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
INFN/LASA, Segrate (MI)
W.C. Heraeus GmbH COPY, Materials Technology Dept., Hanau
Few cavities of the 4th and 6th cavity generation treated accordingly the XFEL recipe have shown performance of ca. 15 MV/m caused by thermal break down without field emission. Effort to post purify some cavities with titanium, that was successfully applied for FLASH cavities, did not improve the performance. The T-map analysis detected the quench areas mainly close to the equator. Optical control by high resolution camera and non-destructive X-Ray radiography have been applied and allowed to monitor the defects in some cases with good correlation to T-map observation. In order to get more detailed information of defects some samples have been extracted from cavity and investigated by light microscope, SEM, EDX and Auger spectroscopy. The detected distinctions are discussed
DESY, Hamburg
The structure, properties and welding parameters of Nb55%Ti -Ti and Nb55%Ti-Nb connection of cavities for XFEL are investigated. These are for example the welding connections of conical disc with helium tank rings (bellow unit and reduction ring) and of conical disc with reference ring. Several samples have been prepared using the electron beam welding equipment of DESY and Lufthansa Technik AG. The metallographic structure analysis, EDX, measurement of gas content, Vickers hardness HV, RRR and electrical resistance measurement have been done. Properties of the welding connection Nb55%Ti-Nb present mainly the bcc body-centered cubic ß-phase according the phase diagram of NbTi alloy. The HV changes rather uniformly, annealing at 1400°C does not lead to changes of the behaviour. The small maximum of the critical superconducting temperature Tc in agreement with the element distribution in the welding connection is observed. Properties of the welding connection Nb55%Ti-Ti present the mixture of the bcc ß-phase and the hexagonal α-phase. Maximum of the HV and increased hydrogen content in the welding connection was observed.
JLAB, Newport News, Virginia
DESY, Hamburg
Performance of 3-Cell Seamless Niobium Cavities P. Kneisel, G.Ciovati, Jefferson Lab and X.Singer, W.Singer, I. Jelezov, DESY In the last several months we have surface treated and cryogenically tested three TESLA-type 3-cell cavities, which had been manufactured at DESY as seamless assemblies by hydroforming. The cavities were completed at JLab with beam tube/flange assemblies. All three cavities performed very well after they had been post-purified with titanium at 1250C for 3 hrs. The cavities, two of which consisted of an end cell and 2 center cells and one was a center cell assembly, achieved gradients of Eacc = 32 MV/m, 34 MV/m and 35 MV/m without quenches. The performance was limited by the appearance of the “Q-drop” in the absence of field emission. This contribution reports about the various measurements undertaken with these cavities.