Compact isolation of a large mirror at low frequency

This study presents a compact isolation system of a large mirror at low frequency. The work is carried out in the framework of the E-TEST project to investigate a new concept of design for the future Einstein Telescope (Europe's next-generation gravitational-wave detector). The isolation system combines an active inertial platform, a passive inverted pendulum platform, and a multi-cascaded pendulum in one system. Given that, the study addresses the isolation system design, its dynamics, and the applied control strategy. For the control strategy, a decoupling method based on a geometric approach is first applied to reduce the magnitude of the off-diagonal elements of the transfer matrix. Then the controller is applied in the Cartesian frame based on the loop shaping method. The simulation results show that the seismic noise could be reduced by about 3 orders of magnitude at 1 Hz in the horizontal and vertical directions when the control is applied. It is also shown that the control is still performing well despite of the residual coupling of the inertial sensor.