Title: Long-term surface evolution models for accelerator materials

Authors: V. Jansson, E. Baibuz, and F. Djurabekova

The Compact LInear Collider (CLIC) is an electron-positron accelerator currently under development in CERN, Switzerland. It is
planned to be a powerful successor of the LHC and well equipped for studies of e.g. the newly discovered Higgs boson. However,
the construction of CLIC still requires certain issues related material science and plasma physics to be properly understood. The
high electric field with a gradient of up to 300 MV/m needed in the accelerating structures causes electric discharges in forms
of arcs to appear inside the structures, despite an ultra high vacuum. These vacuum arcs cause the accelerating field to break
down and decrease the effect of the accelerator with up to 30 %, which is not acceptable. The physical processes causing the
vacuum arcs are not yet known and the phenomenon has been the centre of attention of extensive experimental and theoretical
research efforts.

Part of the efforts to understand the vacuum arc phenomenon has been to develop a Kinetic Monte Carlo model for the longterm
evolution of the copper surface of the accelerator structures in order to study the stability of field-emitter candidate
structures and their possible formation mechanisms in terms of atom diffusion processes on the nanoscale. Such a model have
recently been developed and verified for copper. The same model has also been successfully applied to iron nano-clusters. In this
work, we will give an overview of this model and its applications.