127 DYNAMIC OUTGASSING

Outgassing stimulated by photons, ions and electrons created by high-energy and high-intensity particle beams in accelerators and storage rings represents a serious limitation for the design of vacuum systems. Synchrotron-radiation-induced desorption is particularly important for intense light sources and high current storage rings. Gas desorption-induced by beam generated ions was a serious limitation to the performance of the Intersecting Proton Storage Rings (ISR) at CERN in the past and is a major concern for the vacuum system design of the Large Hadron Collider. 1. SYNCHROTRON RADIATION-INDUCED DESORPTION In high-energy, high-intensity accelerators and storage rings for electrons and positrons, the particle beams emit intense synchrotron radiation in a forward-directed narrow cone. This energetic photon flux produces strong outgassing from the vacuum system and thus a large dynamic pressure increase, which limits the beam lifetime in the machine and may cause increased background in the colliding-beam experiments. For the first time a significant level of synchrotron radiation will also occur with the 7-TeV proton beams in the LHC. 1.1 Basic equations for synchrotron radiation The equations required to evaluate the vacuum conditions with synchrotron radiation for electrons or positrons [1] are: Total radiated power (W): Pγ = 88.6 E4 I ρ (1) where E (GeV) is the beam energy, I (mA) the beam current and ρ (m) the bending radius. The critical energy of the S.R. spectrum (eV) is given by εc = 2.2 ⋅103 E3 ρ. (2) The total photon flux (s-1) around the machine circumference is EI171008.8 ⋅=