Field Quality Improvement of Septum Magnets for SuperKEKB Injection System

Abstract

The SuperKEKB accelerator is a electron-positron collider consisting of the 7 GeV electron ring (high energy ring or HER) and the 4 GeV positron ring (low energy ring or LER). The commissioning of the SuperKEKB accelerator is underway, aiming to supply a great number of interaction events of electrons and positrons to the Belle II detector which is used for discovering the new physics beyond the standard model. The important milestone is to obtain integrated luminosity of 15 ab–1 in the next decade, so that the luminosity should exceed 2 × 1035 cm–2s–1 in several years. To achieve the goal, both rings have to be filled with high current beam of a few amperes, where the high injection efficiency is vitally important because lifetime is expected to be very short. One of the key components of the injection system is the septum magnet. It has been found that a transverse fringe field near the septum plate has sizable multipole components. A tracking simulation shows such fringe fields generate a vertical non-Gaussian tail, which could cause a beam background as well as a bad injection efficiency. Though quadrupole component in the multipoles could be, in principle, cancelled by adjusting the quadrupole magnets in the upstream of the beam line, it turned out to be difficult in actual operation. Even the quadrupole component was perfectly cancelled, components higher than quadrupole would be still harmful, since it generates a beam halo in the ring after injection, which increases the beam backgrounds to the Belle II detector. This paper describes about improvement of the field quality of the septum magnet.