Electron Diagnostics, Timing, Synchronization, and Controls
Paper Title Page
WEB01 Identification and Mitigation of Smoke-Ring Effects in Scintillator-Based Electron Beam Images at the European XFEL 301
 
  • G. Kube, S. Liu, A.I. Novokshonov, M. Scholz
    DESY, Hamburg, Germany
 
  Standard transverse beam profile measurements at the European XFEL are based on scintillating screen monitors using LYSO:Ce. While it is possible to resolve beam sizes down to a few micrometers with this scintillator, the experience during the XFEL commissioning showed that the measured emittance values were significantly larger than the expected ones. In addition, beam profiles measured at bunch charges of a few hundred pC showed a ’smoke ring’ structure. While coherent OTR emission and beam dynamical influence can be excluded, it is assumed that the profile distortions are caused by effects from the scintillator material. Following the experience in high energy physics, a simple model was developed which takes into account quenching effects of excitonic carriers inside a scintillator in a heuristic way. Based on this model, the observed beam profiles can be understood qualitatively. Together with the model description, first comparisons with experimental results will be shown. Possible new scintillator materials suitable for beam profile diagnostics and first test results from beam measurements will be presented.  
slides icon Slides WEB01 [5.057 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEB01  
About • paper received ※ 20 August 2019       paper accepted ※ 28 August 2019       issue date ※ 05 November 2019  
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WEB02 Wire-Scanners with Sub-Micrometer Resolution: Developments and Measurements 307
 
  • G.L. Orlandi, S. Borrelli, Ch. David, E. Ferrari, V. Guzenko, B. Hermann, O. Huerzeler, R. Ischebeck, C. Lombosi, C. Ozkan Loch, E. Prat
    PSI, Villigen PSI, Switzerland
  • N. Cefarin, S. Dal Zilio, M. Lazzarino
    IOM-CNR, Trieste, Italy
  • M. Ferianis, G. Penco, M. Veronese
    Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
 
  Monitors of the beam transverse profile with ever more demanding spatial resolution and minimal invasivity are required by the FEL community. In order to improve the spatial resolution towards the sub-micrometer limit as well as to decrease the impact on the lasing process, nano-fabricated wire-scanners have been manufactured independently at PSI and FERMI by means of a lithographic technique [1,2]. Experimental tests carried out at SwissFEL at a low emittance demonstrated the capability of such innovative wire-scanner solutions to resolve beam transverse profiles with a size of 400-500 nm without being affected by any resolution limit. Status and outlook of nano-fabricated wire-scanners will be presented.
[1] M. Veronese et al., NIM-A, 891, 32-36, (2018).
[2] S. Borrelli et al., Comm. Phys.-Nature, 1, 52 (2018).
 
slides icon Slides WEB02 [11.196 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEB02  
About • paper received ※ 24 August 2019       paper accepted ※ 28 August 2019       issue date ※ 05 November 2019  
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WEB03 Application of Machine Learning to Beam Diagnostics 311
 
  • E. Fol, R. Tomás
    CERN, Meyrin, Switzerland
  • J.M. Coello de Portugal
    PSI, Villigen PSI, Switzerland
  • G. Franchetti
    GSI, Darmstadt, Germany
 
  Machine Learning (ML) techniques are widely used in science and industry to discover relevant information and make predictions from data. The application ranges from face recognition to High Energy Physics experiments. Recently, the application of ML has grown also in accelerator physics and in particular in the domain of diagnostics and control. The target is to provide an overview of ML techniques and to indicate beam diagnostics tasks where ML based solutions can be efficiently applied to complement or potentially surpass existing methods. Besides, a short summary of recent works will be given demonstrating the great interest for use of ML concepts in beam diagnostics and latest results of incorporating these concepts into accelerator problems, with the focus on beam optics related application.  
slides icon Slides WEB03 [5.721 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEB03  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEB04 Few-Femtosecond Facility-Wide Synchronization of the European XFEL 318
 
  • S. Schulz, M.K. Czwalinna, M. Felber, M. Fenner, C. Gerth, T. Kozak, T. Lamb, B. Lautenschlager, F. Ludwig, U. Mavrič, J. Müller, S. Pfeiffer, H. Schlarb, Ch. Schmidt, C. Sydlo, M. Titberidze, F. Zummack
    DESY, Hamburg, Germany
 
  The first facility-wide evaluation of the optical synchronization system at the European XFEL resulted in excellent arrival time stability of the electron bunches at the end of the 2 km long linac, being measured with two individual adjacent femtosecond-resolution bunch arrival time monitors. While each of the monitors is independently linked by a stabilized optical fiber to a master laser oscillator, with one being installed in the injector area and one in the experimental hall, these two reference lasers are tightly synchronized through another few-km long fiber link. Thus, not only the accelerator performance is being benchmarked, but equally the optical synchronization infrastructure itself. Stability on this level can only be achieved by locking the RF for cavity field control to the optical reference and requires an unprecedented synchronization of the master laser oscillator to the main RF oscillator, enabled by a novel RF/optical phase detector. Finally, with the seeders of the experiment’s optical lasers synchronized to the master laser oscillator, first experiments at two independent scientific instruments proved an X-ray/optical timing jitter of few tens of femtoseconds.  
slides icon Slides WEB04 [22.142 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEB04  
About • paper received ※ 20 August 2019       paper accepted ※ 28 August 2019       issue date ※ 05 November 2019  
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WEP003 Balanced Optical-Microwave Phase Detector for 800-nm Pulsed Lasers with Sub-Femtosecond Resolution 322
 
  • K. Şafak, A. Berlin, E. Cano Vargas, H.P.H. Cheng, A. Dai, J. Derksen, M. Neuhaus, P. Schiepel
    Cycle GmbH, Hamburg, Germany
  • F.X. Kärtner
    Deutsches Elektronen Synchrotron (DESY) and Center for Free Electron Science (CFEL), Hamburg, Germany
 
  Novel light-matter interaction experiments conducted in free-electron lasers, ultrafast electron diffraction instruments and extreme light infrastructures require synchronous operation of microwave sources with femtosecond pulsed lasers [1]. In particular, Ti:sapphire lasers have become the most common near-infrared light source used in these facilities due to their wide-range tunability and their ability to generate ultrashort pulses at around 800-nm optical wavelength [2]. Therefore, a highly sensitive optical-to-microwave phase detector operating at 800 nm is an indispensable tool to synchronize these ubiquitous lasers to the microwave clocks of these facilities. Electro-optic sampling is one approach that has proven to be the most precise in extracting the relative phase noise between microwaves and optical pulse trains. However, their implementation at 800-nm wavelength has been so far limited [3]. Here, we show a balanced optical-microwave phase detector designed for 800-nm operation based on electro-optic sampling. The detector has a timing resolution of 0.01 fs RMS for offset frequencies above 100 Hz and a total noise floor of less than 10 fs RMS integrated from 1 Hz to 1 MHz.
[1] M. Xin, K. Shafak and F. X. Kärtner, Optica, vol. 5, no. 12, pp. 1564-1578, 2018.
[2] H. Yang et al., Scientific Reports, vol. 7, no. 39966, 2017.
[3] M. Titberidze, DESY-THESIS-2017-040, 2017.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP003  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEP004 Timing Stability Comparison Study of RF Synthesis Techniques 325
 
  • E. Cano Vargas, F.X. Kärtner
    Deutsches Elektronen Synchrotron (DESY) and Center for Free Electron Science (CFEL), Hamburg, Germany
  • A. Berlin, H.P.H. Cheng, A. Dai, J. Derksen, P. Schiepel, K. Şafak
    Cycle GmbH, Hamburg, Germany
 
  Funding: Deutsches Elektronen-Synchrotron (DESY); Cycle GmbH.
High-precision and low-noise timing transfer from a master clock to different end stations of a free-electron laser (FEL) is an essential task.[1] Timing precisions ranging from few tens of femtoseconds to sub-femtoseconds are required for seeded FELs and attosecond science centers. Mode-locked lasers referenced to RF standards are commonly used as master oscillators, due to their superior stability and timing precision, depicting timing jitter in the attosecond range.[2] In this matter, one of the biggest challenges is to transfer the timing stability of mode-locked lasers to RF sources. Here, we compare and contrast two of the most common techniques used for laser-to-RF synthesis in FEL facilities: (i) RF signal extraction from the optical pulse train using photodiodes, and (ii) VCO-to-laser synchronization. Test setups are built to measure both the absolute phase noise of the generated RF signal and the relative timing jitter with respect to the mode-locked laser. Short-term timing jitter values varying between 10 and 100 fs are achieved for different test setups, while long term timing drift ranging to some hundreds of fs due to environmental influence are observed.
[1] M. Xin, K. Shafak and F.X. Kärtner, Optica, vol. 5, no. 12, pp. 1564-1578, 2018.
[2] J. Kim, F.X. Kärtner, Opt. Lett., vol. 32, pp. 3519-3521, 2007.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP004  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEP006 A PolariX TDS for the FLASH2 Beamline 328
 
  • F. Christie, J. Rönsch-Schulenburg, M. Vogt
    DESY, Hamburg, Germany
 
  Transverse Deflecting RF-Structures (TDS) are successfully used for longitudinal diagnostic purposes at many Free-Electron Lasers (FEL) (LCLS, FLASH, EU-XFEL, FERMI). Moreover, by installing a TDS downstream of the FEL undulators and placing the measurement screen in a dispersive section, the temporal photon pulse structure can be estimated, as was demonstrated at LCLS and sFLASH. Here we describe the installation of a variable polarization X-band structure (PolariX TDS [1]) downstream of the FLASH2 undulators. The installation of such a TDS enables longitudinal phase space measurements and photon pulse reconstructions, as well as slice emittance measurements in both planes using the same cavity due to the unique variable polarization of the PolariX TDS.
[1] P. Craievich et al., "Status of the PolariX-TDS Project", in Proc. IPAC’18, Vancouver, Canada (2018)
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP006  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEP007 Usage of the MicroTCA.4 Electronics Platform for Femtosecond Synchronization Systems 332
 
  • M. Felber, E.P. Felber, M. Fenner, T. Kozak, T. Lamb, J. Müller, K.P. Przygoda, H. Schlarb, S. Schulz, C. Sydlo, M. Titberidze, F. Zummack
    DESY, Hamburg, Germany
 
  At the European XFEL and FLASH at DESY optical synchronization systems are installed providing sub-10 femtosecond electron bunch arrival time stability and laser oscillator synchronization to carry out time-resolved pump-probe experiments with high precision. The synchronization system supplies critical RF stations with short- and long-term phase-stable reference signals for precise RF field detection and control while bunch arrival times are processed in beam-based feedbacks to further time-stabilize the FEL pulses. Experimental lasers are tightly locked to the optical reference using balanced optical cross-correlation. In this paper, we describe the electronic hardware for supervision and real-time control of the optical synchronization system. It comprises various MicroTCA.4 modules including fast digitizers, FPGA processor boards, and drivers for piezos and stepper-motors. Advantages of the system are the high-level of integration, state-of-the-art performance, flexibility, and remote maintainability.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP007  
About • paper received ※ 20 August 2019       paper accepted ※ 26 August 2019       issue date ※ 05 November 2019  
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WEP008 Multi-Beamline Operation at the European XFEL 335
 
  • L. Fröhlich, A. Aghababyan, V. Balandin, B. Beutner, F. Brinker, W. Decking, N. Golubeva, O. Hensler, Y. Janik, R. Kammering, H. Kay, T. Limberg, S. Liu, D. Nölle, F. Obier, M. Omet, M. Scholz, T. Wamsat, T. Wilksen, J. Wortmann
    DESY, Hamburg, Germany
 
  The European XFEL uses a unique beam distribution scheme to direct electron bunches to its three undulator lines. The accelerator delivers up to 600 microsecond long bunch trains, out of which parts or individual bunches can be selected for photon production in any of the FELs. This contribution gives a brief overview of the kicker-septum scheme facilitating this and highlights how even complex bunch patterns can easily be configured via the timing system.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP008  
About • paper received ※ 19 August 2019       paper accepted ※ 29 August 2019       issue date ※ 05 November 2019  
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WEP009 Long Term Stability and Slow Feedback Performance at the European XFEL 339
 
  • R. Kammering
    DESY, Hamburg, Germany
 
  The European XFEL is now routinely running in user operation since more than two years. Up to 8 longitudinal and 9 transversal slow feedback loops are routinely used to keep the accelerators chosen operation conditions. First tests of comparing the machine ’free-floating’ state versus fully fixing all relevant monitoring signals have been carried out and show interesting results. Here we will review the feedback systems in terms of software architecture and conceptual layout but also in respect to feedback and FEL performance.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP009  
About • paper received ※ 20 August 2019       paper accepted ※ 29 August 2019       issue date ※ 05 November 2019  
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WEP010 Femtosecond Laser-to-RF Synchronization and RF Reference Distribution at the European XFEL 343
 
  • T. Lamb, M. Felber, T. Kozak, J. Müller, H. Schlarb, S. Schulz, C. Sydlo, M. Titberidze, F. Zummack
    DESY, Hamburg, Germany
 
  At the European XFEL, optical pulses from a mode-locked laser are distributed in an optical fiber network providing femtosecond stability throughout the accelerator facility. Due to the large number of RF reference clients and because of the expected higher reliability, the 1.3 GHz RF reference signals are distributed by a conventional coaxial RF distribution system. However, the provided ultra-low phase noise 1.3 GHz RF reference signals may drift over time. To remove these drifts, an optical reference module (REFM-OPT) has been developed to detect and correct environmentally induced phase errors of the RF reference. It uses a femtosecond long-term stable laser-to-RF phase detector, based on an integrated Mach-Zehnder amplitude modulator (MZM), to measure and resynchronize the RF phase with respect to the laser pulses from the optical synchronization system with high accuracy. Currently nine REFM-OPTs are permanently operated at the European XFEL, delivering femtosecond stable RF reference signals for critical accelerating field control stations. The operation experience will be reported together with a detailed evaluation of the REFM-OPT performance.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP010  
About • paper received ※ 20 August 2019       paper accepted ※ 28 August 2019       issue date ※ 05 November 2019  
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WEP011 Longitudinal Intra-Train Beam-Based Feedback at FLASH 346
 
  • S. Pfeiffer, Ł. Butkowski, M.K. Czwalinna, B. Dursun, C. Gerth, B. Lautenschlager, H. Schlarb, Ch. Schmidt
    DESY, Hamburg, Germany
 
  The longitudinal intra-train beam-based feedback has been recommissioned after major upgrades on the synchronization system of the FLASH facility. Those upgrades include: new bunch arrival time monitors (BAMs), the optical synchronization system accommodating the latest European XFEL design based on PM fibers, and installation of a small broadband normal conducting RF cavity. The cavity is located prior to the first bunch compressor at FLASH and allows energy modulation bunch-by-bunch (1 us spacing) on the per mille range. Through the energy dependent path length of the succeeding magnetic chicane the cavity is used for ultimate bunch arrival time corrections. Recently the RF cavity operated 1 kW pulsed solid-state amplifier was successfully commissioned. First tests have been carried out incorporating the fast cavity as actuator together with SRF stations for larger corrections in our intra-train beam-based feedback pushing now arrival time stabilities towards 5 fs (rms). The latest results and observed residual instabilities are presented.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP011  
About • paper received ※ 20 August 2019       paper accepted ※ 17 September 2019       issue date ※ 05 November 2019  
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WEP012 THz Spectroscopy with MHz Repetition Rates for Bunch Profile Reconstructions at European XFEL 350
 
  • N.M. Lockmann, C. Gerth, B. Schmidt, S. Wesch
    DESY, Hamburg, Germany
 
  The European X-ray Free-Electron Laser generates most powerful and brilliant X-ray laser pulses. Exact knowledge about the longitudinal electron bunch profile is crucial for the operation of the linear accelerator as well as for photon science experiments. The only longitudinal diagnostic downstream of the main linac is based on spectroscopy of diffraction radiation (DR). The spectral intensity of the DR in the THz and infrared regime is monitored by a four-staged grating spectrometer and allows non-invasive bunch length characterization based on form factor measurements in the range 0.7 - 60 THz. As the readout and signal shaping electronics of the spectrometer allow MHz readout rates, the longitudinal bunch profile of all bunches inside the bunch train can be characterized non-invasively and simultaneously to FEL operation. In this paper, form factor measurements along the bunch train will be described and presented as well as the resulting reconstructed current profiles.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP012  
About • paper received ※ 20 August 2019       paper accepted ※ 29 August 2019       issue date ※ 05 November 2019  
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WEP013 Fast Kicker System for European XFEL Beam Distribution 353
 
  • F. Obier, W. Decking, M. Hüning, J. Wortmann
    DESY, Hamburg, Germany
 
  A special feature of the European XFEL X-ray laser is the possibility to distribute the electron bunches of one beam pulse to different free-electron laser (FEL) beam-lines. This is achieved through a combination of kickers and a Lambertson DC septum. The integration of a beam abort dump allows a flexible selection of the bunch pattern at the FEL experiment, while the superconducting linear accelerator operates with constant beam loading. The driver linac of the FEL can deliver up to 600 µs long bunch trains with a repetition rate of 10 Hz and a maximum energy of 17.5 GeV. The FEL process poses very strict requirements on the stability of the beam position and hence on all upstream magnets. It was therefore decided to split the beam distribution system into two kicker systems, long pulse kickers with very stable amplitude (flat-top) and relatively slow pulses and fast stripline kickers with moderate stability but very fast pulses. This contribution gives a brief overview of the fast kicker system.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP013  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEP014 Long Pulse Kicker for European XFEL Beam Distribution 357
 
  • F. Obier, W. Decking, M. Hüning, J. Wortmann
    DESY, Hamburg, Germany
 
  A special feature of the European XFEL X-ray laser is the possibility to distribute the electron bunches of one beam pulse to different free-electron laser (FEL) beam-lines. This is achieved through a combination of kickers and a Lambertson DC septum. The integration of a beam abort dump allows a flexible selection of the bunch pattern at the FEL experiment, while the superconducting linear accelerator operates with constant beam loading. The driver linac of the FEL can deliver up to 600 µs long bunch trains with a repetition rate of 10 Hz and a maximum energy of 17.5 GeV. The FEL process poses very strict requirements on the stability of the beam position and hence on all upstream magnets. It was therefore decided to split the beam distribution system into two kicker systems, long pulse kickers with very stable amplitude (flat-top) and relatively slow pulses and fast stripline kickers with moderate stability but very fast pulses. This contribution gives a brief overview of the long pulse kicker system.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP014  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEP015 Electro-Optical Bunch Length Detection at the European XFEL 360
 
  • B. Steffen, M.K. Czwalinna, C. Gerth
    DESY, Hamburg, Germany
  • S. Bielawski, C. Evain, E. Roussel, C. Szwaj
    PhLAM/CERCLA, Villeneuve d’Ascq Cedex, France
 
  The electro-optical bunch length detection system based on electro-optic spectral decoding has been installed and is being commissioned at the European XFEL. The system is capable of recording individual longitudinal bunch profiles with sub-picosecond resolution at a bunch repetition rate of 1.13MHz . Bunch lengths and arrival times of entire bunch trains with single-bunch resolution have been measured as well as jitter and drifts for consecutive bunch trains. In addition, we are testing a second electro-optical detection strategy, the so-called photonic time-stretching, which consists of imprinting the electric field of the bunch onto a chirped laser pulse, and then "stretching" the output pulse by optical means. As a result, we obtain is a slowed down "optical replica" of the bunch shape, which can be recorded using a photodiode and GHz-range acquisition. These tests are performed in parallel with the existing spectral decoding technique based on a spectrometer in order to allow a comparative study. In this paper, we present first results for both detection strategies from electron bunches after the second bunch compressor of the European XFEL.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP015  
About • paper received ※ 24 August 2019       paper accepted ※ 28 August 2019       issue date ※ 05 November 2019  
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WEP016 Precise Laser-to-RF Synchronization of Photocathode Lasers 364
 
  • M. Titberidze, M. Felber, T. Kozak, T. Lamb, J. Müller, H. Schlarb, S. Schulz, C. Sydlo, F. Zummack
    DESY, Hamburg, Germany
 
  RF photo-injectors are used in various large, mid and small-scale accelerator facilities such as X-ray Free Electron Lasers (XFELs), external injection-based laser-driven plasma accelerators (LPAs) and ultrafast electron diffraction (UED) sources. Many of these facilities require a high precision synchronization of the photo-injector laser system, either because of beam dynamics reasons or the photo-injector directly impacting pump-probe experiments carried out to study physical processes on femtosecond timescales. It is thus crucial to achieve synchronization in the order of 10 fs rms or below between the photocathode laser and the RF source driving the RF gun. In this paper, we present the laser-to-RF synchronization setup employed to lock a commercial near-infrared (NIR) photocathode laser oscillator to a 2.998 GHz RF source. Together with the first results achieving ~ 10 fs rms timing jitter in the measurement bandwidth from 10 Hz up to 1 MHz, we describe an advanced synchronization setup as a future upgrade, promising even lower timing jitter and most importantly long-term timing drift stability.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP016  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEP019 Concept of a Novel High-Bandwidth Arrival Time Monitor for Very Low Charges as a Part of the All-Optical Synchronization Systems at XFEL and FLASH 368
 
  • A. Penirschke
    THM, Friedberg, Germany
  • W. Ackermann
    TEMF, TU Darmstadt, Darmstadt, Germany
  • M.K. Czwalinna, H. Schlarb
    DESY, Hamburg, Germany
 
  Funding: This work is supported by the German Federal Ministry of Education and Research (BMBF) under contract no. 05K19RO1.
Numerous advanced applications of X-ray free-electron lasers require pulse durations and time resolutions in the order of only a few femtoseconds or better. The generation of these pulses to be used in time-resolved experiments require synchronization techniques that can simultaneously lock all necessary components to a precision in the range of 1 fs only. To improve the experimental conditions at existing facilities and enable future development of seeded FELs, a new all-optical synchronization system at FLASH and XFEL was implemented, which is based on pulsed optical signals rather than electronic RF signals. In collaboration with DESY, Hamburg the all-optical synchronization system is used to ensure a timing stability on the 10 fs scale at XFEL. For a future ultra-low charge operation mode down to 1 pC at XFEL an overall synchronization of (5+1)fs r.m.s. or better is necessary. This contribution presents a new concept for a ultra-wideband pick-up structure for beampipe diameters down to 10 mm for frequencies up to 100 GHz or higher and at the same time providing sufficient output signal for the attached EOMs.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP019  
About • paper received ※ 23 August 2019       paper accepted ※ 28 August 2019       issue date ※ 05 November 2019  
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WEP024 1.3 GHz Solid State Power Amplifier for the Buncher in CTFEL Facility 371
 
  • T.H. He, C.L. Lao, P. Li, X. Luo, L.J. Shan, K. Zhou
    CAEP/IAE, Mianyang, Sichuan, People’s Republic of China
 
  The THz Free Electron Laser facility (CAEP THz FEL, CTFEL) of the China Academy of Engineering Physics uses high quality electron beams to generate high average power terahertz radiations. A 1.3 GHz RF buncher is used in front of the superconducting linear accelerator of the CTFEL facility to improve the electron beams quality. The RF buncher is driven by a solid state power amplifier (SSPA), and the SSPA is feedback controlled by a low level RF (LLRF) control system to ensure the high stability of the amplitude and phase of the bunching field in the buncher cavity. The SSPA operates at 1.3 GHz and outputs 0 to 5 kW of continuous wave power. This paper mainly introduces the principle and composition of the SSPA, and presents some experiments on the RF buncher driven by the SSPA.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP024  
About • paper received ※ 15 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEP030 All-Fiber Photonic, Ultralow-Noise, Robust Optical and Microwave Signal Generators for FELs and UED 382
 
  • J. Kim, I.J. Jeon, D. Kim, D. Kwon
    KAIST, Daejeon, Republic of Korea
 
  Funding: National Research Foundation of Korea (2018R1A2B3001793) and Korea Atomic Energy Research Institute
Optical timing and synchronization is becoming a more important and essential element for ultrafast X-ray and electron science. As a result, compact, ultralow-noise, mechanically robust and long-term stable optical and microwave signal generators are highly desirable for future XFELs and UEDs. Here we show that the combination of mode-locked fiber laser and fiber delay-based stabilization method enables the generation of ultralow-noise optical and microwave signals. We show that all-PM fiber lasers can provide excellent mechanical robustness: stable laser operation over >1 hour is maintained even in continuous 1.5 g vibrations [1]. Using a compactly packaged fiber delay as the timing reference, we could stabilize the repetition-rate phase noise of mode-locked lasers down to -100 dBc/Hz and -160 dBc/Hz at 1 Hz and 10 kHz offset frequency, respectively, at 1 GHz carrier, which corresponds to only 1.4 fs rms absolute timing jitter [1 Hz - 100 kHz] [2]. With DDS-based electronics, low-noise and agile microwave frequency synthesizer was also realized [3]. This new class of photonic signal generator will be suitable for master oscillators in various accelerator-based light sources.
[1] D. Kim et al., Opt. Lett. 44, 1068 (2019)
[2] D. Kwon et al., Opt. Lett. 42, 5186 (2017)
[3] J. Wei et al., Photon. Res. 6, 12 (2018)
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP030  
About • paper received ※ 05 September 2019       paper accepted ※ 22 October 2019       issue date ※ 05 November 2019  
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WEP031 Timing Synchronization Activities for Drift-Free Operation of Ultrafast Electron Diffraction System at KAERI 385
 
  • J. Shin, J. Kim
    KAIST, Daejeon, Republic of Korea
  • I.H. Baek, Y.U. Jeong, H.W. Kim, K. Oang, S. Park
    KAERI, Daejon, Republic of Korea
 
  Funding: This work is funded by KAERI (Grant number: 525350-19)
Precise timing synchronization of an ultrafast electron diffraction facility is essential requirement for femtosecond resolution structure analysis. Recent studies of THz-based electron deflectors have enabled the timing drift measurement between ultrafast electrons and an optical pump beam with few femtosecond resolution [1]. In this work, we will introduce timing synchronization activities to suppress the drift of an electron beam. As timing drift of the electron beam originates from every sub-element, each timing drift contribution from RF transfer, RF-to-optical synchronization, and optical amplification is measured. Timing drift of RF transfer through coaxial cable, which exposed to temperature fluctuation, is actively stabilized from 2 ps to 50 fs by active feedback loop. Further additive drift from RF-to-optical synchronization is maintained below 100 fs. Also optical drift due to the regenerative amplifier, measured by optical correlator, is maintained below 20 fs over an hour. This work allows ultrafast electron diffraction system to operate with less drift correction procedure and increased user availability.
[1] H. Yang et al., "10-fs-level synchronization of photocathode laser with RF-oscillator for ultrafast electron and X-ray sources", Sci. Rep. 7, 39966 (2017).
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP031  
About • paper received ※ 24 August 2019       paper accepted ※ 25 August 2019       issue date ※ 05 November 2019  
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WEP034 Characterization of FEL Spectra Using Specific Figures of Merit 388
 
  • M.A. Pop, F. Curbis
    MAX IV Laboratory, Lund University, Lund, Sweden
  • E. Allaria
    Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
  • F. Curbis
    SLF, Lund, Sweden
 
  By analyzing the spectral content of FEL electron radiation, we can gain new information about the properties of the electron bunch and on the FEL process itself. In this work, we present a peak detection algorithm and its capabilities in characterizing the spectra of seeded FEL.
This work is done in collaboration with FERMI Elettra-Sincrotrone Trieste, Area Science Park, Trieste, Italy
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP034  
About • paper received ※ 20 August 2019       paper accepted ※ 25 August 2019       issue date ※ 05 November 2019  
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WEP035 NIR Spectrometer for Bunch-Resolved, Non-Destructive Studies of Microbunching at European XFEL 392
 
  • S. Fahlström, M. Hamberg
    Uppsala University, Uppsala, Sweden
  • C. Gerth, N.M. Lockmann, B. Steffen
    DESY, Hamburg, Germany
 
  At the European X-ray Free Electron Laser high brilliance femtosecond FEL radiation pulses are generated for user experiments. For this to be achieved electron bunches must be reliably produced within very tight tolerances. In order to investigate the presence of micro-bunching, i.e. charge density variation along the electron bunch with features in the micron range, a prism-based NIR spectrometer with an InGaAs sensor, sensitive in the wavelength range 900 nm to 1700 nm was installed. The spectrometer utilizes diffraction radiation (DR) generated at electron beam energies of up to 17.5 GeV. The MHz repetition rate needed for bunch resolved measurements is made possible by the KALYPSO line detector system, providing a read-out rate of up to 2.7 MHz. We present the first findings from commissioning of the NIR spectrometer, and measurements on the impact of the laser heater system for various bunch compression settings, in terms of amplitude and bunch-to-bunch variance of the NIR spectra as well as FEL pulse energy.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP035  
About • paper received ※ 20 August 2019       paper accepted ※ 29 August 2019       issue date ※ 05 November 2019  
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WEP036 The PolariX-TDS Project: Bead-Pull Measurements and High-Power Test on the Prototype 396
 
  • P. Craievich, M. Bopp, H.-H. Braun, A. Citterio, R. Ganter, T. Kleeb, F. Marcellini, M. Pedrozzi, E. Prat, S. Reiche
    PSI, Villigen PSI, Switzerland
  • R.W. Aßmann, F. Christie, R.T.P. D’Arcy, U. Dorda, M. Foese, P. Gonzalez Caminal, M. Hoffmann, M. Hüning, R. Jonas, O. Krebs, S. Lederer, V. Libov, B. Marchetti, D. Marx, J. Osterhoff, M. Reukauff, H. Schlarb, S. Schreiber, G. Tews, M. Vogt, A. Wagner
    DESY, Hamburg, Germany
  • N. Catalán Lasheras, A. Grudiev, G. McMonagle, W.L. Millar, S. Pitman, K.T. Szypula, W. Wuensch, V. del Pozo Romano
    CERN, Meyrin, Switzerland
  • W.L. Millar
    Cockcroft Institute, Warrington, Cheshire, United Kingdom
 
  A collaboration between DESY, PSI and CERN has been established to develop and build an advanced modular X- band transverse deflection structure (TDS) system with the new feature of providing variable polarization of the deflecting force. The prototype of the novel X-band TDS, the Polarizable X-band (PolariX) TDS, was fabricated at PSI following the high-precision tuning-free production process developed for the C-band Linac of the SwissFEL project. Bead-pull RF measurements were also performed at PSI to verify, in particular, that the polarization of the dipole fields does not have any rotation along the structure. The high-power test was performed at CERN and now the TDS is at DESY and has been installed in FLASHForward, where the first streaking experience with beam will be accomplished. We summarize in this paper the status of the project, the results of the bead-pull measurements and the high power test.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP036  
About • paper received ※ 21 August 2019       paper accepted ※ 26 August 2019       issue date ※ 05 November 2019  
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WEP037 RF Jitter and Electron Beam Stability in the SwissFEL Linac 400
 
  • Z.G. Geng, J. Alex, V.R. Arsov, P. Craievich, C.H. Gough, R. Kalt, T. Lippuner, F. Löhl, M. Pedrozzi, E. Prat, S. Reiche
    PSI, Villigen PSI, Switzerland
 
  The X-ray FEL machine SwissFEL at the Paul Scherrer Institut in Switzerland is commissioned and transiting to user operation smoothly. FEL operation requires stringent requirements for the beam stability at the linac output, such as the electron bunch arrival time, peak current and energy. Among other things, a highly stable RF system is required to guarantee the beam stability. The SwissFEL RF system is designed based on the state-of-the-art technologies that have allowed achieving excellent RF stability. The propagation of RF amplitude and phase jitter to the electron beam are analyzed theoretically and compared with the measurements performed at SwissFEL.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP037  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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WEP038 Commissioning and Stability Studies of the SwissFEL Bunch-Separation System 404
 
  • M. Paraliev, S. Dordevic, R. Ganter, C.H. Gough, N. Hiller, R.A. Krempaská, D. Voulot
    PSI, Villigen PSI, Switzerland
 
  SwissFEL is a linear electron accelerator based, X-ray Free Electron Laser at the Paul Scherrer Institute, Switzerland. It is a user oriented facility capable of producing short, high brightness X-ray pulses covering the spectral range from 1 to 50 Å. SwissFEL is designed to run in two electron bunch mode in order to serve simultaneously two experimental beamline stations (hard and soft X-ray one) at its full repetition rate. Two closely spaced (28 ns) electron bunches are accelerated in one RF macro pulse up to 3 GeV. A high stability resonant kicker system and a Lambertson septum magnet are used to separate the bunches and to send them to their respective beamlines. With the advancement of the construction of the second beamline (Athos) the bunch-separation system was successfully commissioned. In order to confirm that the beam separation process is fully transparent a stability study of the electron beam and the free electron laser in the main beamline (Aramis) was done.  
poster icon Poster WEP038 [0.945 MB]  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP038  
About • paper received ※ 19 August 2019       paper accepted ※ 25 August 2019       issue date ※ 05 November 2019  
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WEP041 Feasibility of Single-Shot Microbunching Diagnostics for a Pre-Bunched Beam at 266 nm 408
 
  • A.H. Lumpkin
    AAI/ANL, Lemont, Illinois, USA
  • D.W. Rule
    Private Address, Silver Spring, USA
 
  Funding: Work supported by U.S. Department of Energy, Office of Science, under Contract No. DE-AC02-06CH11357.
Co-propagating a relativistic electron beam and a high-power laser pulse through a short undulator (modulator) provides an energy modulation which can be converted to a periodic longitudinal density modulation (or microbunching) via the R56 term of a chicane. Such pre-bunching of a beam at the resonant wavelength and the harmonics of a subsequent free-electron laser (FEL) amplifier seeds the process and results in improved gain. We describe potential characterizations of the resulting microbunched electron beams using coherent optical transition radiation (COTR) imaging techniques for transverse size (50 micron), divergence (sub-mrad), trajectory angle (0.1 mrad), spectrum (few nm), and pulse length (sub-ps). The transverse spatial alignment is provided with near-field imaging and the angular alignment is done with far-field imaging and two-foil COTR interferometry (COTRI). Analytical model results for a 266-nm wavelength COTRI case with a 10% microbunching fraction will be presented. COTR gains of 7 million were calculated for an initial charge of 300 pC which enables splitting the optical signal for single-shot measurements of all the cited parameters.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP041  
About • paper received ※ 20 August 2019       paper accepted ※ 29 August 2019       issue date ※ 05 November 2019  
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WEP042 Observations of Short-Range Wakefield Effects in TESLA-Type Superconducting RF Cavities 412
 
  • A.H. Lumpkin, D.R. Edstrom, J. Ruan, R.M. Thurman-Keup
    Fermilab, Batavia, Illinois, USA
 
  Funding: This manuscript has been authored by Fermi Research Alliance, LLC under Contract No. DE-AC02-07CH11359 with the U.S. Department of Energy, Office of Science, Office of High Energy Physics.
The accelerators for high power X-ray free-electron laser (FEL) facilities such as the European XFEL and planned LCLS-II X-ray FEL are employing TESLA-type SCRF cavities. Beam propagation off axis in these cavities can result in both short-range and long-range transverse wakefields which can lead to emittance dilution within the micropulses and macropulses, respectively. The Fermilab Accelerator Science and Technology (FAST) facility has a unique configuration of a photocathode RF gun beam injecting two TESLA-type single cavities (CC1 and CC2) in series prior to the cryomodule. To investigate short-range wakefield effects, we used a vertical corrector between these two cavities to steer the beam off axis at an angle into CC2. A Hamamatsu synchroscan streak camera viewing a downstream OTR screen provided an image of y-t effects within the micropulses with resolutions of ~10-micron spatial and 2-ps temporal. At 500 pC/b, 50 b, and 4 mrad off-axis steering, we observed an ~100-micron head-tail centroid shift in the streak camera image. This centroid shift is consistent with a calculated short-range wakefield effect. Additional results for kick-angle compensation will be presented.
 
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP042  
About • paper received ※ 20 August 2019       paper accepted ※ 28 August 2019       issue date ※ 05 November 2019  
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WEP043 Multi-Energy Operation Analysis in a Superconducting Linac Based on off-Frequency Detune Method 416
 
  • Z. Zhang, C. Adolphsen, Y. Ding, T.O. Raubenheimer
    SLAC, Menlo Park, California, USA
 
  The free-electron laser facilities driven by a superconducting radio-frequency (SRF) linac provide high-repetition-rate electron beam, which makes it feasible to feed multiple undulator lines at the same time. In this paper, we study a method of controlling the beam energy of multiple electron bunches by off-frequency detuning of the SRF linac. Based on the theoretical analysis, we present the optimal solutions of the method and the strategy to allocate linac energy for each possible off-frequency detune. The initial acceleration phases before detuning of the SRF linac can be optimized to reduce the necessary SRF linac energy overhead. We adopt the LCLS-II-HE configuration as an example to discuss possible schemes for two undulator lines.  
DOI • reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP043  
About • paper received ※ 20 August 2019       paper accepted ※ 27 August 2019       issue date ※ 05 November 2019  
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