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 free-electron laser FLASH at DESY and the European XFEL are operated with superconducting radio frequency cavities and supply beam to several user experiments. The switching time between experiments is limited to dozens of microseconds. This contribution will show a regulation with a frequency shifted superconducting cavity to manipulate and change the accelerating properties of electron bunches with 250 kHz.The main challenge of the concept presented in this contribution can be summarized in this statement: finding a way to modulate the energy of individual bunches in a single-source multiple-cavity scheme for a potential CW upgrade of the EuXFEL.

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.

Vacuum-tube amplifiers are the most widespread type of radio frequency (RF) sources used to produce high-power signals needed for beam acceleration in superconducting cavities. At Deutsches Elektronen-Synchrotron (DESY), megawatt-rated klystrons are used to produce millisecond-long RF shots for pulsed operation in particle accelerators. In contrast, inductive output tubes (IOTs) are used to provide a continuous RF signal for continuous-wave (CW) operation. In both cases, the amplifiers suffer from amplitude-dependent nonlinearity between the driving and generated signals. This nonlinearity complicates the setup operations of the low-level RF (LLRF) system and makes it harder to regulate the accelerating field. Therefore, a way to linearize the amplifier is highly valuable. This article covers the design, implementation, and test of a field-programmable gate array (FPGA)-based predistortion linearization unit. The first results on the performance of this component in linearizing the amplifiers of running CW and pulsed superconducting accelerators are presented. Such a component uses programmable interpolating lookup tables (LUT) that are addressed using the squared value of the requested signal amplitude. This component only adds 64-ns latency to the RF control system without relying on any vendor-dependent FPGA component. Other benefits of using programmable interpolating LUT are low usage of FPGA resources and flexibility in terms of the type of amplifier to be corrected. The benefits of using this linearizer for klystrons and IOTs are presented and quantified.

Close to 780 superconducting 1.3-GHz accelerating cavities made of bulk niobium have been installed in the European X-ray free electron laser (E-XFEL) linear accelerator. The linac can operate at a mean gradient (Eacc) of ca. 23 MV/m in the nominal short pulse (SP) operation mode with 1.4-ms pulses and a repetition rate of 10 Hz. An R&D program at Deutsches Elektronen-Synchrotron (DESY) is ongoing since 2011 on the feasibility of a continuous wave (CW) upgrade of the E-XFEL accelerator. In this publication, a modification of the low-level radio frequency (LLRF) control system to the CW mode is presented. Currently, the control system successfully stabilizes accelerating field for the SP mode. The demanding E-XFEL specifications of 0.01% for the field amplitude and 0.01° for the phase stability are required to keep the photon beam quality for all experimental stations. The proposed modification has been implemented and tested to verify the system versatility for SP and CW operation modes. The tests were conducted for a prototype E-XFEL cryomodule XM-3. As for the series cryomodules, this prototype contains eight superconducting 1.3-GHz cavities. The results presented here confirm that the same LLRF system can be used with minor software modifications for either operation mode (CW or SP). However, the CW mode requires a more complex RF-power management and more precise control, especially when cavities operate with very high loaded quality factor (Ql) of the order of 6E7. The achieved stability for such highQl is also presented and discussed.

To investigate the feasibility of the continuous wave (CW) upgrade [1] of the European X-ray free electron laser (XFEL), on-going tests are performed on XFEL prototype and production cryomodules since 2011. For these studies, DESY’s cryomodule test bench (CMTB) has been equipped with a 105 kW CW capable inductive output tube (IOT) in addition to the 10 MW pulsed klystron, making CMTB a very flexible test stand, enabling both CW and pulsed operation. XFEL-like low level radio frequency (LLRF) electronics is used for these tests to stabilize amplitude and phase of the voltage vector sum (VS) of all 8 cavities of the cryomodule under test. The cryomodule most often tested is the pre-series XM-3, unique since it is housing one fine grain niobium and seven large grain niobium cavities. Modified flanges were installed in autumn 2017 on all 8 input couplers to increase the maximum reachable loaded quality factor (QL) beyond 2E7. With higher QL, up to 6E7 for 6 cavities and 3E7 for 2 cavities, we have investigated the VS stability and SRF-performance of this cryomodule under various conditions of cooling down rate and operation temperature 1.65K, 1.8K and 2K, at gradients up to 21.5 MV/m. The results of these tests are presented in 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.

The CryoModule Test Bench (CMTB) is a facility to perform tests on superconducting accelerating modules. The 120 kW Inductive Output Tube (IOT) installed in the facility allows driving the eight superconducting cavities inside the module under test in a vector-sum or single cavity control fashion with average Continuous Wave (CW) gradients higher than 20 MV/m. The scope of these tests is to evaluate the feasibility of upgrading the European X-ray Free Electron Laser (EuXFEL) to CW operation mode. Following the successful tests done on a prototype module XM-3, the initial performance results on production module XM50 will be presented in this paper. Because of EuXFEL requirements, XM50.1 is equipped with modified couplers that allow a variable Loaded Quality factor (QL) to values higher than 4×107. A cost relevant open question is the maximum QL that can be operated at, while maintaining the system within the EuXFEL field stability specifications of 0.01 % in amplitude and 0.01 deg in phase. Because of this, the LLRF system capability of rejecting microphonics and RF disturbances, as well as Lorentz Force Detuning (LFD) related effects in open and closed loop is of prime interest.

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.

In the paper we would like to present the performance of superconducting radio frequency (RF) TESLA 9-cell 1.3 GHz cavity operated at continuous wave (CW). The cavity has been setup for extremely high loaded quality factor (QL of order of 3 ·10) and gradients up to 23 MV/m. The design hardware and firmware components as well as developed high level software procedures allows automatic procedure of cavity ramping up from low to high gradient operation. The microphonics as well as a pendoromotive effects are sensed, identified and applied for cavity detuning correction. The RF and piezo feedbacks are demonstrated and preliminary results are briefly discussed.

The European X-ray Free-Electron Laser (XFEL) at Deutsches Elektronen-Synchrotron (DESY), Hamburg, Germany is a user facility providing ultra-short hard and soft X-ray flashes with a high brilliance. All low level radio frequency (LLRF) stations of the injector, covering the normal conducting RF gun, A1 (8 1.3 GHz superconducting cavities (SCCs)) and AH1 (8 3.9 GHz SCCs), were successfully commissioned by the end of 2015. The commissioning of LLRF stations A2 to A23 (32 1.3 GHz SCCs each) in the XFEL accelerator tunnel (XTL) was concluded in June 2017. Self-amplified spontaneous emission (SASE) light was produced in undulator section SA1 and delivered to the first users in September 2017, marking the beginning of regular user operation. The current state of the LLRF systems, the experience gained during operation and the performance achieved in terms of stability and energy reach are presented.

We present a compact RF control system for supercon-ducting radio frequency (SCRF) single cavities based on MicroTCA.4 equipped with specialized advanced mezzanine cards (AMCs) and rear transition modules (RTMs). To sense the RF signals from the cavity and to drive the high power source, a DRTM-DWC8VM1 module is used equipped with 8 analog field detectors and one RF vector modulator. Fast cavity frequency tuning is achieved by piezo-actuators at-tached to the cavity and a RTM piezo-driver module (DRTM-PZT4). Data processing of the RF signals and the real-time control algorithms are implemented on a Virtex-6 and a Spartan-6 FPGAs within two AMC’s (SIS8300-L2V2 and DAMC-FMC20). The compact single cavity control system was tested at Cryo Module Test Bench (CMTB) at DESY. Software and firmware were developed to support all possible modes, the short pulse (SP), the long pulse (LP) and CW operation mode with duty cycles ranging from 1% to 100%. The SP mode used a high power multi-beam klystron at high loaded quality factor ( Q L ) of 3 · 10 6 . For the LP mode (up to 50% duty cycle) and the CW mode a 120 kW IOT tube was used at Q L up to 1 . 5 · 10 7 . Within this pa-per we present the achieved performance and report on the operation experience on such system.

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

Modern digital low level radio frequency (LLRF) control systems used to stabilize the accelerating field in facilities such as Free Electron Laser in Hamburg (FLASH) or European X-Ray Free Electron Laser (E-XFEL) are based on the Field Programmable Gate Array (FPGA) technology. Presently these accelerator facilities are operated with pulsed RF. In future, these facilities should be operated with continuous wave (CW) which requires significant modifications on the real-time feedbacks realized within the FPGA. For example, higher loaded quality factor of the cavities when operated in a CW mode requires sophisticated resonance control methods. However, iterative learning techniques widely used for machines operated in pulsed mode are not applicable for CW. In addition, the mechanical characteristic of the cavities have now a much more important impact on the choice of the feedback scheme. To overcome the limitations of classical PI-controllers novel real-time adaptive feed forward algorithm is implemented in the FPGA. Also, the high power RF amplifier which is an inductive output tube (IOT) for continuous wave operation instead of a klystron for the pulsed mode has major impact on the design and implementation of the firmware for regulation. In this paper, we report on our successful approach to control multi-cavities with ultra-high precision (dA/A<0.01%, dphi<0.02 deg) using a single IOT source and individual resonance control through piezo actuators. Performance measurements of the proposed solution were conducted at Cryo Module Test Bench (CMTB) facility.