Metrology Department

Linac Laser Alignment System

SLAC’s two mile accelerator was built with an integral alignment system, using Fresnel rectangular zone plates. It was based on a concept by SLAC’s first Director W. K. H. Panofsky. The system consists of a HeNe laser providing a point source at one end of an evacuated light pipe containing 273 movable targets.

The accelerator is mounted on top of this pipe. An imaging system at the other end is used to estimate the positions of the targets, and by extension the position of the accelerator itself. The system was last used to realign the Linac after the Loma Prieta earthquake in 1989. To ensure the system remains operational the imaging hardware was upgraded and the laser was exchanged and re-aimed.



Linac Laser Image

Imaging System

A new imaging system has been constructed, consisting of a CCD camera mounted on a platform that can be positioned with high accuracy in X and Y.  The diffraction image formed by the Fresnel targets falls directly on the CCD surface without the use of a lens. The system is controlled by a program that allows the operator to remotely move the camera while monitoring the image. Each image captured by the system is processed to determine the pixel location of the center of the diffraction pattern. This location is first transformed into the X-Y coordinate system of the camera platform, then it is transformed into relative coordinates of the target in the light pipe. A reference line is established between targets 2-9 and 21-9.

Fresnel Targets

The movable Fresnel targets are contained within the light pipe and are hinged at the top edge such that they can be moved out of the center of the pipe.  One target at a time is inserted into the dispersed laser beam resulting in a diffraction image formed on the surface of the CCD array at the far end of the light pipe. Further details on the aiming process can be found in the IWAA06 poster, "Revisiting the SLAC Linac Laser Alignment System".



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Last update: November 14, 2008