Vacuum measurement of laser-based alignment system at the KEKB injector linac

Towards the Super KEKB project, a new laser-based alignment system is under development at the 600-m-long KEKB injector linac. The alignment system consists of the laser source, position sensing detector and data acquisition system. The reference laser beam propagates inside a cylindrical duct of 115 mm internal diameter to eliminate the laser beam fluctuation due to air flow. For the further of the laser beam, it is necessary that the inside of laser duct is kept at low-presser condition. For this reason, we are investigating the vacuum leakage of all laser ducts installed in the KEKB injector linac. In this report, we report the system of laser duct vacuum measurement and its result in detail. (author)