Optics measurement and correction for future electron circular colliders

Abstract

The development of ultra-low emittance storage rings, such as the e+/e- Future Circular Collider (FCC-ee) with a circumference of about 90 km, aims to achieve unprecedented luminosity and beam size. One significant challenge is correcting the optics, which becomes increasingly difficult as we target lower emittances. The use of stronger quadrupoles and sextupoles makes these machines particularly sensitive to misalignments, which can severely impact performance. This study investigates optics correction methods to address these challenges. We examined the impact of arc and Interaction Region (IR) magnet alignment errors in two optics design options for the FCC-ee, called Baseline and Local Chromatic Correction Optics (LCCO). To establish realistic alignment tolerances, we developed a sequence of correction steps using the Python Accelerator Toolbox (PyAT) to correct the lattice optics, achieve nominal emittance, and large Dynamic Aperture (DA). We focused initially on the Linear Optics from Closed Orbit (LOCO) method, which fits the measured Orbit Response Matrix (ORM) to the lattice model to determine optimal parameters such as quadrupole strengths. We implemented a Python-based numerical code for LOCO correction and evaluated its effectiveness for the FCC-ee. Preliminary results indicate successful optics corrections. We also compared LOCO with phase advance + ηx and coupling Resonance Driving Terms (RDTs) + ηy optics correction, finding that the latter performed better in achieving design emittance values and a large DA area for realistic alignment tolerances, for the studied cases. The code was further optimized and expanded to include more realistic scenarios. Additionally, we applied LOCO to PETRA IV -a fourth generation light source upgrade, and integrated the code into the Python Simulated Commissioning toolkit for Synchrotrons (PySC).