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Forty-four kilometers of the LHC beam vacuum system [1,2] will be equipped with a perforated co-axial liner, the so-called beam screen. Operating between 5 K and 20 K, the beam screen reduces heat loads to the 1.9 K helium bath of the superconducting magnets and minimises dynamic vacuum effects. Constructed from low magnetic permeability stainless steel with a 50 µm inner layer of high purity copper, the beam screen must provide a maximum aperture for the beam whilst resisting the induced forces due to eddy currents at magnet quench. The mechanical engineering challenges are numerous, and include stringent requirements on geometry, material selection, manufacturing techniques and cleanliness. The industrial fabrication of these 16 metre long UHV components is now in its prototyping phase. A description of the beam screen is given, together with details of the experimental programme aimed at validating the design choices, and results of the first industrial prototypes. LHC and EST DivisionsPresented at 1997 Particle Accelerator Conference, Vancouver,[12][13][14][15][16] AbstractForty-four kilometers of the LHC beam vacuum system [1,2] will be equipped with a perforated co-axial liner, the so-called beam screen. Operating between 5 K and 20 K, the beam screen reduces heat loads to the 1.9 K helium bath of the superconducting magnets and minimises dynamic vacuum effects. Constructed from low magnetic permeability stainless steel with a 50 µm inner layer of high purity copper, the beam screen must provide a maximum aperture for the beam whilst resisting the induced forces due to eddy currents at magnet quench. The mechanical engineering challenges are numerous, and include stringent requirements on geometry, material selection, manufacturing techniques and cleanliness. The industrial fabrication of these 16 metre long UHV components is now in its prototyping phase. A description of the beam screen is given, together with details of the experimental programme aimed at validating the design choices, and results of the first industrial prototypes.

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Invited paper to the 1995 Particle Accelerator Conference nd I'€pO. /3vacuugnchamber in a dedicated beam e on the PP-2M electron storage ring at Novosibirsk will be of the behaviour of the LHC vacuum syiem using$8 eV critical energy synchrotron radiation in a 10 K ' Ll-IC vacuum system with its beam screen will be described in detail. In addition, the results of a simulation gggmuch larger than those from the oxide layer. These and o er effects and constraints on the design of the essure at 5 K in excess of 10* Torr and the photon induced as desorption yields from condensed gas can cryopumpy ig, ee pumpin may appear ut in practice introduces several liabilities-for example, o y a few monolayers 0% cryopumped H2 already has a vapour ed bthe cold surface. At first htthis fr beneficial b synchrotron radiation impinging on the walls of the beam screen desorbs gas which is immediately absorb the 0.2 Wm" of synchrotron radiation power at a temperature between 5 K and 20 K. However the CO the corresponding pressure is 3.6 10* Torr. The protons will circulate in a beam screen which will less than 2.8 10* Torr which corresponds to a gas density of 9.3 10' molecule cm", for other gases such as luaas etirne at 8.36 Tesla which need superconductincg magnets operatir;g)at 1.9 K. The beam-resi l g' 7 eV is dominated by nuclear scattering an , for a lifetime of 1 hours, requires a room temperature Hz pressure 43.8 eV. To achieve the required strong bending in the LHC it is necessary to erréploy dipqle fields u to ¥ thus emit a significant amount of synchrotron radiation (9.25 10'° photons s" m") with a critical energy of ln the CERN Large Hadron Collider (LHC) project the relativistic protons have an energy of 7 TeV and