Seismic Attenuation System (AEI-SAS) for the AEI 10 m Prototype

Abstract

An interferometric measurement facility with 10 m arm length, called AEI 10 m Prototype, is being set up at the Albert-Einstein-Institute (AEI) in Hannover, Germany. It will use three optical tables to provide an experimental platform for developing and testing novel techniques for gravitational wave detectors. The experiments in the 10 m Prototype will be housed in a large-scale vacuum system to isolate them from environmental influences. The main optical components of interferometers will be suspended to isolate them from seismic motion. These suspensions isolate above their natural frequencies (around 1 Hz). To reduce the motion at their natural modes, the suspensions require further pre-isolation. For that, a six-degree-of-freedom pre-isolator is needed, with natural frequencies far below these suspension modes. The AEI 10 m Prototype facility will be equipped with three of these isolators, one for each optical table. This thesis describes the assembly, performance measurements, and enhancement of a low-frequency Seismic Attenuation System (SAS) for the 10 m Prototype facility. The design of this system is derived from another SAS prototype, the LIGO HAM-SAS. The specific design for the AEI 10 m Prototype is called AEI-SAS. It is an optimal tool to provide the necessary seismic pre-isolation for the optical suspensions of the interferometer. The techniques described throughout this thesis can, however, be adapted to other applications where ground motion in the 1 Hz–10 Hz frequency range must be reduced. The AEI-SAS provides seismic attenuation by means of mechanical low-frequency oscillators. A major design feature of the SAS is the spatial and mechanical decoupling of vertical and horizontal oscillations. The horizontal isolation is performed by a three-leg Inverted Pendulum (IP). The IP supports the vertical isolation component, composed of three Geometric Anti-Spring (GAS) filters, and the optical bench is then mounted on the GAS filters. The mechanical configuration of these isolation stages allows them to be tuned independently to low frequencies in the order of 0.1 Hz. Two of three AEI-SAS were assembled and tested. Six GAS filters were individually assembled and tuned. The filters achieved a maximum isolation of an unprecedented −80 dB, or a vibration reduction by a factor of 10 000. The isolation performance was achieved with two Centre of Percussion compensators (so-called magic wands), for which a stiffer material (silicone carbide) was introduced. Fluorel™ (ultra-high vacuum compatible rubber) pads were introduced to decouple the optical table from the oscillations of various AEI-SAS components. The fully assembled AEI-SAS reduced the motion of the optical table with respect to ground motion at the natural frequencies of the suspensions (around 1 Hz) by a factor of 10 (−20 dB) in the vertical direction and at least factor of 300 (−50 dB) in horizontal directions. At 4 Hz, the maximum isolation of the system was −50 dB in vertical and −80 dB in horizontal degrees of freedom. The AEI-SAS is a mostly passive mechanical system. However, it is augmented with sensors and actuators for active feedback control of the table motion and damping at the natural modes of the system. The first successful tests of damping are shown. A plan for advancements to the feedback and control system is presented and many of these elements will be implemented in the coming year.