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.
