NEG pumps for SuperKEKB new beam pipes

The SuperKEKB, the upgrade of the KEKB, is an electron-positron collider with asymmetric energies, that is, 7.0 GeV electrons and 4.0 GeV positrons, aiming at a luminosity of 8×10³⁵ cm^-2 s^-1. The construction of the new vacuum system for the SuperKEKB has been ongoing since 2010. The target vacuum pressure in the main ring is on the order of 10^-7 Pa on average at the designed beam currents. The synchrotron radiation (SR) is received by the beam pipes along the ring. Therefore, the distributed pumping scheme is adopted to effectively evacuate the beam pipes. A linear pumping speed of approximately 0.1 m³ s^-1 m^-1 is required if we assume a photo-desorption coefficient (η) of 1×10^-6 molecules photon^-1. A new built-in pump assembly using the multi-layered non-evaporable getter (NEG) was designed for an antechamber space of the new beam pipes as a main pump at the arc sections. The linear pumping speed combined with an RF-screen is expected as approximately 0.1 and 0.18 m³ s^-1 m^-1 for N₂ and CO, respectively. The measured effective pumping speed using a real beam pipe was reasonable one for N₂. The NEG activation in the tunnel after installing the beam pipes are ongoing. The vacuum pressures on the order of 10^-8 Pa on average are mostly achieved after the NEG activation. (author)