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 gas scattering is longer than 1000 s, and the average pressure should be lower than 1 × 10−5 Pa. Nonevaporable getter (NEG) pumps are mainly used with auxiliary ion pumps, and the average effective pumping speed in the arc sections is approximately 0.013 m3 s−1 m−1 (for CO), just after NEG activation. The material of the beam pipes is aluminum alloy. As additional countermeasures against the electron cloud effect, titanium nitride coating and grooved surfacing are adopted. These reduce the secondary electron yield of the inner surfaces of the beam pipes.