The seismic attenuation system (SAS) in TAMA300 consists of a three-legged inverted pendulum and mirror isolation filters in order to provide a high level of seismic isolation. However, the mirror isolation filters have torsion modes with long decay time which disturb the interferometer operation for about half an hour if they get excited. In order to damp the torsion modes of the filters, we constructed a digital damping system using reflective photosensors with a large linear range. This system was installed to all of four SASs. By damping of the target torsion modes, the effective quality factors of the torsion modes are reduced to less than 10 or to unmeasurable level. This system is expected to reduce the inoperative period by the torsion mode excitation, and thus will contribute to improve the duty time of the gravitational wave detector.
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