RasClic: A long-baseline 3-point alignment system for Linear Accelerators

1. RasClic:A long-baseline 3-point alignment system for Linear Accelerators laser + beam expander pixel image sensor diffraction plate λ 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@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 SystemProceedings 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.