Suspensions R&D Activities

Overview

An all-metal test mass quadruple suspension prototype was developed at the University of Glasgow GEO lab and then sent to MIT for testing. All of the solid body modes were identified, and the model for the suspensions developed at Caltech, Stanford, and Glasgow was refined. Further trade studies on the lengths and masses were made based on the updated model. A challenge in the design is to damp the solid-body modes of the suspension without introducing excess noise in the gravitational-wave band (10 Hz and higher). Several approaches are being followed: using passive eddy-current damping, development of a miniaturized interferometric sensor, and an approach using a split feedback system has been developed in VIRGO.

An analysis of the thermal noise of tapered fused silica fibers at Caltech showed that this is an attractive alternative to ribbons for ease of fabrication and ultimate thermal noise performance. Some first samples have been fabricated for tests. Development of ribbons continued at Glasgow as the baseline design. Refinement of the attachment technique of the fused silica suspension fibers to the masses, using hydroxy-catalysis bonding, to sapphire (for the test mass) and high-density glasses (candidate for the penultimate mass) was made with good success.

We have completed the design and fabrication of the first prototype auxiliary optics suspensions; see Figure 1. This suspension design carries the mode-cleaner optics, and will first be exercised at Caltech to check the solid body modes and damping characteristics, and then transferred to the MIT LASTI facility to look at installation and control issues.

Figure 1 Photograph of the prototype of a triple suspension design for the Mode Cleaner mirrors. The dummy optics is made of aluminum with holes bored to match mass and inertia for the final silica optics. The prototype has coil actuators on all three levels, identifiable as white ceramic cylinders.

A significant step in 2002 was the installation of the complete set of triple-pendulum fused-silica fiber suspensions in the GEO-600 interferometer by the GEO project. The Advanced LIGO suspension design is directly derived from the GEO-600 design, and the test of fabrication, installation, and now ultimately performance of the working design will be invaluable for refining the Advanced LIGO design.