Kyo Shibata

Design and construction of the SuperKEKB vacuum system

A two-ring electron-positron collider with asymmetric energies—called the SuperKEKB—has been designed by the High Energy Accelerator Research Organization (KEK) as an upgrade of the KEKB B-factory (KEKB), which completed 12 years of operation in 2010. It is anticipated that the SuperKEKB will reach a luminosity of 8 × 1035 cm−2 s−1, which is approximately 40 times larger than that of the original KEKB. The upgrade of the vacuum system is a key factor that will allow the SuperKEKB to achieve unprecedented high performance. Most of the beam pipes, especially in the positron ring, are newly manufactured to manage the electron cloud effect, and to reduce beam impedance, which is essential to keep the low-emittance beam stable. Our design of the vacuum system implements recent technologies and draws on various experiences and studies during the operation of the original KEKB. The basic design is near completion, and manufacturing of beam pipes and the major vacuum components, such as bellows chambers, gate valves and supports, are in progress. The installation of these components will start in 2013 with the aim of commissioning the SuperKEKB in 2014.A two-ring electron-positron collider with asymmetric energies—called the SuperKEKB—has been designed by the High Energy Accelerator Research Organization (KEK) as an upgrade of the KEKB B-factory (KEKB), which completed 12 years of operation in 2010. It is anticipated that the SuperKEKB will reach a luminosity of 8 × 1035 cm−2 s−1, which is approximately 40 times larger than that of the original KEKB. The upgrade of the vacuum system is a key factor that will allow the SuperKEKB to achieve unprecedented high performance. Most of the beam pipes, especially in the positron ring, are newly manufactured to manage the electron cloud effect, and to reduce beam impedance, which is essential to keep the low-emittance beam stable. Our design of the vacuum system implements recent technologies and draws on various experiences and studies during the operation of the original KEKB. The basic design is near completion, and manufacturing of beam pipes and the major vacuum components, such as bellows chambers, gate val...

Results and problems in the construction phase of the SuperKEKB vacuum system

The SuperKEKB, the upgrade of the KEKB, is an electron–positron collider with asymmetric energies, that is, 7.0 GeV electrons and 4.0 GeV positrons, designed for a luminosity of 8 × 1035 cm−2 s−1. As a key item of the upgrade project, a new vacuum system for the SuperKEKB has been in construction since 2010. Over 1000 beam pipes, vacuum pumps, bellows chambers, and other various vacuum components had almost been fabricated in 2014. All the new beam pipes were baked as a general rule before being installed in the KEKB tunnel. A thin TiN film with a low secondary-electron yield was coated on the inside of the beam pipes for the positron ring as a countermeasure against the electron cloud effect. The performance of the new built-in nonevaporable getter (NEG) pumps and the step-less connection flanges met the expectation. Although several problems such as damage to the bellows chambers due to a large earthquake, cracking of the welding lines of the aluminum beam pipes, and a relatively high air-leak rate at the connection flanges occurred during the construction work, approximately 99% of the beam pipes were successfully installed by the end of October 2015. The activation of the NEG pumps in the tunnel started at the beginning of 2015 and has also been completed in approximately 97% of the ring. The installed beam pipes and the bellows chambers in the tunnel were aligned to their specified positions as the final step of the installation work. The vacuum system was ready in 2015 with the aim of starting the first commissioning in 2016. The various experiences during the construction phase reported here will be a useful reference for the design and construction of other accelerators in the future.

Development of bellows and gate valves with a comb-type rf shield for high-current accelerators: four-year beam test at KEK B-Factory.

Since a comb-type rf shield was proposed in 2003 as a rf shield for future high-intensity accelerators, various types of bellow chambers and gate valves with this rf shield have been installed in the KEK B-Factory rings in series and tested with beams. Through beam tests to check the performance, a structural simplification has been tried in parallel. The temperatures of the bellow corrugations decreased by a factor of 3-6 compared to those with a conventional finger-type rf shield in most cases. The temperatures of the body of the gate valves also decreased by a factor of 2-5. These results demonstrated the availability of the comb-type rf shield. Although a discharge was observed in one simplified model, the latest model has shown no problem up to a stored beam current of 1.8 A (1.3 mA/bunch, 6 mm bunch length). Experiences with the comb-type rf shield in these four-year beam tests are reviewed here.

Development of winged HOM damper for movable mask in KEKB

In a high luminosity lepton machine such as the KEK B-factory (KEKB), the vacuum components are likely to be annoyed by intense higher order modes (HOM) due to the high beam currents. A winged HOM damper equipped with SiC HOM absorbers was developed to absorb unnecessary HOM, especially TE mode like HOM with a power of several kW. Four dampers were installed in the KEKB ring near movable masks and relieved heating of bellows and pump elements effectively at the beam current up to 1.5 A.

Development of movable mask system to cope with high beam current

The KEK B factory (KEKB), a high current electron-positron collider, has a movable mask (or collimator) system to reduce the background noise in the BELLE detector coming from spent particles. The early movable masks, however, had severe problems of heating, arcing, and vacuum leaks over the stored beam current of several hundred mA. The cause is intense trapped higher order modes (HOMs) excited at the mask head, where the cross section of the beam chamber changed drastically. The mask head, made of copper–tungsten alloy or pure copper, was frequently damaged by hitting of the high energy beam at the same time. Since the problems of the mask were revealed, several kinds of improved masks have been designed employing rf technologies in dealing with the HOM and installed to the ring step by step. Much progress has come from adopting a trapped-mode free structure, where the mask was a bent chamber itself. Recently the further improved mask with a reduced HOM design or HOM dampers was developed to suppress th...

Conceptual design of vacuum system for super KEKB

Conceptual design of the vacuum system for the upgrade of KEKB (Super KEKB) is presented. For its large storage current, the synchrotron radiation (SR) power is much higher than ever before. The beam chamber will have an ante-chamber structure to reduce the irradiated SR power density. To accommodate the intense higher order mode (HOM) arising from the short bunch length, the careful R&Ds are required for bellows, movable masks (collimators), HOM absorbers and so on. The first step of R&D has just begun using the KEKB.

Achievements and problems in the first commissioning of superKEKB vacuum system

The first (phase-1) commissioning of SuperKEKB, an asymmetric-energy electron–positron collider at High Energy Accelerator Research Organization, known as KEK, Japan, began in February 2016 and successfully ended in June 2016. Three major tasks of the phase-1 commissioning were as follows: (1) vacuum scrubbing of the new beam pipes in anticipation of a sufficiently long beam lifetime in the next commissioning, (2) checking the stabilities of various new vacuum components at high beam currents of approximately 1 A, and (3) beam tuning to study stability and achieve low emittance. The coefficient of photon stimulated desorption rate, η (molecules photon−1), decreased steadily with increasing photon dose, as expected. The temperature increases of the bellows chambers, connection flanges, etc., were less than 5 °C at 1 A. The effectiveness of an antechamber, a TiN coating, and a clearing electrode to suppress the electron cloud effect (ECE) was confirmed. However, the ECE in the Al-alloy bellows chambers with...

Development of movable mask with reduced-HOM design for KEKB

At the high energy ring of the KEK B-factory (KEKB), it was found that some bellows near the movable masks were overheated due to the higher order mode (HOM) as increasing the beam current over 900 mA. To cope with this problem, a new mask was designed where the length of ramps beside the mask head was expanded from 30 mm to 400 mm. MAFIA T3 simulations showed that the loss factor for the new long mask was about a half of that for the previous short one. The power of the TE mode, on the other hand, which can easily couple with the bellows through the finger-type RF-shield, was expected to reduce to about 6 percent of that for the short one. During the summer shutdown in 2002, two long masks were installed as a test. In the following run the temperature rise of bellows near the long masks was about 20 percent of those near the short ones and the new design was found to be effective to reduce the HOM. The decrease in the temperature rise was larger than the reduction of the HOM power estimated from the loss factor. This result indicates that the overheating of the bellows is mainly due to the TE mode like HOM rather than the total HOM.

Vacuum system of positron damping ring for SuperKEKB

To satisfy the requirements of high beam quality for positron injection into the SuperKEKB electron-positron collider, a new damping ring (DR) is constructed in an upgraded injector system. The DR has two arc sections and two straight sections. In the arc sections, the beam pipes have beam position monitor blocks, pumping ports, radio-frequency-shielded bellows, and special connection flanges to fit narrow spaces between closely located magnets. The beam pipes have antechambers on both sides of a beam channel to deal with synchrotron radiation (SR) which irradiates both sides of the beam pipes. The antechambers are also effective in reducing the electron cloud in the beam channel. To remove the heat caused by SR irradiation, a water cooling system is also required in the arc sections. In the straight sections on the other hand, an antechamber structure and a water cooling system are not necessary, and the beam pipes are much simpler than those in the arc sections. Required beam lifetime due to residual ga...