Within the framework for the Swedish participation in the project preparation for the European X-ray Free Electron Laser (XFEL), planned to be built at DESY, Hamburg, a work package has been set up by the Swedish Centre for Development and Research at Free Electron Laser Facilities, focusing on the beam pipes for the undulator sections of XFEL.
Of the total planned length of 3.4 km, more than 750 meter (divided into five sections) of XFEL will be undulator vacuum chambers in aluminium with possible surface coating. The surface of the undulator vacuum chamber will have to have high conductivity and low surface roughness, due to the high-current-density electron bunches traveling at ultra-relativistic velocities, in order to avoid or minimize excitation of Wake fields. Excited Wake fields acts back on the traveling charge or tailing charges and cause a disturbance that worsen the XFEL properties. High conductivity calls for thin oxide layers on the surface.
A number of prototype undulator vacuum chambers have been manufactured, of which some have been electrochemically polished, and some coated with metal over-layers. Within the collaboration, KTH and Uppsala University will study the growth and thickness of oxide layers on aluminium in order to determine the optimal condition for treating the beam pipes after manufacturing, as well as storing and handling the beam pipes before and during mounting them at XFEL.
A number of experimental techniques available at KTH and Uppsala University have been, is and will be used to study the oxide layer growth and thickness.
Lars Westerberg, Uppsala University
Mats Leandersson, KTH