Conditioning of Surfaces in Particle Accelerators

The electron cloud developing in the vacuum chambers of the LHC during the proton beam operation is responsible for heat load on the cryogenic system of the superconduct- ing magnets. The observed heat load exhibits a strong dispersion between the different LHC arcs, although identical by design. Some of them are currently close to the limit of the cryoplant capacity. Under the effect of the cloud itself, conditioning of the cop- per surface of the LHC beam pipes is expected, decreasing thus the secondary electron yield of the surface and leading to a decrease of the cloud intensity down to operation- compatible levels. Such a process seems therefore to be hindered in some parts of the LHC ring. This work aims to understand the copper conditioning processes occurring in the LHC, to unravel the origin of the heat load dispersion observed along the ring. Copper conditioning mechanisms were studied in the laboratory at room temperature by mimicking the electron cloud by an electron gun. The fundamental role of carbon, among the surface chemical components, in the reduction of the secondary electron yield during conditioning was evidenced. Studying the deconditioning, occurring while exposing a conditioned surface to air (necessary step to extract beam pipes from the LHC) allowed establishing a procedure to limit the erasing of the in-situ conditioning state of such components before the analysis of their surface in the laboratory. The surface of beam pipes extracted from a low heat load magnet were found to have similar characteristics as the ones conditioned in the laboratory. However, beam pipes extracted from a high heat load magnet exhibit cupric oxide CuO and a very low amount of surface carbon. It is demonstrated that these modifications are induced by the LHC operation and lead to a slower conditioning of these surfaces. Therefore, these modifications are currently the best candidate to explain the heat load dispersion observed in the LHC.