GEO 600 triple pendulum suspension system: Seismic isolation and control

In this article we describe aspects of the suspension system for each of the main optics (test masses) in the GEO 600 interferometric gravitational wave detector currently under construction in Germany. In particular we describe the triple pendulum, which is a key aspect of the overall system, discuss the details of the mechanical design, present transfer functions of the isolation performance, and display examples of impulse responses for the local control used to damp the low frequency modes of the triple pendulum. This local control is achieved using co-located sensing and feedback at the highest mass of the triple pendulum, providing a significant attenuation of local control noise to the fused silica test mass, the lowest mass of the triple pendulum. In order to enhance the vertical isolation, the triple pendulum will incorporate two stages of cantilever springs, and fused silica fibers will be used in the lowest pendulum stage in order to minimize thermal noise from the pendulum modes. It is expected that the thermal noise associated with the internal modes of the fused silica test mass (mass ∼6 kg) will set the sensitivity limit for GEO 600 from 50 to ∼200 Hz. The measured performance from individual stages of the prototype suspension system indicates that a seismic noise level which is a factor ∼3 lower than this thermal noise level at 50 Hz can be achieved.