H. Manaud Durand, J-P. Quesnel, T. Touz

1
H. Manaud Durand, J-P. Quesnel, T. Touzé CERN
TS-SU, Geneva, Switzerland Harry van der Graaf,
Henk Groenstege, Elmar Reinhold, Geert Hanraads
Nikhef, Amsterdam, The Netherlands Mark Beker,
Marc Kea, Hidde Westra Nikhef/Delft University of
Technology, The Netherlands Rogier van der
Geer Nikhef/University of Leiden, The
Netherlands Taylan Tozgorto University of
Amsterdam, The Netherlands contact
vdgraaf_at_nikhef.nl
RasClic is a new optical alignment system for
large objects like linear accelerators, based on
the RASNIK alignment system. The system is a
3-point straightness monitor consisting of a
monochromatic light source, a diffraction plate
and a pixel image sensor. By monitoring the
position of a diffraction pattern on the image
sensor, a measure for the relative positions of
the three components is obtained. It has been
shown that such a system can meet the required
alignment tolerances for use as an alignment
system for the Compact Linear Collider (CLIC).
1 H. Dekker, H. van der Graaf, H. Groenstege,
F. Linde, S. Sman, R. Steensma, B. Jongkind, A.
Smeulders The RASNIK/CCD 3-Dimensional Alignment
System Proceedings of the 3rd International
Workshop on Accelerator Alignment, (IWAA 1993),
28 Sept - 1 Oct 1993, Annecy, France
The RasClic vacuum tube in the TT1 tunnel at
CERN. Recently the length increased from 91 to
140 m.
Laser fiber-optic coupling system, eliminating
drift in laser beam position
Typical diffraction pattern seen by pixel image
sensor (Allied Vision Technologies Pike
F100B, 500 x 500 pixels, pitch 7.4 µm.) A
position resolution of 20 nm is reached
Diffraction plate transparant ring with
inner/outer diameter of 80/100 mm
The main practical limitation for increased
sensitivity at low frequency by extending RasClic
to an ideal 20 km is the need for a long vacuum
tube. The relatively cheap laser, diffraction
plate and pixel sensor can work in principle at
any distance, provided they are well coupled to
the earth's crust individually. An interesting
application of RasClic is as an instrument to
monitor the slow deformation of the earth's crust
across a fault line.
Noise spectrum
Linearity tests were performed by varying a
force on the diffraction plate holder of up to 50
N in the vertical direction. These tests show an
excellent position resolution and linearity.
RasClic as a Seismograph The results of the
noise- and sensitivity studies show that RasClic
could be used as a low-frequency seismograph to
monitor the 'earth hum'. There is evidence that
the 1/f noise is to a large part due to
temperature fluctuations a system of T sensors
is set up to eliminate this noise contribution.
Further work needs to be done on the resolution.
Noise behavior of an ideal RasClic compared to
the STS-1 and STS-2 seismographs used by the
Royal Dutch Institute for Meteorology (KNMI)
13. The NHNM and NLNM envelopes are the New
High- and New Low Noise Models, respectively.
These are models for the background noise in
seismic measurements due to actual movements in
the Earth's crust, during periods of high and low
seismic activity, respectively.