Performance of the SPS Ring Vacuum System for Colliding Beam Operation

The SPS ring vacuum system was recently upgraded to the vacuum requirements for proton-antiproton collider operation. The new specification stipulated an average operating pressure in the ring of 3.10-9 mbar, to be reached within 10 days of pumping after a venting to air. The pressure is expressed in nitrogen equivalent gauge reading. Bakeout to rapidly reduce the gas desorption from the system cannot be used in the SPS, except at a few special positions which cover less than 57. of the ring circumference. The specified pumping time is much shorter than the time it usually takes for an unbaked vacuum system to reach its limit pressure. The pressure thus continues to fall for roughly 3 months from the beginning of a pumpdown and finally settles at about 7.10-10 mbar.