A system based on three cascaded stages, each capable of providing about two orders of magnitude of isolation in six degrees of freedom, is being developed. Each stage utilizes active and passive isolation to achieve the performance goals. The preliminary stage is operational, and the two main stages are under construction. Early measurements have been made on the first main stage. The preliminary, single stage, vibration isolation system provides more isolation at 1 Hz than any other research or commercial isolation system. When complete, the three-stage system will provide the quietest terrestrial environment ever achieved. The continuum thermal noise in suspension pendulums, like those planned for gravitational wave interferometers, will be measured by hanging a matched pair of them from the ultra quiet second main stage and determining their relative motion. This advanced active vibration isolation system is being developed to shield sensitive measurement instruments from the effects of seismic and man-made vibration noise at frequencies down to 1 Hz. The objective of this work is to develop techniques which can help reduce the effects of the direct mechanical coupling of ground motions and other vibration on the test masses of gravitational wave interferometers down to the level of the gravitational coupling of disturbances to the test masses over as wide a frequency range as possible. During the third year of work, a conceptual design will be developed for an active vibration isolation system which could be used in advanced low-frequency LIGO detectors. This will include a numerical model of the expected performance. Improved performance at frequencies down to 1 Hz would permit searches for signals which terminate at frequencies of 10 Hz or less.
