Full scale prototype of high Q pendulum for interferometric gravitational wave detectors

A series of measurements were undertaken to measure the quality factor of a full scale prototype of the last stage suspension system (a 21 kg pendulum) of the VIRGO gravitational wave interferometer, by using a structure with negligible recoil losses. The suspension wires consisted of two loops of C85 harmonic steel with a diameter of 200 µm that formed a pendulum with a length of 0.70 m. The measured pendulum Q varied from ~1×104 for the wire simply cradling the mass to 8×105 for the wire being offset from the mass with a prism shaped spacer, while the theoretical value for the pendulum Q was calculated to be 6.7×106. The best measured Q was 1.1×106 for a 20 kg glass mass with metal clamps rigidly attached to it. The main factor that limits the pendulum Q is the dissipation in the wire–test mass attachment. More advanced designs using new materials are needed in order to increase the pendulum Q significantly