rasclic: a long-baseline 3-point alignment system for linear accelerators
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RasClic: A long-baseline 3-point alignment system for Linear AcceleratorsH. 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 WestraNikhef/Delft University of Technology, The Netherlands
Rogier van der GeerNikhef/University of Leiden, The Netherlands
Taylan TozgortoUniversity of Amsterdam, The Netherlands
contact: vdgraaf@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
f x sx, y s
yF ω
x, ω
yei ω
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ysy
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.
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 theNew 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.
Laser fiber-optic coupling system, eliminating drift in laser beam position
RasClic as a SeismographThe results of the noise- and sensitivity studies show that RasClic could be used as a low-frequency seismographto 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.
laser +beam expander
diffractionplate
pixel imagesensor
The RasClic vacuum tube in the TT1 tunnel at CERN. Recently the length increased from 91 to 140 m.
Diffraction plate: transparant ring with inner/outer diameter of 80/100 mm
Typical diffraction pattern seen by pixel image sensor (Allied Vision TechnologiesPike F100B,500 x 500 pixels,pitch 7.4 μm.)A position resolution of 20 nm is reached
λ
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
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