## 2 Overview European XFEL
### The Super-Conducting Electron Accelerator  
The European XFEL accelerator has the task of providing electron bunches for the FEL process. It consists of a photo-injector, the main linac and the different beam line sections. In the 43 m long photo-injector section [19; 20], electron bunches are generated by means of the photoelectric effect from a CsTe cathode. The photocathode is located inside a normal-conducting cavity to immediately accelerate the electrons to 6 MeV before injection into the first super-conducting accelerator module. This module is directly followed by a super-conducting 3.9 GHz acceleration module needed to linearize the longitudinal phase of accelerated electrons. At 130 MeV the electrons enter a diagnostic section enabling the measurement of the phase space properties of individual electron bunches in the bunch train and even of slices of these bunches. At the end of the injector an electron beam dump allows standalone operation of the injector over its full parameter range, such that commissioning and further development can be performed independently of the operation of the main linac. Electron extraction from the cathode is achieved by frequency-quadrupled 257 nm laser pulses supplied by a dedicated Nd:YLF photo-injector laser. The laser is synchronized to the accelerator radio-frequency and delivers a time pattern corresponding to the burst mode repetition rate of electron bunch delivery. The photo-injector performance determines the smallest obtainable emittance of the entire accelerator and its design has been optimized in this regard. Space charge driven emittance growth is the most important limiting effect and is minimized by relatively long laser pulses (up to 20 ps) and very high acceleration gradients of up to 50-60 MV/m at the cathode. Furthermore, the spatial and temporal profile of the laser pulse ideally has a top-hat-like shape when hitting the cathode. The operation with electron bunch charges from 0.02 to 1.0 nC at different emittances and enabling different bunch durations is foreseen. Initial commissioning of the injector was concluded in summer 2016 [21]. The main linac accelerates the electrons to a final energy of up to 17.5 GeV by means of 96 accelerator modules operated at 2.2 K, built by an international collaboration for European XFEL based on the TESLA design [14]. Each module is 12 m long, weighs eight tons and comprises eight nine-cell Nb cavities. A total of 768 couplers provide the radio-frequency (RF) fields generated by 24 RF stations. The accelerator is operated in a 10 Hz pulsed RF mode (see Figure 2) for a maximum beam power of 500 kW, exceeding by far the power that can be reached by any normal conducting machine. The design gradient is 23.6 MV/m and the final installation considers the actual performance of each accelerator module by tuning the RF distribution system. The injector and the three linac sections L1, L2 and L3 are separated by three electron bunch compressors BC0, BC1 and BC2. These are used to compress the electron bunches in steps from their initial approximately 20 ps duration to as short as a few fs, hence reaching the design peak current of 5 kA, depending on the bunch charge. The electron energy at the end of L2 is 2.4 GeV and will be kept constant during operation in order to optimize the performance of the accelerator. Dedicated diagnostics sections for the measurement of  
Figure 1: Overall layout of the European XFEL facility. The electron accelerator leads into two electron beamlines with up to five FEL sources. Each of these has dedicated X-ray transport sections leading towards the experiment hall, where the up to fifteen scientific instruments can be installed. For abbreviations see text. integrated and slice bunch parameters are located after compressors BC1 and BC3. Cool-down of the linac was started end of 2016 and commissioning with electron beam commenced in 2017 [22; 23].