Characteristics of KEKB Vacuum System during First Commissioning.

The commissioning of KEKB accelerator (KEKB-factory) started in December, 1998. The vacuum system is working almost satisfactorily although several troubles occurred near the interaction region. The photon stimulated gas desorption coefficient of copper beam chamber is decreasing steadily and reached to around 1×10-5 molecules photon-1 at the integrated linear photon flux of 1×1024 photons m-1. The beam lifetime is increasing with the integrated beam current but is shorter than that expected by the average pressure especially for the Low Energy Ring. A pressure burst was observed at a straight section of the positron ring. The Helicoflex vacuum sealing is stable. Bellows and gate valves with RF-shield structure showed no excess heating up to the storage current of 500 mA.