FEL2019 - List of Keywords (controls)

Paper	Title	Other Keywords	Page
TUP014	Crossed-Undulator Configuration for Variable Polarized THz Source	undulator, radiation, polarization, focusing	69
	H. Saito, H. Hama, S. Kashiwagi, N.M. Morita, T. Muto, K. Nanbu, H. Yamada Tohoku University, Research Center for Electron Photon Science, Sendai, Japan
	Funding: This work was supported by JSPS KAKENHI Grant Number JP17H01070 and JP15K13401. We have developed crossed-undulator configuration to control the polarization of coherent THz radiation at the femto-second electron beam facility, t-ACTS [1], that has been established at Research Center for Electron Photon Science, Tohoku University. The t-ACTS linac equips a thermionic RF gun, a 3 m accelerating structure and a 50 MW klystron modulator. Ultra-short electron bunch (~80 fs) train can be supplied via velocity bunching scheme. The crossed-undulator system is consisted with two identical transverse undulators intersected by a chicane type phase shifter. Deflecting planes of two undulators are at right angles each other, and the phase shifter makes path length difference between the electrons and the radiation. Target radiation frequency is around 2 THz employing a beam energy of 22 MeV. Since electron bunch trails behind the radiation by the slippage-effect and the nonrelativistic-effect that the electron speed is a bit slower than the speed of light, the radiation from 1st undulator has to be much delayed rather than the electrons. The paper will report the characteristics of polarized radiation and designing work of the phase shifter. [1] H. Hama et al., Int. J. Opt Photonic Eng., 2:004, 2017.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP014
About •	paper received ※ 24 August 2019 paper accepted ※ 17 September 2019 issue date ※ 05 November 2019
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TUP024	Electronic Modulation of the FEL-Oscillator Radiation Power Driven by ERL	FEL, radiation, electron, experiment	98
	O.A. Shevchenko, E.V. Bykov, Ya.V. Getmanov, S.S. Serednyakov, S.V. Tararyshkin BINP SB RAS, Novosibirsk, Russia M.V. Fedin, A.R. Melnikov, S.L. Veber International Tomography Center, SB RAS, Novosibirsk, Russia Ya.V. Getmanov, S.S. Serednyakov NSU, Novosibirsk, Russia
	FEL oscillators usually operate in CW mode and produce periodic train of radiation pulses but some user experiments require modulation of radiation power. Conventional way to obtain this modulation is using of mechanical shutters but it cannot provide very short switching time and may lead to decreasing of the radiation beam quality. Another way could be based on the electron beam current modulation but it cannot be used in the ERL. We propose a simple way of fast control of the FEL lasing which is based on periodic phase shift of electron bunches with respect to radiation stored in optical cavity. The phase shift required to suppress lasing is relatively small and it does not change significantly repetition rate. This approach has been realized at NovoFEL facility. It allows to generate radiation macropulses of desirable length down to several microseconds (limited by quality factor of optical cavity and FEL gain) which can be synchronized with external trigger. We present detailed description of electronic power modulation scheme and discuss the results of experiments.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP024
About •	paper received ※ 20 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019
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WEB03	Application of Machine Learning to Beam Diagnostics	optics, network, diagnostics, target	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 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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WEP007	Usage of the MicroTCA.4 Electronics Platform for Femtosecond Synchronization Systems	laser, FEL, electron, timing	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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WEP011	Longitudinal Intra-Train Beam-Based Feedback at FLASH	feedback, cavity, laser, electron	346
	S. Pfeiffer, L. 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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WEP016	Precise Laser-to-RF Synchronization of Photocathode Lasers	laser, timing, electron, experiment	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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WEP046	The European XFEL Photocathode Laser	laser, FEL, cathode, electron	423
	L. Winkelmann, A. Choudhuri, U. Grosse-Wortmann, I. Hartl, C. Li, C. Mohr, J. Müller, F. Peters, S. Pfeiffer, S.H. Salman DESY, Hamburg, Germany
	We present the Yb:fiber, Nd:YVO4 laser used to generate electrons from the RF photocathode gun at the European XFEL. The laser provides deep UV output pulses in 600 µs bursts with variable internal repetition rate (564 kHz to 4.5 MHz). Due to its robust architecture (mode-locked and synchronized fiber oscillator, Yb:fiber amplifiers and Nd:YVO4 gain blocks), the laser has operated with >99% uptime since January 2017. Using this laser, the XFEL reported energies of 17.5 GeV in July 2018, and simultaneous multi-mJ lasing in its three SASE beamlines. The laser offers two parallel outputs (1064 nm) with single pulse energies of >100 µJ and 11 ps width (FWHM). One output is converted to deep UV with efficiencies > 25%, and the second is used as a laser heater to reduce microbunching instabilities to increase SASE efficiency. Several state-of-art laser controls were implemented, including feed-forward algorithm to flatten electron charge along the bunch, active beam stabilization with < ±10 µm jitter at the photocathode, state machines for hands-off end-user operation, and temporal pulse synchronization and drift compensation to the timing jigger of the electron bunches to less than 45 fs.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP046
About •	paper received ※ 23 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019
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WEP059	Characterizing a Coherent Electron Source Extracted From a Cold Atom Trap	electron, emittance, cavity, laser	469
	H. Luo, P.X. Chu, J. Guo, T. Liu, Y.X. Xu, X. Zhao SWUST, Mianyang City, Sichuan Province, People’s Republic of China X.H. Li, Q.H. Zhou Southwest University of Science and Technology, Mianyang, Sichuan, People’s Republic of China K. Wang USTC, Hefei, Anhui, People’s Republic of China
	Funding: The National Natural Science Foundation of China under Grant No. 11875227. In order to generate a fully coherent free electron laser (FEL) within a compact system, one approach is to interact a coherent electron bunch with a high power laser operating in the quantum FEL regime. The coherent electron source is obtained by ionizing the Rydberg atoms in a magneto-optical trap (MOT). The qualities of the electron source will have direct effects on the brightness, coherence, and line width of the free electron laser. A high quality ultra-cold electron source is obtained by carefully optimizing the extraction electrode structure, the acceleration and focusing system as well as the MOT. Through parameter optimization, a coherent electron source with a temperature lower than 10 K is obtained. Details of the optimization and the characteristics of the coherent electron source are reported in this paper.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP059
About •	paper received ※ 24 August 2019 paper accepted ※ 10 September 2019 issue date ※ 05 November 2019
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WEP085	Field Integral Measurements of DAVV Undulators and Future Measurement Plan	undulator, electron, LabView, insertion	513
	M. Gehlot, S.M. Khan, R. Khullar, G. Mishra Devi Ahilya University, Indore, India J. Hussain Department of Applied Physics, UIT, Bhopal, India F. Trillaud UNAM, México, D.F., Mexico
	Funding: This work is supported by SERB-DST grant EMR/2014/00120 and financial support from UGC [F.15-1/2014-15/PDFWM-2014-15-GE-MAD-26801(SA-II)], Delhi and DGAPA of UNAM, fund PAPIIT TA100617 The Insertion device development and Application (IddA) laboratory of Devi Ahilya University, Indore, India has ongoing activities on undulator design and development. In this paper, we analyze the field integral properties of the two DAVV undulator. The first is the IddA U20 prototype NdFeB-cobalt steel hybrid in house designed device of 20 mm period length with twenty five periods. The uniform gap variable hybrid undulator provides magnetic flux density (in rms) from 2400 G to 500 G in the 10 mm to 20 mm gap range. The second is the NdFeB based U50II undulator of 50 mm period length with 20 number of periods. Hall probe results are described. A short description of the measurement plan of the undulator on the pulsed wire bench and stretched wire bench is described.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP085
About •	paper received ※ 20 August 2019 paper accepted ※ 26 August 2019 issue date ※ 05 November 2019
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WEP103	A Plasma Attenuator for Soft X-Rays in LCLS-II	plasma, electron, FEL, cavity	553
	A.S. Fisher, A.L. Benwell, Y. Feng, B.T. Jacobson SLAC, Menlo Park, California, USA
	Attenuation of X-ray FEL beams is often required to avoid damaging optics and detectors during alignment, and to study fluence-dependent effects. Soft X-rays are commonly attenuated by photoabsorption in a gas such as argon. However, absorbing a mJ pulse along a meter creates a pressure wave that drives gas away from the X-ray propagation axis, until equilibrium recovers in ~1 ms. This timescale matched the 120-Hz pulse spacing of LCLS, but at the high repetition rate (up to 1 MHz) and power (up to 200 W) of LCLS-II, the attenuation of subsequent pulses is reduced. Simulations demonstrate hysteresis and erratic attenuation from gas-density depletion. Instead, we propose to replace the gas column with an argon plasma in a TM010 RF cavity. The density profile then is largely set by the RF mode. X-ray absorption becomes a perturbation compared to the energy in the plasma. An LCLS-II solid-state RF amplifier, generating up to 4 kW at 1.3 GHz, can provide the drive, and the FPGA-based low-level RF controller can be programmed to track tuning with plasma density. Several diagnostics are planned to monitor plasma properties over a fill-pressure range of 10 to 1000 Pa.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP103
About •	paper received ※ 16 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019
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THP061	Bayesian Optimisation for Fast and Safe Parameter Tuning of SwissFEL	FEL, target, undulator, feedback	707
	J. Kirschner, A. Krause, M. Mutný, M. Nonnenmacher ETH, Zurich, Switzerland A. Adelmann, N. Hiller, R. Ischebeck PSI, Villigen PSI, Switzerland
	Parameter tuning is a notoriously time-consuming task in accelerator facilities. As tool for global optimization with noisy evaluations, Bayesian optimization was recently shown to outperform alternative methods. By learning a model of the underlying function using all available data, the next evaluation can be chosen carefully to find the optimum with as few steps as possible and without violating any safety constraints. However, the per-step computation time increases significantly with the number of parameters and the generality of the approach can lead to slow convergence on functions that are easier to optimize. To overcome these limitations, we divide the global problem into sequential subproblems that can be solved efficiently using safe Bayesian optimization. This allows us to trade off local and global convergence and to adapt to additional structure in the objective function. Further, we provide slice-plots of the function as user feedback during the optimization. We showcase how we use our algorithm to tune up the FEL output of SwissFEL with up to 40 parameters simultaneously, and reach convergence within reasonable tuning times in the order of 30 minutes (< 2000 steps).
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP061
About •	paper received ※ 13 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019
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THD03	FEL Optimization: From Model-Free to Model-Dependent Approaches and ML Prospects	FEL, photon, operation, undulator	762
	S. Tomin, G. Geloni EuXFEL, Schenefeld, Germany M. Scholz DESY, Hamburg, Germany
	Users beam-time at modern FEL sources is an extremely valuable commodity. Moreover, maximization of FEL up-time must always be performed accounting for stringent requirements on the photon pulse characteristics. These may vary widely depending on the users requests, which poses issues to parallel operation of high-repetition rate facilities like the European XFEL. Therefore, both model-free or model-dependent optimization schemes, where the model might be given, or provided by machine-learning approaches, are of high importance for the overall efficiency of FEL facilities. In this contribution, we review our previous activities and we report on current efforts and progress in FEL optimization schemes at the European XFEL. Finally, we provide an outlook on future developments.
	Slides THD03 [13.636 MB]
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THD03
About •	paper received ※ 21 August 2019 paper accepted ※ 12 September 2019 issue date ※ 05 November 2019
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Paper

Title

Other Keywords

Page

TUP014

Crossed-Undulator Configuration for Variable Polarized THz Source

undulator, radiation, polarization, focusing

69

H. Saito, H. Hama, S. Kashiwagi, N.M. Morita, T. Muto, K. Nanbu, H. Yamada
Tohoku University, Research Center for Electron Photon Science, Sendai, Japan

Funding: This work was supported by JSPS KAKENHI Grant Number JP17H01070 and JP15K13401.
We have developed crossed-undulator configuration to control the polarization of coherent THz radiation at the femto-second electron beam facility, t-ACTS [1], that has been established at Research Center for Electron Photon Science, Tohoku University. The t-ACTS linac equips a thermionic RF gun, a 3 m accelerating structure and a 50 MW klystron modulator. Ultra-short electron bunch (~80 fs) train can be supplied via velocity bunching scheme. The crossed-undulator system is consisted with two identical transverse undulators intersected by a chicane type phase shifter. Deflecting planes of two undulators are at right angles each other, and the phase shifter makes path length difference between the electrons and the radiation. Target radiation frequency is around 2 THz employing a beam energy of 22 MeV. Since electron bunch trails behind the radiation by the slippage-effect and the nonrelativistic-effect that the electron speed is a bit slower than the speed of light, the radiation from 1st undulator has to be much delayed rather than the electrons. The paper will report the characteristics of polarized radiation and designing work of the phase shifter.
[1] H. Hama et al., Int. J. Opt Photonic Eng., 2:004, 2017.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP014

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paper received ※ 24 August 2019 paper accepted ※ 17 September 2019 issue date ※ 05 November 2019

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TUP024

Electronic Modulation of the FEL-Oscillator Radiation Power Driven by ERL

FEL, radiation, electron, experiment

98

O.A. Shevchenko, E.V. Bykov, Ya.V. Getmanov, S.S. Serednyakov, S.V. Tararyshkin
BINP SB RAS, Novosibirsk, Russia
M.V. Fedin, A.R. Melnikov, S.L. Veber
International Tomography Center, SB RAS, Novosibirsk, Russia
Ya.V. Getmanov, S.S. Serednyakov
NSU, Novosibirsk, Russia

FEL oscillators usually operate in CW mode and produce periodic train of radiation pulses but some user experiments require modulation of radiation power. Conventional way to obtain this modulation is using of mechanical shutters but it cannot provide very short switching time and may lead to decreasing of the radiation beam quality. Another way could be based on the electron beam current modulation but it cannot be used in the ERL. We propose a simple way of fast control of the FEL lasing which is based on periodic phase shift of electron bunches with respect to radiation stored in optical cavity. The phase shift required to suppress lasing is relatively small and it does not change significantly repetition rate. This approach has been realized at NovoFEL facility. It allows to generate radiation macropulses of desirable length down to several microseconds (limited by quality factor of optical cavity and FEL gain) which can be synchronized with external trigger. We present detailed description of electronic power modulation scheme and discuss the results of experiments.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-TUP024

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paper received ※ 20 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019

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WEB03

Application of Machine Learning to Beam Diagnostics

optics, network, diagnostics, target

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 WEB03 [5.721 MB]

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEB03

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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

laser, FEL, electron, timing

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

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paper received ※ 20 August 2019 paper accepted ※ 26 August 2019 issue date ※ 05 November 2019

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WEP011

Longitudinal Intra-Train Beam-Based Feedback at FLASH

feedback, cavity, laser, electron

346

S. Pfeiffer, L. 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

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paper received ※ 20 August 2019 paper accepted ※ 17 September 2019 issue date ※ 05 November 2019

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WEP016

Precise Laser-to-RF Synchronization of Photocathode Lasers

laser, timing, electron, experiment

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

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paper received ※ 20 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019

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WEP046

The European XFEL Photocathode Laser

laser, FEL, cathode, electron

423

L. Winkelmann, A. Choudhuri, U. Grosse-Wortmann, I. Hartl, C. Li, C. Mohr, J. Müller, F. Peters, S. Pfeiffer, S.H. Salman
DESY, Hamburg, Germany

We present the Yb:fiber, Nd:YVO4 laser used to generate electrons from the RF photocathode gun at the European XFEL. The laser provides deep UV output pulses in 600 µs bursts with variable internal repetition rate (564 kHz to 4.5 MHz). Due to its robust architecture (mode-locked and synchronized fiber oscillator, Yb:fiber amplifiers and Nd:YVO4 gain blocks), the laser has operated with >99% uptime since January 2017. Using this laser, the XFEL reported energies of 17.5 GeV in July 2018, and simultaneous multi-mJ lasing in its three SASE beamlines. The laser offers two parallel outputs (1064 nm) with single pulse energies of >100 µJ and 11 ps width (FWHM). One output is converted to deep UV with efficiencies > 25%, and the second is used as a laser heater to reduce microbunching instabilities to increase SASE efficiency. Several state-of-art laser controls were implemented, including feed-forward algorithm to flatten electron charge along the bunch, active beam stabilization with < ±10 µm jitter at the photocathode, state machines for hands-off end-user operation, and temporal pulse synchronization and drift compensation to the timing jigger of the electron bunches to less than 45 fs.

DOI •

reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP046

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paper received ※ 23 August 2019 paper accepted ※ 29 August 2019 issue date ※ 05 November 2019

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WEP059

Characterizing a Coherent Electron Source Extracted From a Cold Atom Trap

electron, emittance, cavity, laser

469

H. Luo, P.X. Chu, J. Guo, T. Liu, Y.X. Xu, X. Zhao
SWUST, Mianyang City, Sichuan Province, People’s Republic of China
X.H. Li, Q.H. Zhou
Southwest University of Science and Technology, Mianyang, Sichuan, People’s Republic of China
K. Wang
USTC, Hefei, Anhui, People’s Republic of China

Funding: The National Natural Science Foundation of China under Grant No. 11875227.
In order to generate a fully coherent free electron laser (FEL) within a compact system, one approach is to interact a coherent electron bunch with a high power laser operating in the quantum FEL regime. The coherent electron source is obtained by ionizing the Rydberg atoms in a magneto-optical trap (MOT). The qualities of the electron source will have direct effects on the brightness, coherence, and line width of the free electron laser. A high quality ultra-cold electron source is obtained by carefully optimizing the extraction electrode structure, the acceleration and focusing system as well as the MOT. Through parameter optimization, a coherent electron source with a temperature lower than 10 K is obtained. Details of the optimization and the characteristics of the coherent electron source are reported in this paper.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP059

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paper received ※ 24 August 2019 paper accepted ※ 10 September 2019 issue date ※ 05 November 2019

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WEP085

Field Integral Measurements of DAVV Undulators and Future Measurement Plan

undulator, electron, LabView, insertion

513

M. Gehlot, S.M. Khan, R. Khullar, G. Mishra
Devi Ahilya University, Indore, India
J. Hussain
Department of Applied Physics, UIT, Bhopal, India
F. Trillaud
UNAM, México, D.F., Mexico

Funding: This work is supported by SERB-DST grant EMR/2014/00120 and financial support from UGC [F.15-1/2014-15/PDFWM-2014-15-GE-MAD-26801(SA-II)], Delhi and DGAPA of UNAM, fund PAPIIT TA100617
The Insertion device development and Application (IddA) laboratory of Devi Ahilya University, Indore, India has ongoing activities on undulator design and development. In this paper, we analyze the field integral properties of the two DAVV undulator. The first is the IddA U20 prototype NdFeB-cobalt steel hybrid in house designed device of 20 mm period length with twenty five periods. The uniform gap variable hybrid undulator provides magnetic flux density (in rms) from 2400 G to 500 G in the 10 mm to 20 mm gap range. The second is the NdFeB based U50II undulator of 50 mm period length with 20 number of periods. Hall probe results are described. A short description of the measurement plan of the undulator on the pulsed wire bench and stretched wire bench is described.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP085

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paper received ※ 20 August 2019 paper accepted ※ 26 August 2019 issue date ※ 05 November 2019

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WEP103

A Plasma Attenuator for Soft X-Rays in LCLS-II

plasma, electron, FEL, cavity

553

A.S. Fisher, A.L. Benwell, Y. Feng, B.T. Jacobson
SLAC, Menlo Park, California, USA

Attenuation of X-ray FEL beams is often required to avoid damaging optics and detectors during alignment, and to study fluence-dependent effects. Soft X-rays are commonly attenuated by photoabsorption in a gas such as argon. However, absorbing a mJ pulse along a meter creates a pressure wave that drives gas away from the X-ray propagation axis, until equilibrium recovers in ~1 ms. This timescale matched the 120-Hz pulse spacing of LCLS, but at the high repetition rate (up to 1 MHz) and power (up to 200 W) of LCLS-II, the attenuation of subsequent pulses is reduced. Simulations demonstrate hysteresis and erratic attenuation from gas-density depletion. Instead, we propose to replace the gas column with an argon plasma in a TM010 RF cavity. The density profile then is largely set by the RF mode. X-ray absorption becomes a perturbation compared to the energy in the plasma. An LCLS-II solid-state RF amplifier, generating up to 4 kW at 1.3 GHz, can provide the drive, and the FPGA-based low-level RF controller can be programmed to track tuning with plasma density. Several diagnostics are planned to monitor plasma properties over a fill-pressure range of 10 to 1000 Pa.

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-WEP103

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paper received ※ 16 August 2019 paper accepted ※ 28 August 2019 issue date ※ 05 November 2019

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THP061

Bayesian Optimisation for Fast and Safe Parameter Tuning of SwissFEL

FEL, target, undulator, feedback

707

J. Kirschner, A. Krause, M. Mutný, M. Nonnenmacher
ETH, Zurich, Switzerland
A. Adelmann, N. Hiller, R. Ischebeck
PSI, Villigen PSI, Switzerland

Parameter tuning is a notoriously time-consuming task in accelerator facilities. As tool for global optimization with noisy evaluations, Bayesian optimization was recently shown to outperform alternative methods. By learning a model of the underlying function using all available data, the next evaluation can be chosen carefully to find the optimum with as few steps as possible and without violating any safety constraints. However, the per-step computation time increases significantly with the number of parameters and the generality of the approach can lead to slow convergence on functions that are easier to optimize. To overcome these limitations, we divide the global problem into sequential subproblems that can be solved efficiently using safe Bayesian optimization. This allows us to trade off local and global convergence and to adapt to additional structure in the objective function. Further, we provide slice-plots of the function as user feedback during the optimization. We showcase how we use our algorithm to tune up the FEL output of SwissFEL with up to 40 parameters simultaneously, and reach convergence within reasonable tuning times in the order of 30 minutes (< 2000 steps).

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THP061

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paper received ※ 13 August 2019 paper accepted ※ 27 August 2019 issue date ※ 05 November 2019

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THD03

FEL Optimization: From Model-Free to Model-Dependent Approaches and ML Prospects

FEL, photon, operation, undulator

762

S. Tomin, G. Geloni
EuXFEL, Schenefeld, Germany
M. Scholz
DESY, Hamburg, Germany

Users beam-time at modern FEL sources is an extremely valuable commodity. Moreover, maximization of FEL up-time must always be performed accounting for stringent requirements on the photon pulse characteristics. These may vary widely depending on the users requests, which poses issues to parallel operation of high-repetition rate facilities like the European XFEL. Therefore, both model-free or model-dependent optimization schemes, where the model might be given, or provided by machine-learning approaches, are of high importance for the overall efficiency of FEL facilities. In this contribution, we review our previous activities and we report on current efforts and progress in FEL optimization schemes at the European XFEL. Finally, we provide an outlook on future developments.

Slides THD03 [13.636 MB]

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reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2019-THD03

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paper received ※ 21 August 2019 paper accepted ※ 12 September 2019 issue date ※ 05 November 2019

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