Bergische Universität Wuppertal, Wuppertal
DESY, Hamburg
Enhanced field emission (EFE) from particulate contaminations or surface irregularities is one of the main field limitations of the high gradient superconducting niobium cavities required for XFEL and ILC [1]. While the number density and size of particulates on metal surfaces can be much reduced by high pressure water rinsing, dry ice cleaning [2] and clean room assembly of the accelerator modules, the influence of surface defects of the actually electropolished and electron-beam-welded Nb surfaces on EFE has been less studied yet. Therefore, we have systematically measured the surface roughness of typically prepared Nb samples some of which were cut out of a nine-cell cavity by means of optical profilometry and AFM. Pits up to 800 μm diameter with crater-like centers (~Ø100μm) and sharp rims (5-10 μm height) as well as scratch-like protrusions up to 10 μm in height were found even on mirror-like surfaces. In the iris-weld region an absolute surface roughness up to 10 μm was determined. In order to study the EFE expected for such defects, correlated field emission scanning microscopy (FESM) and high resolution SEM images will be performed on selected samples after HPR at DESY.
[1] A. Dangwal et al., Phys. Rev. ST Accel. Beams 12, 023501 (2009).
[2] A. Dangwal et al., J. Appl. Phys. 102, 044903 (2007).
