuation in the low frequency region, from \sim0.1Hz to \sim50Hz, which is necessary to obtain small off band residual motions of the payload. The extremely soft IP requires minimal control force, which simplifies any needed actuation (e.g. at \sim10mHz a load of 500Kg requires control forces of \sim2mN for a 1mm excursion).