Dedicated beam position monitor pair for model-independent lattice characterization at NSLS-II

• Published in: arXiv.org 2024-06
• Main Authors: Li, Yongjun, Ha, Kiman, Padrazo, Danny, Kosciuk, Bernard, Bacha, Belkacem, Seegitz, Michael, Rainer, Robert, Mead, Joseph, Yang, Xi, Tian, Yuke, Todd, Robert, Smaluk, Victor, Cheng, Weixing
• Format: Article
• Language: English
• Subjects: Light diffraction; Light sources; Parameters; Poincare maps; Quadrupoles
• ISSN: 2331-8422

This paper reports recent lattice characterization results obtained at the National Synchrotron Light Source II (NSLS-II) storage ring, conducted without reliance on a lattice model. A pair of beam position monitors (BPMs) with bunch-by-bunch (B\(\times\)B) resolution, were recently installed in a section of the storage ring free of magnetic fields. The new BPM pair measured the beam, or bunch's transverse Poincaré map precisely after the beam was excited. Linear one-turn-matrices (OTM) were then derived, and from these, the 4-dimensional coupled Twiss parameters were extracted at the locations of the BPM pair. By normalizing beam oscillation amplitudes with the Twiss parameters, the global action-variables were obtained. These action-variables facilitated the measurement of the local Twiss parameters observed by other BPMs independent on lattice model. This method is general, and particularly useful in certain scenarios such as a round beam mode in a diffraction-limited light source ring. We applied it to assess both weakly and strongly coupled lattices at the NSLS-II ring. Through analysis of the strongly coupled lattice, the quadrupole tilt errors were estimated to be less than 400 \si{\mu}rad. Utilizing the BPMs' B\(\times\)B resolution, for the first time we observed the variations of the linear lattice along a long bunch-train.