Simulations of synchrotron-radiation-induced electron production in the CESR vacuum chamber wall

Abstract

We report on calculations of electron production by synchrotron radiation absorbed in the vacuum chamber walls of the Cornell Electron Storage Ring (CESR). These photo-electrons are the source of electron clouds which limit the performance of storage rings by causing betatron tune shifts, instabilities and emittance growth. Until now, cloud buildup modeling codes have used ad hoc models for the production of the seed electrons. We have employed the photon scattering code Synrad3D developed at Cornell University to quantify the pattern of absorbed photons around the CESR ring, including the distribution in azimuthal location on the wall of the beampipe. The reflectivity of the wall, including its dependence on photon incident angle and energy, is modeled for various materials using online look-up tables. Micro-groove structure in the vacuum chamber wall is also accounted for. The resulting absorbed photon energy and incident angle information are used as input to Geant4-based simulations of electron emission from the walls, in which the material composition of the wall is also taken into account. The quantum effciency is found to vary dramatically with the location of the absorption site, owing to the distribution in photon impact energies and angles. The electron production energy spectrum plays an important role in the modeling of electron cloud buildup, where the interplay of production energy and acceleration by the beam bunches determines the time structure and multipacting characteristics of the cloud.