Design and Performance of the Alignment System for the CMS Muon Endcaps

The alignment system for the CMS muon endcap detector employs several hundred sensors such as optical 1-D CCD sensors illuminated by lasers and analog distance- and tilt-sensors to monitor the positions of one sixth of 468 large cathode strip chambers. The chambers mounted on the endcap yoke disks undergo substantial deformation on the order of centimeters when the 4 T field is switched on and off. The muon endcap alignment system is required to monitor chamber positions with 75-200 mum accuracy in the Rphi plane, ~400 mum in the radial direction, and ~1 mm in the z-direction along the beam axis. The complete alignment hardware for one of the two endcaps has been installed at CERN. A major system test was performed when the 4 T solenoid magnet was ramped up to full field for the first time in August 2006. We present the overall system design and first results on disk deformations, which indicate that the measurements agree with expectations.