TECHNOLOGICAL PROBLEMS RELATED TO THE COLD VACUUM-SYSTEM OF THE LHC

The Large Hadron Collider (LHC) project, now in its design phase at CE RN, comprises two proton storage rings with colliding beams of 7 TeV e nergy. The machine will be housed in the existing LEP tunnel with a ci rcumference of 26.7 km and requires a bending magnetic field of 8.4 T with 14 m long superconducting magnets. The beam vacuum chambers compr ise the inner ''cold bore'' walls of the magnets. These magnets operat e at 1.9 K, and thus serve as very good cryo-pumps. In view of reducin g the cryogenic power consumption, both the heat load from synchrotron radiation emitted by the proton beams and the resistive power dissipa tion by the beam image currents have to be absorbed on a ''beam screen '', which operates between 5 and 20 K and is inserted inside the vacuu m chamber. The design of this beam screen represents a technological c hallenge in view of the numerous and often conflicting requirements an d because of the very tight mechanical tolerances imposed. The synchro tron radiation produces strong outgassing from the walls. The design p ressure necessary for operation must provide a beam lifetime of severa l days. An additional stringent requirement comes from the power depos ition in the superconducting magnet coils due to protons scattered on the residual gas which could lead to a magnet quench and, therefore, i nterrupt the machine operation. Cryo-pumping of gas on the cold surfac es provides the necessary low gas densities but it must be ensured tha t the vapour pressures of cryo-sorbed molecules, of which H-2 and He w ill be the most critical species, remain within acceptable limits. In the warm straight sections of the LHC the pumping speed requirement is determined by ion induced desorption and the resulting vacuum stabili ty criterion. Copyright (C) 1996 Elsevier Science Ltd.