Status of SuperKEKB construction

SuperKEKB is a major upgrade of the asymmetric B-factory collider, KEKB, to provide 40 times higher luminosity than that achieved at KEKB. The design of SuperKEKB adopts the nano-beam scheme for the collision of two beams, which was originally proposed for the Italian SuperB project. The nano-beam scheme is realized with low-emittance optics, a very small β-function, and a large crossing angle at the collision point. For the low-emittance design, a substantial part of the positron ring (LER) was reconstructed. All dipole magnets in the LER were replaced with longer ones. The beam pipes in the LER were redesigned to mitigate the electron cloud problem. Around the interaction point, about 300 m of both rings were completely redesigned for the new collision scheme. The commissioning of SuperKEKB is divided into three phases. Phase 1 is used for vacuum scrubbing of new beam pipes without the Belle-II detector and the final focus magnets. The final focus magnets and Belle-II are installed in Phase 2. However, the central vertex detector (VXD) of Belle-II is not installed. Phase 2 is devoted to background studies and collision tuning. After the installation of VXD, Phase 3 starts for the new luminosity frontier. The construction of SuperKEKB for Phase 1 is nearing completion. The preparation of hardware for Phase 2 and Phase 3 is in steady progress. From various aspects, the nano-beam scheme SuperKEKB is a challenging accelerator. The tolerances for the hardware to realize nano-scale beam sizes and to keep the collision are very tight. The optics design is still being optimized to maximize the dynamic aperture. It is hard to predict how the commissioning will proceed.