Bergische Universität Wuppertal, Wuppertal
DESY, Hamburg
One major issue of operating laser driven rf guns with high duty cycles as electron sources for free electron lasers like FLASH or the future European XFEL is the dark current emitted from the gun body and the photocathode. It is lost at various places along the beam line and part of it even reaches the undulator. When dark current is lost electromagnetic radiation and neutrons are created and may damage diagnostic components and electronic devices close to the beam line. Imperfect photocathode regions with enhanced field emission and their contact area to the rf cavity are considered as main dark current sources at typical electric surface fields of about 100 MV/m. We have constructed a novel UHV scanning anode field emission microscope (SAFEM) as part of the systematic quality control of freshly prepared photocathodes at DESY. It is designed to achieve dc surface fields of at least 200 MV/m. In addition it provides the localization of field emitters with a spatial resolution of about 1 μm. In this contribution we report on completed construction and actual commissioning tests of the SAFEM.
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).