European XFEL Linac RF System Conditioning and Operating Test

96 accelerating modules with 768 TESLA / European XFEL type superconducting cavities were installed in the European XFEL ([1] – [3]) LINAC tunnel (XTL) in fall 2016. Warm conditioning of the RF system High/Low Level RF System and main input couplers begun even before finishing the accelerator installation works. All modules were conditioned and tested prior to the installation in the tunnel in the AMTF test stand at DESY. Nevertheless, due to some repair activities on warm input coupler parts, warm conditioning was needed on a few modules/couplers. Cooling down to 2K begun in December 2016 and was finished in January 2017. Since then cold conditioning and tests are running. A few input couplers did have problems with conditioning and were disconnected, limiting otherwise the system performance. Some cavities in the modules showed multipacting (MP) effects, mostly because the cavity vacuum was vented with dry nitrogen gas because of mentioned repairs on couplers in some modules. Such MP effects did appear in AMTF as well. All MP effects were successfully conditioned until now. INTRODUCTION The European XFEL layout is described in ([1] – [3]). Prior to Linac accelerating modules installation in the tunnel SRF cavities and modules were tested, their performance and limits evaluated and documented ([3] – [7]). Tunnel installation is almost finished now. High power RF system (HPRF, klystrons) are commissioned, low lever RF (LLRF) system is being commissioned [8]. Parallel to LLRF commissioning fundamental power couplers (FPC) conditioning is done, followed by SRF accelerating cavities conditioning and operating test. FUNDAMENTAL POWER COUPLERS Figure 1: European-XFEL FPC layout. Last developments for the European XFEL FPC are described for example in [9]. Solving Push-Rod Vacuum Leak Problem During the module tests in AMTF 35 warm parts (WP) were replaced because of inner screw contact problems and problems with the conditioning, cold 70K window overheating and not conditionable discharges. Another recurrent problem were the tuning push-rod bellow (inside the FPC warm window, Fig. 1) vacuum leaks – 30 push-rods were replaced until the problem was understood and solved finally in the tunnel. Figure 2: CST MWS FPC simulation: push-rod bellow. Figure 3: FPC push-rod bellow vacuum leak spot cut-out. Figure 4: FPC copper coax-gasket (red). 18th International Conference on RF Superconductivity SRF2017, Lanzhou, China JACoW Publishing ISBN: 978-3-95450-191-5 doi:10.18429/JACoW-SRF2017-MOPB111