Beam optics distortion caused by orbit deviation at strong sextupole magnets in SuperKEKB

Abstract

The SuperKEKB is an electron-positron double-ring asymmetric-energy collider at the High Energy Accelerator Research Organization (KEK) in Japan. It uses a novel collision method referred to as the nano-beam scheme to avoid the so-called hourglass effect. The highest peak luminosity achieved thus far is 4.65 × 1034 cm-2s-1, which was reached on June 8th, 2022. Although the SuperKEKB holds the record for the world's highest instantaneous luminosity, several unresolved issues prevent a further increase in luminosity. One of the major issues is inadequate stability and reproducibility of the beam orbit and optics. This paper discusses optics distortion caused by orbit deviation near strong sextupole magnets. Numerical estimations indicate that a horizontal orbit deviation of a few tens of micrometers induces a sizable distortion of the optical functions at the interaction point. Analyses of the amount of betatron tune feedback and orbit at sextupole magnets used for crab waist and vertical local chromaticity correction indicate that the beam-current dependence of the vertical tune shift is attributable to the change of the beam orbit at the sextupoles. The orbit fluctuation at the sextupoles causes beta-beating and makes stable operation at high beam currents more difficult. The results show that orbit tuning at the sextupoles improves both the injection efficiency and the detector background level. Optics deterioration within a few days is another urgent issue in high-beam-current operation. A vertical orbit change of a few tens of microns at sextupole magnets installed in arc cells is a matter of concern according to numerical estimations.