ULTIMATE PRESSURES OF THE LARGE ELECTRON-POSITRON COLLIDER (LEP) VACUUM-SYSTEM

When pumping a 12 m long standard LEP dipole chamber with a Non Evaporable Getter (NEG) strip and a 30 l s-1 sputter-ion pump, an ultimate pressure of about 2 x 10(-12) torr is obtained. The main constituents of the residual pressure are the gases which are not pumped by NEG, i.e argon and methane. It is shown in this paper that by adding six more 30 l s-1 sputter-ion pumps, the ultimate pressure decreases to less than 5 x 10(-13) torr. In this case hydrogen becomes the leading gas, while both argon and methane should fall below 10(-13) torr.