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Beam commissioning of the European X-ray Free Electron Laser (European XFEL project) is ongoing. Commissioning the XFEL cryogenic system has started by cooling down the XFEL injector section in December 2015. The stationary operation was continued until August 2016. After intermediate warming up of the complete XFEL cryogenic system, the commissioning of remaining components including the 1.5 km long superconducting XFEL linear accelerator (linac) has commenced and was completed in beginning of December 2016. After conclusive pressure and leak tests, and flushing the cooldown started on 11 December 2016. Stable 4.5 K operation both for the linac and injector was established on 28 December. In this paper the XFEL cryogenic system is introduced and the first cooldown of the XFEL linac is reported. The cooldown sequences are described and the measured cooldown evolution is presented. Thermal losses of single circuits are given. Preliminary conclusions with the review of critical points are drawn. 1. System overview A conceptual design of the XFEL cryogenic system was presented in 2006 [1]. The final XFEL cryogenic system is subdivided into three groups of components linac, injector and refrigerator (see figure 1.) 1.1 XFEL refrigerator Two former HERA refrigerators each consisting of a line of screw compressors (3LP, 1HP) and one cold box were overhauled and modified by Linde Kryotechnik AG (LKT, Switzerland) to supply the XFEL with the required cooling power. Several new components were added: low temperature purifier XLTP, distribution box DB54, transfer line XRTL, cold compressor (CC) box CB44 and XFEL distribution box XLVB. The cold boxes CB41 and CB43 were upgraded to integrate the 2 K circuit and meet the design specification for a future 20 GeV beam operation of the linac or an equivalent load including additional margins (see table 1.) The actual goal is to operate the XFEL linac at 17.5 GeV. In June 2017 the linac is running at 14 GeV. The cold boxes are identical and can be operated in parallel or stand alone. Calculated and specified values (including margins) are compared with the measured capacities both for single and parallel operation (see table 1.) Each of the boxes can be operated either in 80 K or in 4.5 K mode. In the 80 K mode the last 4.5 K turbine of the cold box (Turbine 7) is not in operation and bypassed. The mode allows using the maximal output pressure of the screw compressors up to 1.9 MPa. The lowest achievable temperature in this mode is around 40 K. In the 4.5 K mode Turbine 7 is in operation resulting in a supplied pressure of about 0.45 MPa and temperature of 5.5 K. The low temperature purifier XLTP (LKT) is intended for continuous purification of raw helium exiting the XFEL compressor units by absorption of air impurities with adsorbents cooled to LN2 temperature. The XLTP is designed for purification of 1 kg/s (at 1.6 MPa and 300 K) of helium contaminated by 100 ppm (weight) of air to an impurity level less than 1 ppm. Charging and regeneration time for each of the two adsorber units amount to respectively 24 and 16 hours. Figure 1. Layout of XFEL cryogenic components. Table 1. XFEL Refrigerator capacities. The table shows the specified and measured capacities of the XFEL refrigerator. In some cases the test equipment limited the measurements. These cases are marked with (*). The real values should exceed the numbers in the table. Circuit 17.5 GeV operation calculated [kW] 17.5 GeV design [kW] Single CB operation measured [kW] Parallel CB operation measured [kW] 2 K 1.46 1.896 >2.0 (*) >2.56 (*) 5/8 K 2.4 3.593 2.78 >4 (*) 40/80 K 16 24 18 >26.7 (*) The distribution box DB54 (LKT) is intended to combine/distribute flows from/to the cold boxes CB41 and CB43. The transfer line XRTL (DeMaCo, Netherlands) connects DB54 with CB44 and carries 7 process pipes: forward and return pipes for the 5/8 and 40/80 K circuits (respectively 5KF, 8KR, 40KF and 80KR), forward pipe for the 2 K circuit 2KF, helium vapour return pipe 4KR and return pipe 20KR from the cold compressors outlet. The process pipes are surrounded by the 40/80 K thermal shield. The cold compressors box CB44 (LKT) contains a chain of 4 cold compressors intended to compress ~110 g/s of helium at 3.5 K from a pressure of about 2.7 to 60-120 kPa. The compressed helium at a temperature of about 23 K is then returned to the thermodynamic cycle of the cold boxes. CB44 has available a CC bypass circuit intended to keep the constant mass flow of 110g/s across the CCs. Furthermore CB44 contains an 860 l phase separator to supply saturated liquid helium at 120 kPa for the 2KF circuit. The XFEL distribution box XLVB (BINP, Russia) is connected to CB44 by a slightly inclined short transfer line. XLVB collects all cryogenic circuits to the XFEL linac and injector and contains a JT heat exchanger for cooling the 2KF flow with the 2KR vapour. The 2KR vapour can then be processed either by the cold compressors or by the AMTF warm pumps. This option helps to keep the XFEL linac still at 2 K during maintenance periods of the cold compressors. The 2KR pipe is additionally connected to the 4KR pipe to keep superconducting components of XFEL linac also at 4.5 K. XLVB is equipped with control valves to mix warm and cold helium flows for controllable cooldown or warmup of all XFEL linac circuits. XLVB houses two DN250 valves in the 2KR circuit. 1.2 XFEL injector The XFEL injector (BINP) [2] cryogenic system is designed to supply two parallel injectors. For the time being only one injector is installed and in operation. The injector consists of transfer line XITL, injector box XIVB, feed box XI1FB, transfer line XI1TL, feed cap XI1FC and end cap XI1EC. The transfer line XITL connects the XLVB with the XIVB and houses the 2KR, 5KF, 4KR, 40KF and 80KR process pipes surrounded by a 40/80 K thermal shield. The XIVB box collects all cryogenic circuits to both injectors and houses a 300 l sub-cooler with two integrated heat exchangers. XIVB has available provisions for mixing warm and cold helium flows to carry out controllable cooldown or warmup of both injectors independently from each other. The installed injector consists of a standard 1.3 GHz XFEL and a 3.9 GHz cryomodule. Close to the cryomodules the 2 K liquid supply is expanded from the 5 K supply via a JT valve in the appendix of the feed cap XI1FC. Figure 2. Cryomodules inside XFEL tunnel.

Commissioning and first cooldown of XFEL linac