Surface oxide and roughness on test samples for the Ultra High Vacuum section of the Laser Heater for the European XFEL

The European X-ray Free Electron Laser has recently started with operation for users. An approximately 3 m long ultra high vacuum laser heater section is implemented to overcome possible electron bunch instabilities. We describe the process of determining the oxide layer thickness and surface roughness on test samples of the internal surface material in the laser heater vacuum chambers using elastic recoil detection analysis and optical surface profiling. The results are compared to specified values and show that surface roughness on the samples is larger than the requested maximum, with RMS deviations from a mean plane of up to 1.76 μm for 0.60 × 0.45 square millimeter scans. The maximum oxide layer thickness is 5.5 nm on non-electropolished surfaces assuming cuprous oxide with density 6.0 g per cubic centimeter and 4.0 nm on electropolished surfaces.