DC FIELD EMISSION OF NB SAMPLES PREPARED IN TESLA CAVITIES

Field emission loading is still a major obstacle of the superconducting niobium cavities at the TESLA Test Facility especially at accelerating fields above 20 MV/m, i.e. peak electric surface fields above 40 MV/m. One of the reasons is that the cleaning of the actual ninecell cavities is much more difficult than that of small samples which do not show field emission up to surface fields of 100 MV/m after open surface preparation. Therefore, we have investigated Nb samples, which have been wet-che-mically prepared inside TESLA cavities, by means of a dc field emission scanning microscope. The field emission of such samples started at surface fields of about 20 MV/m and provided a typical density of 10 emitters per cm2 at 80 MV/m, which are mostly microparticles or surface defects. Promising results obtained by modifications of the wet preparation technique are presented.