SuperKEKBにおける衝突点光学補正用回転六極電磁石の開発とその性能

抄録

SuperKEKB is an asymmetric-energy electron positron collider based on the “nano-beam scheme,” where two low emittance beams collide with a large crossing angle at the interaction point (IP). Chromatic x-y coupling at the IP needs to be corrected to realize such low emittance beams and recover luminosity. The chromatic x-y coupling correction can be done using skew sextupole magnets, as was done at KEKB B-factory in 2009. SuperKEKB adopted correction by skew sextupole field generated by changing the roll angles of the sextupole magnets, instead of adding skew sextupole magnets and power supplies. In December 2021, one of the chromatic x-y coupling parameters was varied using different setups of the rolling angles of the 24 sextupole magnets for the first time in a collider. It was found that the vertical emittance growth at the primary and secondary synchro-beta resonance lines can be controlled by rolling the sextupole magnets. Luminosity increase followed after optimizing the chromatic x-y coupling parameter and finding a better tune working point. The tilting system is described, and the study results are summarized in the report.