A two-dimensional low-frequency vibration attenuator using X-pendulums.pdf

A two-dimensional low-frequency vibration attenuator using X-pendulums D. Tatsumi  , Mark A. Barton y , T. Uchiyama and K. Kuroda Institute for Cosmic Ray Research, University of Tokyo, Tanashi, Tokyo 188, Japan Abstract We have designed and constructed an improved two dimensional X-pendulum vibration- isolation table. It achieved a lower resonant frequency (7 s) than previous prototypes, and the eects of many parasitic elastic resonances have been reduced by careful balancing, leading to much improved vibration isolation around a few Hertz. 1 Introduction To detect gravitational waves using an interferometric detector, mirrors should be isolated from seismic motion. The seismic vibration is a particular problem at low frequency, because the mirror is suspended with pendulum with a resonant frequency around 1 Hz. The Japanese in- terferometric gravitational wave detector, TAMA300, will have a vibrational attenuation system consisting of three stages: a stack, the two-dimensional X-pendulum attenuator presented here, and a double pendulum suspension system. The stack and the double pendulum suspension are only eective at relatively high frequencies, so the X-pendulum system will be used to improve the isolation at low frequency (less than 10 Hz). Since the stack and the double pendulum suspension systems have several resonances around a few Hz, we need to attenuate the amplitude of horizontal vibration by an order of magnitude (20 dB) to let the interferometer achieve its maximum sensitivity. The simplest way to do this would be to construct a very long period simple pendulum. However such a pendulum needs a tall supporting frame and this needs to be placed in a vacuum chamber (the TAMA chambers are 1 m in diameter and not quite 2 m in height). Our X-pendulum system is designed to have a suitably long period but is much more compact vertically and easily meets the space requirements [1, 2]. The basic X-pendulum is seen in Fig. 1. When a center of mass of the load