Test mass suspensions

In this chapter we describe the designs used for the suspension systems for Advanced LIGO and Advanced Virgo, which support the test masses that serve as the mirrors for the interferometers. These systems are attached to the last stage of the seismic isolation systems for the detectors. Their designs are chosen to minimize thermal noise, residual seismic noise and technical noise, while allowing control of position and orientation of the test masses, and enabling the interferometers to function. Advanced LIGO and Advanced Virgo differ in their approach to seismic isolation and control. Thus, although the final stages of the suspension systems have clear similarities, there are significant differences in the overall suspension systems. Hence we discuss the designs of the two systems separately. In a concluding section we compare the two systems and note that although the two projects have taken different approaches in certain areas, there are key concepts that show similarities and that both designs should meet their requirements.