Extreme-ultraviolet to x-ray free-electron lasers (FELs) in operation for scientific applications are up to now single-user facilities. While most FELs generate around 100 photon pulses per second, FLASH at DESY can deliver almost two orders of magnitude more pulses in this time span due to its superconducting accelerator technology. This makes the facility a prime candidate to realize the next step in FELs-dividing the electron pulse trains into several FEL lines and delivering photon pulses to several users at the same time. Hence, FLASH has been extended with a second undulator line and self-amplified spontaneous emission (SASE) is demonstrated in both FELs simultaneously. FLASH can now deliver MHz pulse trains to two user experiments in parallel with individually selected photon beam characteristics. First results of the capabilities of this extension are shown with emphasis on independent variation of wavelength, repetition rate, and photon pulse length.

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 future 12 GeV upgrade of CEBAF requires new cryomodules in both linacs to increase the energy gain per pass to 1090 MeV [1]. Until recently, the design of new cryomodules, which should deliver on average operational voltage of 70 MV each, was based on 7-cell superconducting cavities that are an extended version of the 5-cell structures currently used in the machine. The 5cell cavities were constructed 20 years ago at Cornell University (Original Cornell-shape) for the Cornell Electron Storage Ring (CESR). The geometry of these structures [2] met specifications at the time CESR was constructed but is not optimized for the future operation of CEBAF. Two improved cavity shapes have been proposed. This contribution presents the RF features of both new shapes and discusses advantages for the machine operation resulting from the improvement. In addition, we comment on the measurements on copper models of both new cavities and present results of the multipacting calculations.