The European XFEL (EuXFEL) is a free-electron laser in the X-ray range for users. Its high availability is one of the key aspects of the machine. In 2022, it entered in the sixth year of operation. The EuXFEL linac is based on the TESLA superconducting RF technology, operating at 1.3 GHz with an RF pulse length of 650 μs and a repetition rate of 10 Hz. The LLRF system is based on the MicroTCA standard and relies on a high level of automation. In this contribution, we review the LLRF operation at the EuXFEL and the development of new tools and features to improve the monitoring and extend the usability of the LLRF system.

The European XFEL is a hard X-ray free-electron laser (FEL) based on a high-electron-energy superconducting linear accelerator. The superconducting technology allows for the acceleration of many electron bunches within one radio-frequency pulse of the accelerating voltage and, in turn, for the generation of a large number of hard X-ray pulses. We report on the performance of the European XFEL accelerator with up to 5,000 electron bunches per second and demonstrating a full energy of 17.5 GeV. Feedback mechanisms enable stabilization of the electron beam delivery at the FEL undulator in space and time. The measured FEL gain curve at 9.3 keV is in good agreement with predictions for saturated FEL radiation. Hard X-ray lasing was achieved between 7 keV and 14 keV with pulse energies of up to 2.0 mJ. Using the high repetition rate, an FEL beam with 6 W average power was created. The first operation of the European X-ray free-electron laser facility accelerator based on superconducting technology is reported. The maximum electron energy is 17.5 GeV. A laser average power of 6 W is achieved at a photon energy of 9.3 keV.

The European X-Ray Free Electron Laser (E-XFEL) accelerator is a pulse machine. The typical time duration of a radio frequency (RF) pulse is about 1.3 ms. The RF power transmitted to the superconducting RF (SCRF) cavity as a set of successive pulses (10 Hz repetition rate), causes strong mechanical stresses inside the cavity. The mechanical deformations of the RF cavity are typically caused by the Lorentz force detuning (LFD). The cavity can be tuned to 1.3 GHz resonance frequency during the RF pulse using the fast piezo tuners. Since the E-XFEL will use around 800 cavities (each cavity with double piezos), a distributed architecture with multi-channel digital and analog control circuits seems to be essential. The most sought-after issue is high-voltage, high-current piezo driving circuit. The driving electronics should allow a maximum piezo protection against any kind of failure. The careful automation of the piezo tuners control and its demonstration for the high gradient conditions is also a real challenge. The first demonstration of the piezo controls applied for chosen RF stations of the E-XFEL linear accelerator (linac) are presented and obtained results are briefly discussed within this paper.

After its successful commissioning during the first half of 2017, the European X-ray free electron laser is now in regular operation delivering photons to users since September 2017. This contribution presents an overview on the experience gathered during the first couple of years of operation. In particular, the focus is set on RF operation, advanced commissioning and RF related machine studies.

This paper describes some achievements of the low-level radio frequency (LLRF) tests performed to evaluate the superconducting cryomodules in preparation of their installation in the European X-ray Free Electron Laser (XFEL) accelerator. The software developed to characterize the cryomodules tested at the Accelerator Module Test Facility (AMTF) will be presented. The purpose of these tests is to evaluate some of the key parameters of the eight Tera Electronvolt Superconducting Linear Accelerator cavities, housed in every cryomodule. These parameters include: cavity quench gradients, cavity fundamental resonant modes, performance of the slow and fast frequency tuners, and characteristics of the input power coupler. These results impact the final decision on the cryomodule acceptance and its installation inside the linear accelerator (linac). In this approach, the MicroTCA. 4-based LLRF system is used to control and assess the cryomodule performance. Middle layer servers are developed to verify the tests initial conditions, carry the test, and log the results in a dedicated database. The tests results provide a basis for the cryomodule validation, but also serve as a reference useful during the commissioning and operating phase of the accelerator.

This volume contains a subset of contributions presented at LLRF2017: the 8th Low-Level RF Workshop held in Barcelona, Spain, on October 16-19, 2017.

The European X-ray Free-Electron Laser (XFEL) at Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany is a user facility under commissioning, providing ultrashort X-ray flashes with a high brilliance in the near future. All LLRF stations of the injector, covering the normal conducting RF gun, A1 (8 1.3 GHz superconducting cavities (SCs) and AH1 (8 3.9 GHz SCs), were successfully commissioned by the end of 2015. The injector was operated with beam transmission to the injector dump since then. After the conclusion of the construction work in the XFEL accelerator tunnel (XTL), the commissioning of 22 LLRF stations (A2 to A23) started with the beginning of 2017. Every station consists of a semi-distributed LLRF system controlling 32 1.3 GHz SCs. Stable operation with beam transport to the main dump (TLD) was achieved. The commissioning procedure applied, experience gained and performance reached are described.

The superconducting cavities operated at high Q level need to be precisely tuned to the RF frequency. Well tuned cavities assure the good field stability and require a minimum level of RF power to reach the operating gradient level. The TESLA cavities at XFEL accelerator are tuned using slow (step motors) and fast (piezo) tuners driven by the control system. The goal of this control system is to keep the detuning of the cavity as close to zero as possible even in the presence of disturbing effects (LFD - Lorentz Force Detuning and microphonics). The step motor tuners are used to coarse cavity tuning while piezo actuators are used to fine-tuning and disturbance compensation. The crucial part of the piezo control system is the piezo driver. To compensate LFD the piezo driving with relatively high voltage (up to 100V) and high current (up to 1A) is needed. Since the piezo components are susceptible to destruction with overvoltage, overcurrent, and also overtemperature, one has to pay special attention to keep the piezos healthy. What makes things worse and more critical, is that the piezo exchange is not possible after the module is assembled. Therefore the special hardware must be assisting the power amplifier, detecting the dangerous conditions and disabling piezo operation when needed. It must be fail-safe, so even in a case of failure the piezos shall survive. It must be also robust and it must not disturb or disable normal operation. Due to many channels (16 for master/slave RF), the hardware solution must be well scalable. The paper discuss the design of XFEL's piezo driver together with PEM (Power and Energy Monitor) supervising the driver operation and preventing piezos from destruction. The achieved results and operation of the complete system are demonstrated.

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

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 European X-ray free electron laser (XFEL) at the Deutsches Elektronen-Synchrotron (DESY), Hamburg Germany is in its construction phase. Approximately a third of the super-conductive cryomodules have been produced and tested. The RF gun is installed since 2013; periods of commissioning are regularly scheduled between installation phases of the rest of the injector. The first linac, L1, consisting of 4 cryomodules powered by one 10 MW klystron is installed and being commissioned. This contribution reports on the installation and preparation work of the low-level radio frequency system (LLRF) to perform the commissioning of the XFEL first components. The commissioning plans, schedule and first results are presented.

The Free–Electron Laser in Hamburg (FLASH) at Deutsches Elektronen–Synchrotron (DESY), Hamburg Germany is a user facility providing ultra–short, femtosecond laser pulses down to the soft X–ray wavelength range. For the precise regulation of the radio frequency (RF) fields within the 60 superconducting cavities, which are organized in 5 RF stations, digital low level RF (LLRF) control systems based on the novel MicroTCA.4 standard were implemented in 2013. With the newly installed system, an outstanding LLRF performance was achieved so far, positively impacting the FLASH stability and operation capabilities. Also valuable experiences with failures potentially due to radiation, overheating, and ageing as well as with the general operation of the control system could be gained. These have a direct impact on the operation and on the performance of FLASH and will allow future improvements. The lessons learned are not only important for FLASH but also in the scope of European X–ray Free–Electron Laser (XFEL), which will be operated with the same LLRF control system.