J Winterflood, Z Zhou, and D G Blair

Seismic isolation for gravitational wave interferometers is a challenging problem and we are currently designing and building a suspension system which minimises all residual motion of the test mass for our 80 meter advanced research interferometer. We have built a vertical pre-isolation stage which is capable of suspending up to a tonne of mass with a period of approx 10 secs. Incorporated into this structure is inverse pendulum horizontal preisolation giving full 3-D pre-isolation.  This design is rigid to tilt and is thus cascadable. Seismic tilting motion renders cascading of pre-isolators ineffective unless tilt is servo controlled to a high degree.  For this reason we are developing a 2-D tilt sensor incorporating a novel laser walk-off angular readout for high sensitivity. With active tilt control, a second stage of pre-isolation becomes useful, and we intend to use our Scott-russel and torsion crank structures for this. In addition to pre-isolation, we are incorporating a novel passive damping scheme in our all-metal isolation stacks which we have termed self-damping.  It viscously couples the tilting normal modes of the stack with the pendulum modes and damps both of them very effectively. These various designs will be illustrated and most recent measurements presented.