Speaker
Description
This presentation describes the impact on the sensitivity of the ET low-frequency detector that the vertical thermal noise of the room-temperature seismic isolation system produces. It is shown that longer arms and more tilted tunnels produce larger contributions along the interferometer arm, which in any configuration is larger than the ET desired sensitivity. A method that has been proposed to mitigate this problem is the use of silicon blade springs in the mirror suspension and active anti-springs acting on them to reduce their resonant frequencies to useful values below 1 Hz, and thus filter the excess of noise. The active anti-spring would comprise an optical displacement sensor, an electrostatic actuator and a feedback filter. It is calculated that given the extremely large quality factor of silicon at low temperatures, the thermal noise of these blade springs remains below the desired level. A major difficulty of this strategy is the very high sensitivity required for the displacement sensor, to the extent that devising other strategies are most likely needed.