figure 1: Vertical crossing VELO with the two seperated vacuum systems.

The main volume (beam vacuum) is part of the LHC accelerator, the second volume (detector vacuum) encloses the detector. This secondary volume is mainly enclosed inside the vessel. The pressure in both volumes should be as the low as possible. The current aim is a pressure 10-9 mbar for the beam vacuum and a pressure better than 10-4 mbar for the detector vacuum. The detector inside the secondary volume can move in- and outwards with respect to the IP to allow injection of the beam.

figure 2: The Vacuum Vessel.	figure 3: The two seperated vacuum systems in 3D.

The vacuum vessel (figure 2) consists of two cylindrical flanges connected by a central part. The connecting shell will be 6 mm thick. The central part of the vessel has on the left and right hand side rectangular flanges with slots. On these slots aluminium detector hoods will be mounted. These detector hoods houses the vacuum feed-throughs, flat cables to the detector, and the CO2 cooling system.

The detector vacuum is connected by a rectangular bellow at the upstream side of each detector hood. Four special flanges at the bottom, and two at the top side of the vessel, are connected by a edge welded bellow to the detector supports to allow translation in all directions of the two detectors. The four conflat flanges (CF200) on the topside of the vessel are equipped with blind flanges. They will be used to allow access to the wake field suppressors (which connect the thin containers of the detectors with the beam pipe of the LHC).

figure 4: The Vacuum Vessel on the stainless steel stand.

The four conflat flanges also can be used to mount glass windows for visual inspection. At the bottom of the vessel are two CF150 flanges on which two getter pumps are hanging. Downstream (z-direction) a 2 mm thick aluminium exit window mounted will be mounted. A small bellow, near the center, will prevent that forces of the beam-pipe will go into the exit window. This exit window will be produced by CERN.

Upstream (negative z-direction), the vessel cover closes the vacuum system of the vessel, which is equipped with flanges and bellows to connect to the different vacuum systems. Four conflat flanges (CF 200 and CF150) on the spherical head will be used to mount a turbo pump station and to allow access the upstream wake field suppressor. The conflat flange (CF63) in the center of the spherical head forms the connection to the beam pipe of the LHC accelerator

The vessel will be supported by two adjustable supports at the bottom (figure 4). These supports are rigidly mounted on a stable stainless steel stand. At the downstream side of the vessel, this support is rigid. The support at the upstream side is flexible in the horizontal direction to allow expansion of the vessel due to temperature variations.