

Seismic noise at a reasonably quiet site on the earth follows a
spectrum in all three dimensions close to
(where here and elsewhere we measure
f
in Hz) and thus if the disturbance to each test mass must be
less than
at, for example, 30 Hz then the reduction of seismic noise
required at that frequency in the horizontal direction is greater
than
. Since there is liable to be some coupling of vertical noise
through to the horizontal axis, along which the gravitational
wave induced strains are to be sensed, a significant level of
isolation has to be provided in the vertical direction also.
Isolation can be provided in a relatively simple way by making
use of the fact that, for a simple pendulum system, the transfer
function to the pendulum mass of the horizontal motion of the
suspension point falls off as
above the pendulum resonance. In a similar way isolation can be
achieved in the vertical direction by suspending a mass on a
spring. In the case of the VIRGO detector system the design
allows operation to below 10 Hz and here a seven stage
horizontal pendulum arrangement is adopted with six of the upper
stages being suspended with cantilever springs to provide
vertical isolation [16]. Similar systems are being developed in Australia [56] and at Caltech [28]. For the GEO 600 detector, where operation down to
50 Hz is sought, a triple pendulum system is used with the
first two stages being hung from cantilever springs to provide
the vertical isolation necessary to achieve the desired
performance. This arrangement is then hung from a plate mounted
on passive `rubber' isolation mounts and on an active
(electro-mechanical) anti-vibration system [75], [99] as shown in Fig.
4
.
Figure 4:
Schematic view of one suspension system for use in the
GEO 600 interferometer.
In order to cut down motions at the pendulum frequencies,
active damping of the pendulum modes has to be incorporated, and
to reduce excess motions at low frequencies around the
micro-seismic peak, low frequency isolators have to be
incorporated. These low frequency isolators can take different
forms - tall inverted pendulums in the horizontal direction and
cantilever springs whose stiffness is reduced by means of
attractive forces between magnets for the vertical direction in
the case of the VIRGO system [63], Scott-Russell mechanical linkages in the horizontal and
torsion bar arrangements in the vertical for an Australian
design [110], and a seismometer/actuator system for GEO 600 [76].


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Gravitational Wave Detection by Interferometry (Ground
and Space)
Sheila Rowan and Jim Hough
http://www.livingreviews.org/lrr-2000-3
© Max-Planck-Gesellschaft. ISSN 1433-8351
Problems/Comments to
livrev@aei-potsdam.mpg.de
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