SuperKEKB Commissioning Begins, Successful Storage of Beams in its Electron and Positron Rings Reported

After five years of large-scale upgrade work, Japan’s High Energy Accelerator Research Organization (KEK) began test operations on SuperKEKB, an electron-positron colliding accelerator. SuperKEKB represents a major upgrade from the previous KEKB accelerator and is the culmination of construction that started in the second half of 2010. Commissioning began on February 1. On February 10, SuperKEKB succeeded in circulating and storing a positron beam in the positron ring, and it had succeeded in circulating and storing an electron beam in the electron ring by February 26.

Before beginning test operation of the storage rings, machine tuning of the electron-positron injector started at the end of January. The beam transport lines, the positron ring and the electron ring were then successively commenced. Machine tuning will continue until the end of June, followed by installation of the Belle II detector upgrade and new superconducting electromagnets for the final focus at the collision point. Machine tuning will then be carried out to achieve collisions of the electron and positron beams.

SuperKEKB has adopted and will realize for the first time a nano-beam large-angle crossing collision scheme, with the beam size at the collision point reduced to 1/20th of the beam size of KEKB. The stored beam current will also be increased to twice that of KEKB, with the final aim of achieving a collision performance 40 times higher than that of KEKB.

The rate of collisions produced by SuperKEKB will also be several tens of times larger than that of KEKB. The aim is to pursue the mystery surrounding the disappearance of anti-matter during the early, developmental processes of the universe and to discover and clarify new physical laws that go beyond the Standard Model of particle physics.

The SuperKEKB accelerator tunnel in the Fuji straight section. Image: KEK

The SuperKEKB accelerator tunnel in the Fuji straight section. Image: KEK

In 2001, the KEKB Factory accelerator (KEKB), the predecessor to SuperKEKB, achieved the best beam collision performance (the highest luminosity) in the world. Following that achievement, KEKB continued to set new luminosity records. By analyzing the large quantities of data generated by the collisions of electron and positron beams at KEKB, the Belle group experimentally verified the Kobayashi-Maskawa theory by observing the first signal of CP asymmetry in B mesons. Thus, KEKB has been contributing to the study of elementary particle physics and accelerator physics for many years.

The Kobayashi-Maskawa theory explains the difference between matter and anti-matter that was experimentally discovered in 1964. Since there is almost no anti-matter presently found in nature—while equal quantities of matter and anti-matter were present at the beginning of the universe—there must be a fundamental difference between matter and anti-matter. However, the Kobayashi-Maskawa theory alone cannot completely explain the deficit of anti-matter. Hence, it is necessary to increase the experimental data samples and do further research.

For this purpose, after the operation of KEKB terminated in June 2010, the KEKB accelerator group undertook the project of upgrading KEKB to SuperKEKB, for higher luminosity. Since then, the KEKB electron and positron rings, each of which has a circumference of 3,016 m, have been upgraded. The various modifications include replacing the beam pipes with new types, upgrading and increasing the number of electromagnets and the power sources for the electromagnets and increasing the power of the radio-frequency accelerator system, while reusing the existing facilities as much as possible. The 600 m-long electron and positron injector has also been upgraded by developing a new electron gun and positron source and by improvement of the precision alignment of the accelerator beam pipes and electromagnets.

In SuperKEKB, the electron and positron bunches circulate around each ring by using electromagnets that control the particle orbits along with radio-frequency accelerating cavities that provide the beams with power and various other types of devices. A large number of bunches must be stably stored in the rings for physics experiments at high luminosity. The electron and positron bunches travel around the rings with velocities close to that of light, with the beam currents being measured by current transformers.

Schematic of SuperKEKB. Image: KEK

Schematic of SuperKEKB. Image: KEK

Phase 1 operations will last until June. During this time, the accelerator will be tuned to achieve stable storage of low emittance beams (beams with narrow spread). Vacuum scrubbing—cleaning the surface of the beam pipes using synchrotron radiation generated by the beams—will be carried out as well. Phase 1 commissioning is required so that the Belle II detector can be rolled in to the collision point for upcoming Phase 2 operations.

The Belle II detector is being assembled by the Belle II collaboration, an international research organization hosted by KEK IPNS (Institute of Particle and Nuclear Studies). Installation of the Belle II detector upgrade at the collision point is planned after the completion of Phase 1 operation. Prior to this, and still during Phase I, seven types of compact sensors (BEAST measurement instruments) will be installed at the collision point to investigate the effects of electromagnetic showers generated by beam particles that deviate from their orbits during the physics experiment.

In addition to the installation of the Belle II detector, superconducting electromagnets that focus the beams to a few tens of nanometers at the collision point will be installed and beam tuning with squeezed beams to increase luminosity is scheduled to begin in the fall of 2017 (Phase 2 operation). For Phase 2 operation, the positron beams must have low emittance before injection into the positron ring. To accomplish this, a new positron damping ring, with a circumference of 135 m, is being built. The installation and tuning of the damping ring in the completed tunnel and buildings are underway.