DIAGNOSTIC OF DEFECTS IN HIGH PURITY NIOBIUM

The investigation of the quench area of TTF cavity D6 confirms a necessity of a quality control of the Nb sheets. A small cluster with high Ta content was detected in D6 by means of X-ray micro radiography. The identification and estimation of Ta concentration was done via Synchrotron Radiation Fluorescence Analysis (XAFS, SURFA). Some methods of nondestructive diagnostic of Nb sheets (X-ray radiography, neutron radiography, neutron activation analysis NAA, XAFS, SURFA, ultrasonic-, eddy current inspection, microhardness measurement) were analysed for check of efficiency. The eddy current method was chosen as most suitable for detection of defects (rather fast, sensitive to different sorts of defects, has a high resolution). An eddy current scanning system (with probes special developed for this purpose) is created and about 700 Nb sheets are tested. The penetration depth of the signal is about 500 µm. SURFA and NAA are applied for supplemental nondestructive identification and investigation of detected defects. The first method is more efficient for analysis of layers close to the surface (with a penetration depth between few µm and few hundred µm), NAA delivers the information about bulk Nb and demonstrates very high sensitivity to Ta inclusion in Nb. Some results are discussed. For example some small iron spots probably imbedded during rolling were detected in one Nb sheet with the help of the eddy current system. The sort of the inclusion and its three dimensional profile was determined by SURFA. Foreign material inclusion in TTF cavity D6 Traditionally the quality control of high purity niobium deals with three aspects: purity, workability and surface quality. Another problem, which becomes more important for high gradient cavities, is the non homogeneous distribution of foreign elements in Nb. The cold test in the vertical cryostat of cavity D6 before and after post purification with Ti has shown, that the dependence of Q on field E almost does not change and is limited at 13MV/m. The worst performance shows the cell 5. The application of a rotating T-R mapping system, which is applied at DESY for diagnostic of the hot spots in TTF 9-cell cavities, detected the sharp temperature increase in the definite area of the 5th cell /1/ (figure 1). The eddy current inspection of this cavity from outside with an extremely sensitive probe was done at the BAM (Bundesanstalt fuer Materialforschung und -pruefung, Berlin). A signal deviation at the same area was found. At the same time a careful inspection of the