Paper VT-TuP2
Vacuum System of Positron Damping Ring for SuperKEKB

Tuesday, November 8, 2016, 6:30 pm, Room Hall D

SuperKEKB, which is an upgrade of the KEKB B-factory (KEKB), is a next-generation high-luminosity electron-positron collider. Its design luminosity is 8.0x10³⁵ cm^-2s^-1, which is about 40 times than the KEKB’s record. To satisfy tight requirements on beam quality for positron injection, a new damping ring (DR) with a circumference of 135.5 m is constructed in the positron injection system. For beam quality improvement, the positron beam extracted from an injection linac with an energy of 1.1 GeV stays in the DR for 40 ms. Maximum stored current is 70.8 mA, and the number of bunches is 4.

The DR has two arc sections (~110 m) and two straight sections (~20 m). In the arc sections, walls on both sides of the beam pipes are irradiated with synchrotron radiation (SR, critical energy: ~1 keV). To deal with SR and photoelectrons, the beam pipes in the arc sections have antechambers on both sides of a beam channel. The antechamber is also effective in reduction of the beam impedance. The height of the beam channel in the arc sections is 24 mm and the width including the antechambers is 90 mm. To remove the heat by SR irradiation, a water cooling system is also required in the arc sections. In the straight section, on the other hand, an antechamber structure and a water cooling system are not necessary. The cross-section of the beam pipe is octagon with an inscribed circle diameter of 46 mm.

Required beam lifetime due to residual gas scattering is longer than 1000 sec and averaged pressure should be lower than 1x10^-5 Pa. In the arc sections, pumping speed should be much larger than that in the straight sections because a major dynamic gas load during the beam operation is photon stimulated desorption by SR. Non-evaporable getter (NEG) pumps are mainly used with auxiliary ion pumps, and the average effective pumping speed in the arc sections is about 0.05 m³s^-1m^-1 (CO) just after NEG activation. If the photon stimulated desorption coefficient drops to below 1x10^-4 molecules photon^-1 by the sufficient SR irradiation (i.e. scrubbing), the target pressure can be achieved even after the average pumping speed reduces by nearly half.

The material of the beam pipes is aluminum alloy. The number of the beam pipes is about 100, and almost all beam pipes are coated with titanium nitride films as a countermeasure against the electron cloud issue. Moreover, the beam pipes in the arc sections have the grooved surfaces on upper and lower sides of the beam channel to reduce the secondary electron yield structurally.