Installation and First Commissioning of the LLRF System for the European XFEL

The installation of the European X-ray Free Electron Laser (XFEL) is finished, leaving place for its commissioning phase. This contribution summarizes the low-level radio frequency (LLRF) commissioning with a special emphasis on the development of automation tools to support the commissioning of such a large scale accelerator. First results of the LLRF commissioning in the main linac are also given. THE EUROPEAN XFEL AND ITS LLRF The European XFEL is based on a 17.5 GeV pulsed superconducting accelerator, consisting of 101 cryomodules organized in 26 RF stations. Its injector has been commissioned and is in operation since December 2013 [1]. The installation phase for the main linac stretched between 2014 and 2016. A description of the LLRF system for the XFEL is given in [2, 3]. Reports of the installation planning and progress can be found in [4, 5]. It was decided not to install the last 4 cryomodules of the main linac (RF station 26), as they need to undergo substantial repair work which would delay the overall installation plan. The energy loss from the missing cryomodules is acceptable, since the target accelerator energy can still be reached with the installed cryomodules. LLRF systems were installed for all RF stations, but operation only up to A20 is allowed at this time. INSTALLATION SUMMARY The main LLRF installation steps are: crate preparation in the lab (≈1 month for 6 crates), rack preparation and inner rack cabling (1-2 weeks), installation of the LLRF racks inside the tunnel, external RF cabling (2 months), connection to mains, cooling water, Ethernet and LLRF precommissioning (2-3 weeks). The LLRF system is then ready to drive the klystron in open loop and monitor cavity forward and reflected signals. The installation of the first RF station started in January of 2015. Each subsequent installation followed a cryostring (CS) granularity (i.e. 3 RF stations at a time). The first complete CS installation took 250 days (including cyromodules, klystrons etc.), while the last one was completed in less than 150 days. Most of the tasks could ∗ julien.branlard@desy,de be performed in parallel, resulting in a total installation time just below 2 years. Although the LLRF installation itself only represents a fraction of that time, the cryostring installation dictated the LLRF installation schedule. The core LLRF installation team consisted of 6 people, sharing tasks related to infrastructure, MicroTCA.4 system setup, firmware and server installation, system integration and troubleshooting. Both innerand outer-rack cabling was handed over to a professional cabling company. Basic LLRF checks could be performed parasitically during warm coupler conditioning, hence identifying hardware failures as early as possible, in order to minimize the cold commissioning time and best make use of open tunnel access times. At the end of the installation, the complete RF distribution chain was measured, from master oscillator down to each local reference distribution point along the tunnel. Power levels, insertion losses, and signal spectra were documented. Due to delays in the design and production, piezo drivers were not ready in time before tunnel closure and will be installed later in 2017. COMMISSIONING OVERVIEW From a LLRF point of view, commissioning of the injector covers the normal conducting RF gun, the first accelerating cryomodule A1, the third harmonic cryomodule AH1 and the normal conducting transverse deflective structure TDS. Commissioning of the first linac corresponds to RF station A2; the second linac comprises RF stations A3, A4 and A5, and the commissioning of the third linac consists of commissioning RF stations A6 through A20. The operation of RF stations A21-A23 was not yet approved by German authorities and was only scheduled for end of April 2017. Operation of the last installed RF stations A24 A25 has no firm date at this time. The commissioning of the LLRF system relies on many pre-commissioning steps performed during installation, at the board, crate, rack and system level. These steps are referred to as warm commissioning [6] since they are performed before accelerator cool-down. The LLRF cold commissioning can be subdivided into the following steps: 1) initial LLRF system verification, 2) LLRF signals dynamic range optimization, 3) cavity frequency tuning, 4) coupler THOAA3 Proceedings of IPAC2017, Copenhagen, Denmark Pre-Release Snapshot 19-May-2017 10:10 ISBN 978-3-95450-182-3 0 Co py rig ht © 20 17 CC -B Y3. 0 an d by th er es pe ct iv ea ut ho rs -P re -R ele as eS na ps ho t1 9M ay -2 01 7 10 :1 0 06 Beam Instrumentation, Controls, Feedback and Operational Aspects