Abstract

SuperKEKB is an electron-positron collider with asymmetric energies located at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan. After more than five years of upgrading work on KEKB, phase-1 commissioning commenced in February 2016 and ended in June of the same year. Following a 20-month shutdown for the installation of a particle detector, BELLE II, phase-2 commissioning commenced in March 2018 and ended in July of the same year. This paper describes one major issue faced by SuperKEKB: the electron cloud effect (ECE) in the positron ring, which was observed during phase-1 commissioning. In the high-beam-current region, electron clouds, i.e., the source of the ECE, existed in the beam pipes at drift spaces of the ring, which had antechambers and a titanium nitride (TiN) film coating as countermeasures against the ECE. Permanent magnets and solenoids used to generate magnetic fields in the beam direction were attached to the beam pipes as additional countermeasures before the next commissioning phase commenced. Consequently, during phase-2 commissioning, experiments showed that the threshold of the current linear density for exciting the ECE increased by a factor of at least 2 compared to that during phase-1 commissioning. While the countermeasures were strengthened, the effectiveness of the antechambers and TiN film coating of the real beam pipe was reevaluated. Through various simulations and dedicated experiments during phase-2 commissioning, the antechambers were found to be less effective than anticipated with regard to reducing the number of photoelectrons in the beam channel. In contrast, the TiN film coating had a low secondary electron yield, as expected.

Highlights

  • SuperKEKB is an electron-positron collider with asymmetric energies located at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, and is a successor to the former KEKB (KEKB B-factory) [1,2,3]

  • The electron cloud effect (ECE) was excited in the SuperKEKB LER during phase-1 commissioning

  • The more serious ECE observed in the high-current region was caused by the electron cloud in the beam pipes at drift spaces, which already had the titanium nitride (TiN) film coating and antechambers

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Summary

INTRODUCTION

SuperKEKB is an electron-positron collider with asymmetric energies located at the High Energy Accelerator Research Organization (KEK) in Tsukuba, Japan, and is a successor to the former KEKB (KEKB B-factory) [1,2,3]. The electron cloud effect (ECE) has been a serious problem in recent high-intensity proton and positron rings, including the SuperKEKB LER [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]. While the countermeasures were strengthened, the effectiveness of the antechambers and TiN film coating of the real beam pipes in the ring was reevaluated through various simulations and dedicated experiments during phase-2 commissioning. The results from the reevaluation of the effectiveness of the antechambers and TiN film coating for suppressing the ECE in the real beam pipe are reported

COUNTERMEASURES ADOPTED FOR SUPERKEKB
ECE at high current
ADDITIONAL COUNTERMEASURES
ECE IN PHASE-2 COMMISSIONING
REEVALUATION OF THE EFFECTS OF ANTECHAMBERS AND TIN COATING
Calculation of β
From the photon distribution
From the measured ne
From the beha vior of ne with respect to I
From the dependence of ne on the train length
Results of reevaluation
Findings
SUMMARY
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