Overview of the KEKB accelerators

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Abstract

An overview of the KEKB accelerators is given as an introduction of the following articles in this issue, first by summarizing the basic features of the machines, and then describing the improvements of the performance since the start of the physics experiment.

Introduction

We have witnessed a steady improvement in the performance of KEKB from the start of the physics experiment in June 1999 to the present time. To review this improvement, we list here milestone dates of the peak luminosity: the peak luminosity of KEKB exceeded 1.0×1033cm−2s−1 in April 2000, reached 2.0×1033cm−2s−1 in July 2000, surpassed 3.0×1033 and 4.0×1033cm−2s−1 in March and June 2001, and finally reached 5.47×1033cm−2s−1 in November 2001. Along with the increase in the peak luminosity, integrated luminosity per day and per month were also increased up to 281pb−1/day and 6120pb−1/month in November 2001. This indicates that KEKB has indeed entered into a new realm of collider machines and has shown that it can work as a machine capable of producing real physics. Fig. 1 summarizes the increase in peak and integrated luminosity, stored currents in the rings and the accumulated luminosity by Belle from the start of the physics experiment in June 1999 up to November 2001.

This steady improvement in the performance of KEKB and the achievement of unprecedented luminosity of 5.47×1033cm−2s−1 are the culmination of almost a 10-year effort by KEKB staff members. There still remain many things to be improved in KEKB, to first reach the design luminosity of 1×1034cm−2s−1, and then to eventually exceed it. The papers compiled here summarize our efforts made for KEKB and were written to serve as a basis of further improvements of KEKB. We sincerely hope that these papers are of some help to people working on other colliders and accelerators.

Table 1 summarizes the main design parameters of KEKB and the achieved performances.

Section snippets

Features of KEKB

The construction of KEKB started in 1994, utilizing the existing tunnel for TRISTAN, a 30GeV×30GeV electron–positron collider. After 32 months of dismantling of TRISTAN, the construction of KEKB was completed in November 1998, and commissioning started in December 1998.

Improvements

During the summer shutdown of 2000, we made some modifications to the machine. First, in addition to the original four superconducting cavities, four superconducting cavities were added to the HER so as to increase its current limit up to 900mA. Second, we replaced movable masks of the LER with much more robust ones. During operation before the summer shutdown of 2000, movable masks in the LER suffered from arcings, which caused a few cases of vacuum leakage. The new mask is actually a special

Electron cloud instability

In KEKB, the most serious instability which limits its performance is the electron cloud instability (ECI), which has been observed in the LER. Synchrotron light from the beam hits the inner wall of the vacuum chambers and produces photoelectrons. These photoelectrons are attracted by the positively charged beam to form clouds around the beam orbit. Also, electrons created by multipacting form clouds around the beam. The clouds then excite head–tail-type oscillation within a bunch, and the beam

Future prospects and conclusion

KEKB now has the capability to deliver 5–6fb−1 per month; therefore, it is reasonably expected that by the end of 2002, Belle will have accumulated 100fb−1, and in 4–5 years, the accumulated luminosity will be up to, or higher than, 300fb−1. Recently, discussions have started about upgrading it to a super B-Factory with a goal luminosity of 1035cm−2s−1.

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