At the Free-Electron Laser in Hamburg (FLASH) and the European X-Ray Free-Electron Laser, superconducting TeV-energy superconducting linear accelerator (TESLA)-type cavities are used for the acceleration of electron bunches, generating intense free-electron laser (FEL) beams. A long rf pulse structure allows one to accelerate long bunch trains, which considerably increases the efficiency of the machine. However, intrabunch-train variations of rf parameters and misalignments of rf structures induce significant trajectory variations that may decrease the FEL performance. The accelerating cavities are housed inside cryomodules, which restricts the ability for direct alignment measurements. In order to determine the transverse cavity position, we use a method based on beam-excited dipole modes in the cavities. We have developed an efficient measurement and signal processing routine and present its application to multiple accelerating modules at FLASH. The measured rms cavity offset agrees with the specification of the TESLA modules. For the first time, the tilt of a TESLA cavity inside a cryomodule is measured. The preliminary result agrees well with the ratio between the offset and angle dependence of the dipole mode which we calculated with eigenmode simulations.

The preservation of low-emittance electron beams will continue to be a challenge and an objective in rflinac-driven accelerators where off-axis steering can lead to both transverse long-range wakefields (LRWs) and short-range wakefields (SRWs) that dilute the emittance [1]. Earlier experimentalists using normal conducting S-band and L-band accelerators have mitigated these effects by steering the beam optimally through the cavities while watching downstream imaging screens [2] or streak camera images [3]. One can even tune the wakefields to cancel some of the effects in a few normal conducting L-band structures by not centering the beam on a screen or BPM after each structure in simulations [4]. Since the transverse wakefields depend on 1/a where a is the cavity bore radius, it was somewhat surprising to identify both LRWs including higher order modes (HOMs) [5] and SRWs [6] in the superconducting rf TESLA-type cavities with their larger 35-mm radii. These cavities are used in major accelerator facilities (FLASH and the European XFEL), the under-construction LCLS-II XFEL, the Superconducting Test Facility in Japan, and proposed for the conceptual International Linear Collider (ILC) in Japan. Recent tests at Fermilab showed that near-resonance conditions of an HOM frequency with a beam harmonic resulted in submacropulse centroid oscillations at 100 kHz that diluted macropulse-averaged beam size [5]. More importantly, the same off-axis steering resulted in the generation of SRWs whose submicropulse transverse head-tail kicks produced projected beam size dilutions of 40% and greater in the sampled distributions, an effect at least 5x larger than that of the HOMs [6]. Such effects would also dilute the emittance values, and they would be a particular problem for ultra-low emittance preservation. These effects were seen after only two TESLA-type cavities with beam injected at 4.5 MeV and a final energy of 41 MeV. The transverse wakes depend on charge, beam offset, and the SQRT of bunch length, but inversely on beam energy. Thus, the emittance dilution threat is highest at the lower energies in the first accelerator cavities after the gun such as occurs in the LCLS-II injector with <1 MeV into the cryomodule. This same principle applies to all accelerators in labs around the country at Fermilab, SLAC, and Argonne. In the case of the SC rf cavities, HOM couplers provide online signals of dipolar modes dependent on beam offset and downstream streak camera images or a rf transverse deflecting cavity (TDC) plus screen provide submicropulse information. Such a scenario of multiple options for steering and tracking beam effects should be a prime application for machine learning techniques with extensions to virtual diagnostics [7].

With FLASH2020+, a major upgrade of the FLASH facility has started to meet the new requirements of the growing soft X-ray user community. The design of the FEL beamlines aims at photon properties suitable to the needs of future user experiments with high repetition rate XUV and soft X-ray radiation. By the end of the project, both existing FEL lines at FLASH will be equipped with fully tunable undulators capable of delivering photon pulses with variable polarization. The use of external seeding at 1 MHz in burst mode is part of the design of the new FLASH1 beamline, while FLASH2 will exploit novel lasing concepts based on different undulator configurations. The new FLASH2020+ utilise electron beam energies up to 1.35 GeV which will extend the accessible wavelength range to the oxygen K-edge with variable polarization. This will be complemented by new laser sources for pump and probe experiments and new experimental stations. FLASH FEL USER FACILITY Since the first operations of the FLASH FEL for users started more than 15 years ago [1, 2] the facility has experienced a series of upgrades in order to extend its capabilities and to fulfil the increasing demands of the growing X-ray user community. The last major upgrade involved the implementation of a second FEL line [3] allowing to serve two experiments simultaneously with parameters as reported in Table 1 and in [4]. Table 1: Current FLASH FEL Parameters

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.

TESLA cavities are used at the free-electron laser in Hamburg (FLASH) to accelerate electron bunches for generating intense free-electron laser beams. Two specially designed couplers mounted at both ends of each cavity damp the higher order modes (HOMs) excited by the beam in these cavities. By using a specific dipole mode signal extracted from these couplers, one can determine the transverse beam position as well as the dipole mode polarizations and center in each cavity. We introduce a new method based on fitting the dipole mode signal to correlate the dipole mode amplitude and phase to the measured beam position. With this method, the TESLA cavities can be used as HOM-based beam position monitors (HOMBPMs), delivering consistent results over several months, with a resolution better than 10 mu m RMS. We implemented this method for a whole accelerating module for beam orbit monitoring. The method also delivers the polarization axes and the center of the dipole mode for all eight cavities in this module. Our measurements show that the cavities did not move inside the module within almost one year.

It is important to verify both by simulation and experiments the wakefields in superconducting radio frequency (SRF) cavities, which can degrade the electron beam quality considerably or impose excessive heat load if left undamped. In this paper, we investigate the Higher Order Mode (HOM) spectra of the 3.9 GHz SRF cavities, which are assembled in a cryogenic module and are used to linearize the longitudinal phase space of the electron beam in the injector of the European XFEL. The HOM spectra are significantly different from the ones from a single cavity due to the coupling of the modes amongst cavities. The measurements not only provide direct input for the beam dynamics studies but also for the beam instrumentation utilizing these modes. The mode spectra are also investigated with a number of numerical simulations and the comparison with measurements shows favorable agreement.

Historically the FLASH (Free Electron Laser in Hamburg) facility at DESY (Deutsches Elektronen-Synchrotron) in Germany has foreseen operation in a charge range from 1 nC-3 nC for which a VME based BPM (Beam Position Monitor) system has been in operation since 2005 with a later upgrade to lower charges. In the past few years the standard machine operation settled at a few hundreds of pC with a tendency to smaller charges down to 100 pC and less. The performance of the BPM system at charges below 300 pC was in many locations along the machine unsatisfactory, making the operation of the linac more unstable and less reliable. Therefore a new BPM electronic system based on the utca (Micro Telecommunication Computing Architecture) for physics MTCA.4 standard has been developed to overcome the limitations of the old electronics and has already been successfully in operation in FLASH 2. A substantially improved version of the RF (radio frequency) front-end and digital electronics/firmware has been developed in 2016 and tested successfully. The peak detector electronics have been extended to a double peak detection in four channels and the fully customized Firmware is working machine synchronous. In summer 2017 all old button and strip-line BPM electronics have been replaced with the new type. This paper summarizes the features and performance of the new BPM system, compares the beam jitter free resolution of the old and new BPM system and highlights its high single shot resolution of better than 10 μm rms at a charge of 15 pC.

The control of beam phase relative to the accelerating RF field within a superconducting cavity is important in many accelerator applications and is of particular importance for a free electron laser facility. As standard practice, the phase is usually inferred from the beam-induced transient field with respect to a timing reference. We report here on an alternative and novel means of beam phase determination based on beam-excited higher order electromagnetic modes and the accelerating electromagnetic mode, which are conveniently available from the same coupler. The monopole modes are immune to the electron beam offset and therefore are best suited for the task. A coupled circuit model is used to assist the development and to rapidly assess the facility of the method. Simulations based on the circuit model indicate that the resolution of this system depends critically on the signal to noise ratio. Beam-based measurements with a test setup were carried out at the European X-ray Free Electron Laser (XFEL), Germany. Based on this new method, we have routinely obtained a resolution of 0.1°. The best resolution observed with the current setup was 0.03°. These results agree very well with the predictions from those predicted by a circuit model. The system investigated here can be used to provide diagnostic information for the current low level RF system employed in the European XFEL. To this end, the associated electronics are under development. This monitor is the first of its kind that can deliver direct and online measurements of the beam phase with respect to the RF field.

Chains of superconducting radio-frequency resonators are key components of modern particle accelerators such as the European XFEL, which is currently under construction in the north of Germany. In addition to the accelerating mode of the resonators, their beam excited higher order modes are of special interest, because they can harm the beam quality. In contrast to the accelerating mode, these modes are in general not confined within single resonators of the cavity string. For instance, eigenmodes can be localized between adjacent cavities or can be distributed along the entire chain of cavities. Therefore, the full chain has to be considered for a reasonable investigation of its resonant spectra. Accounting for such complex structures is computationally challenging and is therefore often avoided. In this article, the challenge is faced by using the so-called state-space concatenation approach, which is a combination of domain decomposition and model-order reduction. The technique allows for a reduction of the number of degrees of freedom by a factor of approximate to 1.471 x 10(-4). The method is employed to generate a compendium of eigenmodes in the chain of third harmonic cavities for the European XFEL. The results are discussed in detail and are compared with experimental measurements. The compendium serves as a reference for experiments (inter alia for diagnostics based on higher order modes) at the third harmonic cavity string of the European XFEL, it allows for qualitative understanding of resonant effects appearing in chains of cavities, and it is meant to be a proof of principle of the state-space concatenation approach to handle very long and complex radio-frequency structures. To the authors' knowledge, it is the first time that a modal compendium of a structure with the given complexity is generated. The article presents geometrical details of the chain, defines quantities relevant to superconducting radio-frequency cavities, and describes the employed computational approach.

Received 31 May 2017DOI:https://doi.org/10.1103/PhysRevAccelBeams.20.079901Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetismMethods in electromagnetismRadio frequency calculationsRadio frequency techniquesSuperconducting RFAccelerators & Beams

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 beam phase relative to the accelerating field is of vital importance for the quality of photon beams produced in modern Free Electron Lasers based on superconducting cavities. Normally, the phase is determined by detecting the transient field induced by the beam. In this way the phase of each cavity is checked and adjusted typically every few months. In this paper, we present an on-line method of beam phase determination, based on higher order modes (HOMs) excited in the 2 monopole band by the beam inside these cavities. A circuit model of this HOM band is also presented. Various effects on the resolution have been studied. The results indicate that the resolution is strongly dependent on the signal to noise ratio and the sampling rate. Preliminary experimental results, based on a broadband setup, reveal a resolution of ca. 0.1 RMS. These are in good agreement with simulation results. The work will pave the way for a dedicated system of beam phase monitoring, which is under development for the European XFEL which requires the beam phase to be measured to within 0.01 RMS. This will be the first implementation of a dedicated on-line beam phase monitor, based on beam-excited HOMs in accelerating cavities.

The FLASH (Free Electron Laser in Hamburg) facility at DESY (Deutsches Elektronen-Synchrotron) in Germany has been extended by the new undulator line FLASH2 providing twice as many experimental stations for users in the future. After the acceleration of the electron bunch train up to 1.2GeV in FLASH, a part of the beam can be kicked into FLASH2, while the other is going to the old undulator line of FLASH I. The commissioning phase of FLASH2 started in early 2014 and is continued parasitically during user operation in FLASH1. One key point during first beam commissioning is the availability of standard diagnostic devices such as Beam Position Monitors (BPMs). In the last couple of years new electronics for button and strip-line BPMs have been developed, based on the MTCA.4 standard. This new Low Charge BPM (LCBPM) system is designed to work with bunch charges as small as 100 pC in contrast to the old systems at FLASH initially designed for bunch charges of 1nC and higher. This paper summarizes the performance of the BPM system and discusses the applied methods based on beam based correlation techniques to study the system.

The 3.9 GHz cryomodule and RF system for the XFEL Injector is being assembled and delivered to the underground building in summer 2015, for the injector commissioning in Fall 2015. This contribution outlines the status of the activity and reports the preparation stages of the technical commissioning of the system.

HOMBPMs (HOM based Beam Position Monitors) are installed at the FLASH facility at DESY, Hamburg. These are aimed at aligning the beam and monitoring the beam position. Over time, the accuracy of beam position prediction is degraded. This is due to instability issues in the 1.3 GHz and 3.9 GHz superconducting cavities and associated electronics. In this paper, we demonstrate for the first time a measurement technique which is stable and can be relied upon over a period of three months with unprecedented resolution (below 4 μm horizontally and 2 μm vertically). We attribute this improvement in stability to a focused campaign on various signal processing and analysis techniques. These techniques include SVD (Singular Value Decomposition), ANN (Artificial Neural Network) and PLS (Partial Least Square). We found the best resolution and computational power using the latter method, PLS. These techniques are directly applicable to the HOMBPM system at the European XFEL that is currently under construction. However, they are in many ways generic and hence applicable to other measurement methods.

The FLASH (Free Electron Laser in Hamburg) facility at DESY (Deutsches Elektronen-Synchrotron) in Germany has been extended by a new undulator beam line called FLASH2 to provide twice as many experimental stations in the future [1]. After the acceleration of the electron bunch train up to 1.2 GeV, a part can be kicked into FLASH2, while the other is going to the old undulator beam line. In order to tune the wavelength of the SASE (Self Amplified Sponta-neous Emission), the new line is equipped with variable gap undulators. The commissioning phase of FLASH2 started in early 2014 and continues mostly parasitically during user operation in FLASH1. One key point during first beam commissioning is the availability of standard diagnostic devices such as BPM (Beam Position Monitor) [2]. In this paper we present the design and first operational experience of a new BPM system for button and strip-line monitors based on MTCA.4 [3]. This is referred to as LCBPM (low charge BPM) in contrast to the old systems at FLASH initially designed for bunch charges of 1 nC and higher. We sum-marize the recent analog and digital hardware development progress [4,5] and first commissioning experience of this new BPM system at FLASH2 and present a first estimation of its resolution in a large charge range from 1 nC down to 100pC and smaller.

This article presents a comparison between measured and simulated scattering parameters in a wide frequency interval for the third harmonic accelerating module ACC39 in the linear accelerator FLASH, located at DESY in Hamburg/Germany. ACC39 is a cryomodule housing four superconducting 3.9 GHz accelerating cavities. Due to the special shape of the cavities (in particular its end cells and the beam pipes) in ACC39, the electromagnetic field in the module is, in many frequency ranges, coupled from one cavity to the next. Therefore, the scattering parameters are determined by the entire string and not solely by the individual cavities. This makes the determination of the scattering properties demanding. As far as the authors can determine, this paper shows for the first time a direct comparison between state-of-the-art simulations and measurements of rf properties of long, complex, and asymmetric structures over a wide frequency band. Taking into account the complexity of the system and various geometrical unknowns, the agreement between experimental measurements and simulations is remarkably good for several distinct measurements, although a variety of effects (e. g. cavity deviations from the ideal shape or interactions with not modeled parts of the structure) is not considered in the computer simulation. After a short introduction, the paper provides detailed descriptions of simulations and experimental measurements performed at the module. In this context, the estimation of the cable properties is discussed as well. As a central part of the article, the comparison between measured and simulated transmission spectra and quality factors is presented. This study represents one of the first detailed comparisons between simulations and measurements for a coupled accelerator cavity system.

Summary CBPMs at FLASH2 Introduction The development of the CBPM system for the European XFEL is in an advanced state. An E-XFEL pre-series version of the CBPM pickups and electronics has been installed and tested in FLASH1 and FLASH2, and already fulfills the requirements for E-XFEL. Future activities will focus on improvement of lab and beam-based calibration techniques, as well as on improved automated range control and digital signal processing to further improve the CBPM system performance. • Commissioning of FLASH2 has started, a new soft Xray FEL undulator line at the DESY FLASH facility. • 17 cavity beam position monitor (CBPM) pick-ups and electronics [1] developed for the European XFEL (EXFEL) included. • Four CBPMs are available at FLASH1 for test and development. • The CBPM system enables an unprecedented position and charge resolution at FLASH. • Results of first beam measurements as well as correlations with other FLASH diagnostics systems are reported. CBPMs at FLASH1 • Cavity BPMs with sub-micron noise and drift will be used for the alignment of the electron beam with the photon beam in the undulator area at the European X-FEL [2]; for a detailed description of a cavity BPM see [3,4]. • Two types of CBPMs: 10 mm aperture and 100 mm length, a second CBPM type with 40.5 mm aperture and 255 mm length will be installed in some locations. • A test area for the verification of the performance of both CBPM types has been installed at FLASH1 after the last undulator. • The CBPM electronics, including its embedded FPGA firmware and software, is provided in an In-kind contribution from PSI. • Both CBPM have the same electronics because the BPM pickups have the same frequency of 3.3 GHz and similar loaded Q for their position and reference resonator. • In addition to FLASH1, a second undulator beamline FLASH2 [5] has been built to extend the capability of the FLASH soft X-ray FEL facility [6] with 17 CBPMs. FLASH Undulator BPM Commissioning and Beam Characterization Results.