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).
DESY, Hamburg
About 50 nine-cell cavities of the recent fine-grain niobium cavity productions have been analysed with respect to maximum gradient and critical field emission onset in the first and final vertical acceptance test, respectively. Parameters of the analysis were the manufacturer of the cavities, the location of the main EP (=> industry or in-house), the final surface treatment (=> final 40μm EP or short 10μm "flash" BCP) and the cavity preparation strategy (=> vertical acceptance test with or without He-Tank welded). Moreover the effect of a re-processing of field emission loaded cavities by additional ultra pure high pressure water rinsing has been investigated.
DESY, Hamburg
Surface removal by electropolishing (EP) and final cleaning by High Pressure Ultra Pure water rinsing (HPR) are baseline technologies in order to achieve reproducible high surface field in superconducting niobium cavities. In the last years the standard horizontal EP process applying hydrofluoric (HF) and sulfuric acid (H2SO4) has been transferred to industry at KEK and DESY successfully. Alternative approaches based on the HF/H2SO4 mixture are vertical EP (Cornell University, CEA Saclay) as well as low voltage EP (CEA Saclay). As an alternative approach electrolytes free of HF are under investigation (INFN Legnaro, Accel Co/DESY and others). HPR has been established with various mechanical set-ups, water pressures, nozzle configurations and nozzle designs worldwide. Carbon dioxid (CO2) snow is an additional cleaning approach developed at DESY. In contradiction to HPR it is a dry cleaning process, which allows the cleaning of water sensitive components as well as horizontal cleaning of niobium accelerator cavities. The recent developments of these processes will be discussed.
CEA, Gif-sur-Yvette
DESY, Hamburg
First promising results concerning Electro-Polishing at lower voltage of 5V (abbreviated as LV-EP) has previously been reported [*,**]. This effort is being pursued and a 1-cell Tesla Shape 1.3 GHz cavity has been dedicated to LV-EP and has reached improved gradient exceeding 39MV/m. Furthermore, a second cavity has alternately been electro-polished at 5V and 17V. It did not encounter any decrease in performance after LV-EP. This process is then especially promising for the treatment of large cavities for proton applications. Moreover, long-time EP experiments on niobium flat samples show that high-voltage EP is more likely to generate impurities in the EP mixture that might contaminate cavities. Some results will also be presented concerning efficient field emission removal by chloroform rinsing of 1-cell cavities.
* F. Eozénou et al, Proc. of 13th workshop on SRF, China, (2007) TUP80
** F. Eozénou et al, CARE-Report-2008-022-SRF