Vacuum Chamber Surface Electronic Properties Influencing Electron Cloud Phenomena

摘要

In the vacuum science community, it is now commonly accepted that, for the present and next generation accelerators, the surface electronic properties of the vacuum chamber material have to be studied in detail. Moreover, such studies are of valuable help to define the cleaning procedures of the chosen materials and to identify the most efficient vacuum commissioning. In the case of the Large Hadron Collider (LHC) the proton beam stability, in the presence of an electron cloud, is analysed using Beam Induced Electron Multipacting (BIEM) simulations requiring a number of surface related properties, such as photon reflectivity, electron and photon induced electron emission, heat load, etc. and their modification during machine commissioning and operation. Such simulation codes base their validity on the completeness and reliability of the aforementioned input data. In this work we describe how a Surface Science approach has been applied to measure, total electron yield (SEY) as well as energy distribution curves excited by a very low energy electron beam (0-320 eV), from the industrially prepared Cu co-laminated material, the adopted LHC beam screen material, held at cryogenic temperatures (about 10K). The data show that the SEY converges to unity at zero primary electron energy and that the ratio of reflected to secondary electrons increases for decreasing energy below about 70 eV, and becomes dominant below electron energies of about 20 eV. These observations lead to the notion of long-lived low-energy electrons in the accelerator vacuum chamber, which could be an issue for the LHC, damping rings and future accelerators.