Chairperson : Fulvio Ricci


The suspensions of the optical elements of the interferometer must provide the necessary attenuation from seismic and acoustic noise and must implement the control strategy necessary to keep the interferometer at its working point. The last stage of the suspension plays also another role: all the mechanical elements which are connected to the mirror must be designed not to degrade the intrinsic mechanical losses of the mirror itself. This is necessary because of the well known relation between mechanical dissipations and thermal motion in macroscopic systems. Until now this thermal noise has been reduced by developing sophisticated suspension systems with materials with low mechanical dissipation and low friction mechanical clamps. However, at the goal sensitivity of third generation detectors, the only way to have a further reduction of thermal noise will be the use of cryogenics. This choice is quite natural considering also the large amount of power which will be stored in the F-P cavities: it could be found indeed that cryogenics will be the only method available to further reduce the thermal lensing effects at the incident power of third generation interferometers.

The first step of the conceptual design will be a study which will define material, size, optical properties, input power, desired attenuation and desired temperature for the mirror. For instance, the mass of the mirror itself must be decided according to the necessity to limit both thermal noise and the motion noise due to the recoil of the mirror from radiation pressure.


The second step will be the design of the upper suspension chain. A choice will be necessary between an active and a passive systems, which will be guided also by the definition of the best control strategy and by the required sensing and actuation elements, including reading and driving electronics.

To define the lower part of the suspension, a choice between different cryogenic systems will be made (PT refrigerators, cryogenic fluid bath, Claudet bath, mixed tecniques). In all cases, there will be no need to cool down also the upper part of the suspension, while some care will be needed in the design of the interface between the upper and the lower part. It will be necessary also to have active and passive mechanical filters connecting the refrigeration system to the mirror. The design of these filters will be a trade-off between the necessity to avoid a decrease in sensitivity due to the injected noise and the need to preserve enough refrigeration power to reach the desired temperature on the mirror.


A very important item of this WP is the development of new sensing and control systems of the optics, taking advantage of the superconducting ultra low noise technologies which are available at low temperatures.

The final result of the study will be a conceptual design of the suspension including many highlights on the control strategies to be implemented and a consistent model of the overall suspension, with well identified and realistic parameters and a comprehensive noise budget. This result will be a powerful tool for the design of the actual parts during a preparatory phase.

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