Design of the vacuum system for KEKB

The construction of a new asymmetrical collider with 3.5 GeV positron beam and 8 GeV electron beam has started at KEK. The collider consists of two rings with the circumference of 3 km intersecting at the colliding point. The vacuum ducts deal with intense heat from synchrotron radiation because of a high design current, 2.6 A for a positron and 1.1 A for an electron. As a result of the compact design dimensions for a bunch, the requirement for smoothness of the inner surface is tight. We adopt copper as a material for the vacuum chambers of KEKB. Acid etch or chemical polishing is applied to clean the extruded surface. Using NEG strips as the main pump, a pumping speed is designed as 100 1 s-1 m-1. When the photo-desorption coefficient is 10-6, a pressure of 10-7Pa will be realized. All chambers are baked before installation. By adopting ''dry-hood'' technique, in situ bake out will be omitted. The pumping slots are backed up by mesh to prevent the penetration of a beam induced field which causes pump elements to heat up. The gap between flanges is filled using a Helicoflex vacuum seal. Contact force of a RF finger in a bellows is assured by a spring finger. (author)