As a source of heat load on cryogenic sections, the electron cloud is currently a major limitation to the intensity of some modern particle accelerators such as the LHC and its high luminosity upgrade at CERN. During LHC operation, conditioning of the copper beam pipe surface occurs, leading to a decrease of the cloud intensity. To understand the role of the different chemical surface components of air exposed copper in the electron conditioning process, air exposed copper samples as well as specific model surfaces produced in the laboratory, namely sputter-cleaned copper and carbon-free cuprous oxide (Cu 2 O), were conditioned by low energy electron irradiation. Conditioning of air exposed copper results in a decrease of the maximum secondary electron yield (SEY) below 1.1. Surface cleaning by electron stimulated desorption and carbon graphitization without increase of the carbon surface concentration are observed by x-ray photoelectron spectroscopy. After conditioning, the maximum SEY of both sputter-cleaned copper and Cu 2 O remains higher than 1.1. No significant surface modification is observed by x-ray photoelectron spectroscopy during irradiation for these two surfaces. These results prove that neither an increase of the amount of surface carbon nor oxide modification is responsible for the SEY reduction observed during electron irradiation of air exposed copper. They confirm that graphitic carbon is required to decrease the maximum SEY of copper below 1.1.

Beam-induced heat loads on the cryogenic regions of the Large Hadron Collider (LHC) exhibit a wide and unexpected dispersion along the accelerator, with potential impact on the performance of its High-Luminosity upgrade. Studies related the heat load source to the avalanche multiplication of electrons at the surface of the beam vacuum chamber, a phenomenon known as electron could build-up. Here, we demonstrate that the topmost copper surface of beam pipes extracted from a low heat load region of the LHC consists of native Cu2O, while the pipe surface from a high heat load region had been oxidized to CuO during LHC operation and maintenance cycles. Experiments show that this process increases the secondary electron yield and inhibits efficient surface conditioning, thus enhancing the electron cloud intensity during LHC operation. This study relates the abnormal LHC heat loads to beam-induced surface modifications of its beam pipes, enabling the development of curative solutions to overcome this critical limitation.

Energy-resolved secondary electron spectroscopy has been performed on air-exposed standard Cu samples and modified Cu surfaces that are tested and possibly applied to efficiently suppress electron cloud formation in the high-luminosity upgrade of the Large Hadron Collider at CERN. The Cu samples comprise pristine oxygen-free, carbon-coated and laser-structured surfaces, which were characterized prior to and after electron irradiation and rare-gas ion bombardment. Secondary-electron and reflected-electron yields measured with low charge dose of the samples exhibit a universal dependence on the energy of the primary impinging electrons. State-of-the-art models can successfully be used to describe the spectroscopic data. The supplied spectral dependence of electron emission and integrated electron yield as well as the derived parametrization can serve as a basis for forthcoming simulations of electron cloud formation and multipacting.

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