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Mitigation of beam backgrounds via collimators is critical for the success of the Belle II experiment at the SuperKEKB electron-positron collider. We report on an improved simulation methodology, which includes a refined physical description of the collimators and beam pipe, our first implementation of collimator tip scattering, and in which the existing beam particle tracking software has been embedded into a new sequential tracking framework. These improvements resolve longstanding discrepancies between measured and predicted Belle II background levels, and significantly reduce the computing time required to optimize the collimation system in simulation. Finally, we report on collimator aperture scans, which confirm the accuracy of the simulation and suggest a new method for aligning the collimators.

Movable collimators for the SuperKEKB main ring, which is a two-ring collider consisting of a 4 GeV positron and 7 GeV electron storage ring, were designed to fit an antechamber scheme in the beam pipes, to suppress background noise in a particle detector complex named Belle II, and to avoid quenches, derived from stray particles, in the superconducting final focusing magnets. We developed horizontal and vertical collimators having a pair of horizontally or vertically opposed movable jaws with radiofrequency shields. The collimators have a large structure that protrudes into the circulating beam and a large impedance. Therefore, we estimated the impedance using an electromagnetic field simulator, and the impedance of the newly developed collimators is lower compared with that of our conventional ones because a part of each movable jaw is placed inside the antechamber structure. Ten horizontal collimators and three vertical collimators were installed in total, and we installed them in the rings taking the phase advance to the final focusing magnets into consideration. The system generally functioned, as expected, at a stored beam current of up to approximately 1 A. In high-current operations at 500 mA or above, the jaws were occasionally damaged by hitting abnormal beams, and we estimated the temperature rise for the collimator’s materials using a Monte Carlo simulation code. In order to avoid damage to the jaw, we have started developing a collimator with low atomic number materials.

The operation of the SuperKEKB has been ongoing since 2016. The vacuum systems of the main ring (MR) which consists of a 7-GeV electron ring (HER) and a 4-GeV positron ring (LER), the damping ring (DR) for 1.1 GeV positrons in the middle of the injector linac have been working well as a whole. As of June 2022, the maximum stored beam currents of MR are 1.46 and 1.14 A for the LER and the HER, respectively, and approximately 30 mA for the DR. The pressure increase per unit beam current is steadily decreasing and the new vacuum components developed for the SuperKEKB have been working as expected. No significant electron cloud effect has been observed in the LER after installing solenoids in drift spaces in 2017 which apply magnetic fields in the beam direction. The recent pressure behavior of the LER with increased beam current is explained by considering thermal gas desorption induced by the beam as well as photon-stimulated gas desorption. Currently, the beam lifetime is primarily limited by the Touschek effect rather than the vacuum pressure, and their degrees of contribution are evaluated. The challenges associated with high beam currents, such as damage to beam-collimator heads and excess heating of beam pipes at wiggler sections have become more apparent as beam currents are increased. The status of the SuperKEKB vacuum system and the experiences during the past 6 years of operation are presented here.

In the low energy ring (LER) for positrons in the SuperKEKB, a vertical beam size blow-up was observed when the bunch current was approximately 1 mA. If a beam size blow-up occurs, the design luminosity cannot be achieved. Therefore, beam size blow-ups must be pre-vented. According to calculations, the bunch current threshold of the transverse mode coupling instability (TMCI) is 2 mA or more, and the observed value is 50% or smaller. Ordinary TMCI cannot explain this vertical beam size blow-up. This paper shows that the cause of the vertical beam size blow-up can be determined by analyzing factors such as beam oscillation. The study results showed that the vertical beam size blow-up in the LER was caused by a -1 mode instability.

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.

The first (Phase-1) commissioning of SuperKEKB, an asymmetric-energy electron-positron collider at KEK, began in February 2016, after more than five years of upgradation work on KEKB and successfully ended in June 2016. A major task of the Phase-1 commissioning was the vacuum scrubbing of new beam pipes in anticipation of a sufficiently long beam lifetime and low background noise in the next commissioning, prior to which a new particle detector will be installed. The pressure rise per unit beam current decreased steadily with increasing beam dose, as expected. Another important task was to check the stabilities of various new vacuum components at high beam currents of approximately 1 A. The temperature increases of the bellows chambers, gate valves, connection flanges, and so on were less than several degrees at 1 A, and no serious problems were found. The effectiveness of the antechambers and TiN coating in suppressing the electron-cloud effect (ECE) in the positron ring was also confirmed. However, the ECE in the Al-alloy bellows chambers was observed where TiN had not been coated. The use of permanent magnets to create an axial magnetic field of approximately 100 G successfully suppressed this effect. Pressure bursts accompanying beam losses were also frequently observed in the positron ring. This phenomenon is still under investigation, but it is likely caused by collisions between the circulating beams and dust particles, especially in the dipole magnet beam pipes.

SuperKEKB is an electron–positron asymmetric-energy double-ring collider, which was built in Japan. It has been operated to explore new phenomena in B-meson decays. Hence, extremely higher luminosity is required. A collision scheme of low emittance with a large Piwinski angle called a “nano-beam scheme” has been adopted to achieve higher luminosity by squeezing the vertical beta function at the interaction point to be smaller than the bunch length. A “crab waist collision scheme” proposed by P. Raimondi et al. has also been adopted to improve the luminosity performance. The article presents an overview of the operation of the nano-beam and crab waist collision schemes at SuperKEKB.

Chum salmon (Oncorhynchus keta) is an important fisheries resource around the North Pacific region. However, the genetic characteristics of this species mostly remain to be elucidated with competent molecular genetic markers for their effective resource management and sustainable use. A total of sixteen novel microsatellite markers in chum salmon were isolated using a next-generation sequencing approach. All loci were polymorphic with 6–21 alleles, with the observed and expected heterozygosities of 0.41667–0.97917 and 0.66184–0.92434, respectively. These new loci will provide a tool for examining the genetic diversity and population structure of chum salmon, which are an important information for the adaptive management of this species.

A total of 18 novel microsatellite markers in masu salmon, an important fisheries resource around the Far East, were isolated using a next-generation sequencing approach. The number of alleles and expected heterozygosity per locus ranged 5–15 and 0.55847–0.92944, respectively. Four loci also were polymorphic in three other salmonid species after cross-species amplification. These newly isolated loci will provide important information for the sustainable adaptive management of masu salmon.