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

TL;DR

This paper introduces a model-independent method using a specialized BPM pair at NSLS-II to accurately characterize the lattice and measure local Twiss parameters, even in complex coupling scenarios.

Contribution

A novel BPM pair with bunch-by-bunch resolution was developed for model-independent lattice characterization at NSLS-II.

Findings

Accurately measured linear lattice variations along a long bunch-train.

Estimated quadrupole tilt errors below 400 μrad.

Demonstrated the method's effectiveness in both weakly and strongly coupled lattices.

Abstract

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 Poincar\'e 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.