Another finding from the operation

Another finding from the operation of TAMA-SAS was that one has to make sure that all the low-frequency resonances are appropriately damped by passive/active damping mechanisms. In case of TAMA, the operation of the interferometer was often disturbed by yaw rotation of the suspended optics, which arises through excitation of the torsion modes of the suspension wires. Since the GAS filters and the lower stages are connected through single-wire suspensions, they form a multi-stage torsion pendulum with very low resonant frequencies (in the order of ∼ 10 mHz). The torsion modes have long decay times (in the order of 10 3 -10 4 sec) unless they are appropriately damped. Consequentyly, TAMA-SAS retrofitted motion sensors to detect the torsion modes and actively damp them [67].