Gas dynamics modelling for particle accelerators

Abstract

Design of an accelerator vacuum chamber requires an input from different scientific disciplines such as surface science, material science, gas dynamics, particle beam dynamics, and many others. Although vacuum scientists work on the boundary field between these disciplines, gas dynamics is the one that allows joining them to the vacuum science for particle accelerators. The vacuum requirements (usually UHV or XHV) in particle accelerators are defined by beam–gas interactions that should be negligible compared to other phenomena that limit the quality of the beam. At such low pressures the main source of gas in the vacuum chamber is a molecular desorption from materials used for the vacuum chamber and its components. The outgassing rates vary over a very wide range and depend on material, cleaning procedure, treatments, temperature, bombardment by particles and accumulated irradiation dose. The gas dynamics is used to design the research facilities to accurately measure and to study outgassing rates at different conditions. By applying these data to the accelerator vacuum design, one would have to consider that outgassing is often non-uniform and changes with time with different functions. The most time-efficient way of beam vacuum optimization is using a 1D diffusion model where all parameters are defined as a function of longitudinal coordinate (along the beam path). A full 3D modelling with TPMC codes provides much more accurate results, however, being time consuming work is not ideal for pumping and design optimization and is used for complex components and for finalized design.