Implementation of a Software Feedback Control for the CMS Monitoring Lasers
Light monitoring will play a crucial role in maintaining the energy resolution of the CMS lead tungstate crystal electromagnetic calorimeter (ECAL) in situ at LHC. Since 2001, a laser based monitoring system has been used in the CMS ECAL beam tests at CERN. While the stability of the laser pulse energy and width, measured in 24 hours, is at a level of 3%, a long term degradation of both the laser pulse energy and the width were observed, as well as a drift of the laser pulse center timing at 2 ns/day. This degradation and drift, caused by the natural aging of the DC Kr lamp, would affect respectively the monitoring precision and the synchronization between the laser pulse and the ECAL ADC clock. This paper presents a design and implementation of a software feedback control which stabilizes the laser pulse energy, width and timing. With the software feedback implemented, a stability of the laser pulse energy and width at 3% level and a pulse timing jitter at 2 ns were observed in laser runs lasted for more than 3 months. The 0.5% energy resolution of the CMS PbWO4 crystal ECAL is maintained after applying the laser monitoring corrections in a beam irradiation test.
Additional Information© Copyright 2008 IEEE. Reprinted with permission. Manuscript received December 7, 2006; revised November 16, 2007. [Posted online: 2008-02-08] This work was supported in part by the U.S. Department of Energy under Grant DE-FG03-92-ER-40701 and in part by the U.S. National Science Foundation Awards PHY-0612805 and PHY-0516857. Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. The authors would like to thank the CMS ECAL group, especially members of Caltech and Saclay groups, for fruitful collaborations and many useful discussions. Special issue: 15th IEEE-NPSS Real Time Conference (RT2007) on Computing Applications in Nuclear and Plasma Sciences, Fermilab, Batavia, IL, April 29–May 4, 2007. IEEE Transactions on Nuclear Science 55(1) Parts 1-3, February 2008
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