[en] Niobium is a mainly used material for the fabrication of superconducting accelerating RF cavities. The proposed e⁺e^- linear collider TESLA demands about 500 tons of niobium. High purity Nb should reach the TESLA specification. The required high thermal conductivity can be additionally improved by the post-purification. The quality control includes the residual resistivity ratio measurement, microstructure analysis, analysis of interstitial and metallic impurities, hardness measurement, tensile test, examination of the surface roughness and search for clusters. An eddy current scanning system with rotating table is applied for diagnostic of cracks and foreign material inclusions in niobium sheets. 100% of sheets are eddy current tested. Synchrotron fluorescence analysis and neutron activation analysis used for supplemental non-destructive identification and investigation of detected defects. More than 1000 niobium sheets for TESLA test facility were examined. A SQUID based scanning system has potentially higher sensitivity as a conventional pick up coil. First prototype of SQUID apparatus demonstrates sensitivity sufficient to detect inclusions as small as 0.1 mm in diameter

[en] In the framework of SRF cavity development, Fermilab is creating the infrastructure needed for the characterization of the material used in the cavity fabrication. An important step in the characterization of ''as received'' niobium sheets is the eddy current scanning. Eddy current scanning is a non-destructive technique first adopted and further developed by DESY with the purpose of checking the cavity material for sub-surface defects and inclusions. Fermilab has received and further upgraded a commercial eddy current scanner previously used for the SNS project. The upgrading process included developing new filtering software. This scanner is now used daily to scan the niobium sheets for the Fermilab third harmonic and transverse deflecting cavities. This paper gives a status report on the scanning results obtained so far, including a discussion of the typology of signals being detected. We also report on the efforts to calibrate this scanner, a work conducted in collaboration with DESY

[en] We have fabricated and tested a total of six single cell niobium cavities, made from single crystal, high purity niobium. Two of the three cavities of the TESLA shape (1300 MHz) were made from Heraeus niobium by extending a smaller single crystal by rolling and annealing steps; the third cavity was made by spinning from CBMM material. The three other cavities of the scaled 'Low Loss' (LL) shape (two) and 'High Gradient' (HG) shape (one) resonated at 2.3 GHz and were fabricated from 'as received' single crystals, both from Heraeus and CBMM niobium. After appropriate surface treatments by buffered chemical polishing and electropolishing most cavities performed quite nicely and peak surface magnetic fields of ∼ 160 mT or above corresponding to accelerating gradients between 38 MV/m and 45 MV/m were reached. This paper reports about the performance of these cavities.

[en] In the framework of SRF cavity development, Fermilab is creating the infrastructure needed for the characterization of the material used in the cavity fabrication. An important step in the characterization of ''as received'' niobium sheets is eddy current scanning. Eddy current scanning is a non-destructive technique first adopted and further developed by DESY with the purpose of checking the cavity material for subsurface defects and inclusions. Fermilab has received and further upgraded a commercial eddy current scanner previously used for the SNS project. This scanner is now used daily to scan the niobium sheets for the Fermilab third harmonic, the ILC, and the Proton Driver cavities. After optical inspection, more than 400 squares and disks have been scanned and when necessary checked at the optical and electron microscopes, anodized, or measured with profilometers looking for surface imperfections that might limit the performance of the cavities. This paper gives a status report on the scanning results obtained so far, including a discussion of the classification of signals being detected

No abstract available

[en] Eddy current scanning (ECS) has been used to screen niobium sheets to avoid defective material being used in costly cavity fabrication. The evaluation criterion of this quality control tool is not well understood. Past surface studies showed some features were shallow enough to be removed by chemical etching. The remaining features were identified to be small number of deeper inclusions, but mostly unidentifiable features (by chemical analysis). A real cavity made of defective niobium material has been tested. The cavity achieved high performance with comparable results to the cavities made from defect free cavities. Temperature mapping could help to define the control standard clearly.

[en] The main aim of the DESY large grain R and D program is to check whether this option is reasonable to apply for fabrication of ca. 1000 XFEL cavities. Two aspects are being pursued. On one hand the basic material investigation, on the other hand the material availability, fabrication and preparation procedure. Several single cell large grain cavities of TESLA shape have been fabricated and tested. The best accelerating gradients of 41 MV/m was measured on electropolished cavity. First large grain nine-cell cavities worldwide have been produced under contract of DESY with ACCEL Instruments Co. All three cavities fulfill the XFEL specification already in first RF test after only BCP (Buffered Chemical Polishing) treatment and 800 degrees C annealing. Accelerating gradient of 27-29 MV/m was reached. A fabrication method of single crystal cavity of ILC like shape was proposed. A single cell single crystal cavity was build at the company ACCEL. Accelerating gradient of 37.5 MV/m reached after only 112 microns BCP and in situ baking 120 degrees C for 6 hrs with the quality factor higher as 2x1010. The developed method can be extended on fabrication of multi cell single crystal cavities

No abstract available

[en] A development project was carried out at DESY, aimed at reducing secondary electron emission and multipactor effects in TESLA couplers by TiN layers generation on surfaces which were not protected in this way previously. Thin TiN films on ceramic or metallic surfaces were produced by deposition from Ti vapor in low pressure ammonia. Selection of processing parameters and their effect on multipactor suppression in RF field have been studied using a multipactor test resonator at DESY. Appropriate values of deposition rate, substrate temperature, final layer thickness and chemical conversion procedure were selected. A significant reduction of multipactor time during RF tests was reached due to surface coating. Chemical analysis of TiN layers on both ceramic and metallic substrates has been performed using SIMS method. TiN coating of more than 80 wave guide and cylindrical RF windows were performed for TTF2, TTF3 and TTF4 versions of TESLA couplers. Surface processing of flat wave guide windows for TTF2 resulted in significant reduction of multipactor effects and improvement of power transmission. Their good performance remained unchanged after 24-hour exposition to air. TiN coating of all vacuum-facing surface of TTF2 coupler led to reduction of the indispensable RF conditioning time from typically 3 days down to 4-6 hours. (author)

[en] Analysis of the strategy for superconducting cavity material procurement and quality management is done on the basis of the experience with the cavity production for the European x-ray free electron laser (EXFEL) facility. An adjustment of the material specification to EXFEL requirements, procurement of material, quality control (QC), documentation, and shipment to cavity producers have been worked out and carried out by DESY. A multistep process of qualification of the material suppliers included detailed material testing, single- and nine-cell cavity fabrication, and cryogenic radiofrequency tests. Production of about 25 000 semi-finished parts of high purity niobium and niobium-titanium alloy in a period of three years has been divided finally between companies Heraeus, Tokyo Denkai, Ningxia OTIC, and PLANSEE. Consideration of large-grain (LG) material as a possible option for the EXFEL has resulted in the production of one cryogenic module consisting of seven (out of eight) LG cavities. LG materials fulfilled the EXFEL requirements and showed even 25% to 30% higher unloaded quality factor. A possible shortage of the required quantity of LG material on the market led, however, to the choice of conventional fine-grain (FG) material. Eddy-current scanning (ECS) has been applied as an additional QC tool for the niobium sheets and contributed significantly to the material qualification and sorting. Two percent of the sheets have been rejected, which potentially could affect up to one-third of the cavities. The main imperfections and defects in the rejected sheets have been analyzed. Samples containing foreign material inclusions have been extracted from the sheets and electrochemically polished. Some inclusions remained even after 150 μm surface layer removal. Indications of foreign material inclusions have been found in the industrially fabricated and treated cavities and a deeper analysis of the defects has been performed. (paper)