The production of over 800 1.3-GHz superconducting (SC) cavities for the European X-ray Free Electron Laser (EXFEL), the largest in the history of cavity fabrication, has now been successfully completed. In the past, manufacturing of SC resonators was only partly industrialized; the main challenge for the EXFEL production was transferring the high-performance surface treatment to industry. The production was shared by the two companies RI Research Instruments GmbH (RI) and Ettore Zanon S.p.A. (EZ) on the principle of "build to print". DESY provided the high-purity niobium and NbTi for the resonators. Conformity with the European Pressure Equipment Directive (PED) was developed together with the contracted notified body TUEV NORD. New or upgraded infrastructure has been established at both companies. Series production and delivery of fully-equipped cavities ready for cold rf testing was started in December 2012, and finished in December 2015. More than half the cavities delivered to DESY as specified (referred to "as received") fulfilled the EXFEL specification. Further improvement of low-performing cavities was achieved by supplementary surface treatment at DESY or at the companies. The final achieved average gradient exceeded the EXFEL specification by approximately 25%. In the following paper, experience with the 1.3-GHz cavity production for EXFEL is reported and the main lessons learned are discussed.

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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-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. Large-grain (LG) material as a possible option for the EXFEL has been considered and resulted in the production of one cryogenic module consisting of seven (out of eight) LG cavities. They 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 leaded, however, to the choice of conventional fine grain material. The eddy-current scanning has been applied as an additional QC tool for the niobium sheets and contributed significantly to the material qualification and sorting. 2% of the sheets have been rejected what potentially could affect up to 1/3 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.

The influence of heat treatments at 122, 400, and 800 C on the field emission of large-grain and single-crystal high-purity niobium samples has been investigated. Buffered chemical polishing of 40 m and high pressure ultrapure water rinsing under clean-room conditions resulted in smooth surfaces with a linear surface roughness of 46 to 337 nm. By means of field emission scanning microscopy, an increasing number of emitters up to 40=cm 2 with temperature were found at surface fields up to 160 MV=m. Two different mechanisms of emitter activation were found, i.e. activation by the applied electric field and activation by temperature. Some emitters with an onset surface field of 50 to 100 MV=m appeared already after the low-temperature bakeout. Correlated scanningelectron-microscopy/energy-dispersive-x-ray measurements revealed particles and surface defects as emitters. Their activation will be discussed with respect to the thickness of the insulating oxide layer.

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