Technological problems related to the cold vacuum system of the LHC

Abstract

The Large Hadron Collider (LHC) project, now in its design phase at CERN, comprises two proton storage rings with colliding beams of 7 TeV energy. The machine will be housed in the existing LEP tunnel with a circumference of 26.7 km and requires a bending magnetic field of 8.4 T with 14 m long superconducting magnets. The beam vacuum chambers comprise the inner “cold bore” walls of the magnets. These magnets operate at 1.9 K, and thus serve as very good cryo-pumps. In view of reducing the cryogenic power consumption, both the heat load from synchrotron radiation emitted by the proton beams and the resistive power dissipation 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 vacuum chamber. The design of this beam screen represents a technological challenge in view of the numerous and often conflicting requirements and because of the very tight mechanical tolerances imposed. The synchrotron radiation produces strong outgassing from the walls. The design pressure necessary for operation must provide a beam lifetime of several days. An additional stringent requirement comes from the power deposition in the superconducting magnet coils due to protons scattered on the residual gas which could lead to a magnet quench and, therefore, interrupt the machine operation. Cryo-pumping of gas on the cold surfaces provides the necessary low gas densities but it must be ensured that the vapour pressures of cryo-sorbed molecules, of which H 2 and He will 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 stability criterion.