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Beam Energy Measurement at the ILC
For the physics program at the ILC, an excellent bunch-to-bunch control of the beam energy is mandatory. Several techniques will be implemented at the ILC in order to achieve this request. More in details, an energy spectrometer upstream as well as downstream of the electron/positron interaction point has been proposed. The present default option for the upstream spectrometer is a BPM-based spectrometer. In the year 2006/2007, a prototype test setup (experiment T474/491) was implemented in the End Station A beam line at the Stanford linear accelerator center in order to study its performance. DESY Zeuthen was a member of the collaboration this experiment, and our tasks concerned monitoring of the B-field integral of the magnets (ILC-SLACESA TN-2008-1) and the evaluation of the resolution of the spectrometer (A Lyapin et al 2011 JINST 6 P02002). In addition, we studied the feasibility of a novel method based on laser Compton backscattering. In fact the experience at LEP suggests to implement complementary methods in order to cross-check the results of the BPM-base spectrometer. The results of this study were published in NIM A (Nucl. Instrum. Meth., A607:340-366, 2009), respectively, arXiv:0812.0925v2 [physics.ins-det].