VUV synchrotron radiation studies of candidate LHC vacuum chamber materials

Abstract

For the design of future accelerators, in particular, the beam vacuum of the large hadron collider (LHC), a 27 km circumference proton collider to be built at CERN, ‘white light’ (WL) and monochromatic VUV synchrotron radiation (SR) have been used to measure, both qualitatively and quantitatively, electron emission from candidate vacuum chamber materials. Emphasis has been placed on measuring the photoelectron yields (PY) and the energy distribution of the emitted electrons. These parameters are relevant to gas desorption, the LHC cryogenic cooling capacities and the possible creation of an electron cloud which may cause beam instabilities. Proposed materials, prepared on an industrial scale, such as 50 μm OFE Cu co-laminated onto a high Mn-content stainless steel, exhibit significant modifications when exposed to the WL spectrum from the BESSY TGM7 beamline. Surface cleaning treatments, such as sample annealing and ion bombardment, induce substantial changes to the electron emission which therefore indicate that such surfaces would not be constant, in terms of electron emission, during machine operation. Surfaces which are considered to be constant electron emitters, such as annealed TiZr alloys and commercial non-evaporable getters, were also investigated. These results and their implications for the choice of the material to be used for the LHC are discussed.