Reflectivity and photoelectron yield from copper in accelerators

Abstract

For the design of high-performance particle accelerators and vacuum systems for high-energy colliders, the choice of materials and of their surface treatment is an essential prerequisite. Physical and optical properties of such materials and their coatings are input parameters for programs to simulate their performance and to validate machine design. Materials behavior after exposure to synchrotron radiation (SR), produced by the circulating particles, need to be studied because of its detrimental consequences, such as photo induced desorption, heat load, vacuum and beam induced instabilities. Reflectivity, its angular distribution and photo yield, i.e., the number of photoelectrons produced per incident photon, are essential ingredients to simulation codes. Such parameters must be studied not only on materials as they are in accelerators, but also in conditions as close as possible to the operative ones. In this work, we present results of such an experimental campaign, carried out at the Optics Beamline of BESSY-II. This experimental setup, designed to investigate quasiperfect x-ray-optical elements by “at-wavelength” metrology (from 35 eV to 1850 eV), is also an ideal tool to perform reflectivity and photo yield studies of vacuum chamber materials. As will be discussed, different roughness and various nano-, micro- or macro- modifications of a Cu-surface significantly influence the parameters under study. Energy- and angle-integrated values for the total reflectivity and photo yield are derived. Such integrated values are representative for material behavior under “white-light” irradiation for the various accelerators discussed here.