FEL2017 - Classification: Electron Beam Dynamics

Paper	Title	Page
TUP002	Numerical Studies on RF-Induced Trajectory Variations at the European XFEL	251
	T. Hellert, B. Beutner, W. Decking, N. Walker DESY, Hamburg, Germany
	At the European X-Ray Free-Electron Laser, superconducting TESLA-type cavities are used for acceleration of the driving electron bunches. Due to the high achievable duty cycle, a long radio frequency (RF) pulse structure can be provided, which allows to operate the machine with long bunch trains. The designated pointing stability of the FEL radiation places stringent restrictions on the acceptable trajectory variations of individual electron bunches. Therefore a transverse intra-bunch-train feedback system (IBFB) is located upstream of the undulator section. However, intra-bunch-train variations of RF parameters and misalignment of RF structures induce significant trajectory variations that may exceed the capability of the IBFB. In this paper we give an estimate of the expected RF-induced intra-bunch-train trajectory variations for different machine realizations and investigate methods for their limitation.
DOI •	reference for this paper ※ https://doi.org/10.18429/JACoW-FEL2017-TUP002
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TUP003	First Beam Halo Measurements Using Wire Scanners at the European XFEL	255
	S. Liu, V. Balandin, B. Beutner, W. Decking, L. Fröhlich, N. Golubeva, T. Lensch DESY, Hamburg, Germany
	Beam halo measurements and collimations are of great importance at the European XFEL, especially for the operation at high repetition rates (27000 pulses/s). First beam halo measurements have been performed during the commissioning using the wire scanners installed before and after the ~200 m long post-linac collimation section. We present the measurement results and the comparison of beam halo distributions before and after the collimation section.
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TUP004	Longitudinal Phase Space Optimization for the Hard X-ray Self-Seeding	259
	S. Liu, W. Decking, G. Feng, V. Kocharyan, I. Zagorodnov DESY, Hamburg, Germany G. Geloni, S. Serkez XFEL. EU, Hamburg, Germany
	For the implementation of Hard X-ray Self-Seeding (HXRSS) at European XFEL, short electron-beam bunches (FWHM ≤ 50 fs) are preferred to mitigate spatio-temperal coupling effect and to fit to the seeding bump width. Therefore, operations with low charges (< 250 pC) are preferred. Longitudinal phase-space optimization has been performed for the 100 pC case by flattening the current distribution. Start-to-end simulations show that, with the optimized distribution, for the photon energy of 14.4 keV, the HXRSS output power, pulse energy and spectral intensity can be increased by a factor of approximately 2 as compared to the nominal working point.
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TUP005	Studies of the Transverse Beam Coupling in the European XFEL Injector	263
	M. Scholz, B. Beutner DESY, Hamburg, Germany
	Coupling between the transverse plains leads to an increase of the horizontal and vertical electron beam emittances. The coupling can be measured with dedicated multi quadrupole scans while the correlations of the beam are observed on a screen. In this paper we show the results from first coupling studies in the European XFEL injector.
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TUP009	The Effect of Transverse Space Charge on Beam Evolution and Photon Coherence	266
	Q.R. Marksteiner LANL, Los Alamos, New Mexico, USA
	An electron beam experiences a transverse electric field which tends to act like a defocusing force on the electron beam. This defocusing force will act with different strengths at different locations in the electron beam because the current varies along the beam. A simple, quasi-analytic method is presented to calculate the impact of this force on beam projected emittance. In addition, estimates are made regarding the degree to which this could degrade the transverse coherence of x-rays in an XFEL.
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TUP010	Double-Bunches for Two-Color Soft X-Ray Free-Electron Laser at the MAX IV Laboratory	269
	J. Björklund Svensson, O. Lundh Lund University, Lund, Sweden J. Andersson, F. Curbis, M. Kotur, F. Lindau, E. Mansten, S. Thorin, S. Werin MAX IV Laboratory, Lund University, Lund, Sweden
	The ability to generate two-color free-electron laser (FEL) radiation enables a wider range of user experiments than just single-color FEL radiation. There are different schemes for generating the two colors, the original being to use a single bunch and two sets of undulators with different K-parameters. A development of the scheme has recently been shown, where two separate bunches in the same RF bucket are used for lasing at different wavelengths. We here investigate the feasibility of accelerating and compressing a double-bunch time structure generated in the photocathode electron gun for subsequent use in a soft X-ray FEL at the MAX IV Laboratory.
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TUP013	Experience and Initial Measurements of Magnetic Linearisation in the MAX IV Linac Bunch Compressors	273
	S. Thorin, J. Andersson, M. Brandin, F. Curbis, L. Isaksson, M. Kotur, F. Lindau, E. Mansten, D. Olsson, R. Svärd, S. Werin MAX IV Laboratory, Lund University, Lund, Sweden J. Björklund Svensson Lund University, Division of Atomic Physics, Lund, Sweden
	The MAX IV Linac is now in routine operation for injection into two storage rings, and as a high-brightness driver for a Short Pulse Facility (SPF). In short-pulse mode the electron bunch is created in a photo cathode gun and compressed in two double achromat bunch compressors that also linearize longitudinal phase space with the second order transfer matrix element T566. T566 in the compressors can be tweaked with weak sextupoles located at high dispersion. In this paper we present the current experience from operating the bunch compressors at MAX IV and results from initial measurements of longitudinal phase space using the zero-crossing method.
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TUP015	Coherent Transition Radiation from Transversely Modulated Electron Beams	276
	A. Halavanau, P. Piot Northern Illinois University, DeKalb, Illinois, USA S.P. Antipov, W. Liu, N.R. Neveu, J.G. Power, C. Whiteford, E.E. Wisniewski ANL, Argonne, Illinois, USA A.I. Benediktovitch BSU, Minsk, Belarus, Belarus S.N. Galyamin, A.V. Tyukhtin Saint Petersburg State University, Saint Petersburg, Russia D. Mihalcea, P. Piot Fermilab, Batavia, Illinois, USA N.R. Neveu IIT, Chicago, Illinois, USA
	A transverse laser-shaping optical setup using microlens arrays (MLAs), previously developed and employed at Argonne Wakefield Accelerator (AWA), allows the formation of both highly uniform and modulated (patterned) beams. In the latter case, transverse modulation is imposed in the sub-millimeter scale. In the present study, we report the simulations of backward coherent transition radiation (CTR) emitted from a transversely modulated beam. We compare the case of a uniform round beam against different transverse modulation wavelengths by generating CTR on a steel target and measuring the autocorrelation function of the resulting radiation with an interferometer. We particularly focus on the differences between round and patterned beam distributions and discuss possible future applications of this setup in THz radiation generation.
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TUP016	Beam-Dynamics Analysis of Long-Range Wakefield Effects on the SCRF Cavities at the Fast Facility	280
	Y.-M. Shin Northern Illinois University, DeKalb, Illinois, USA K. Bishofberger, B.E. Carlsten, F.L. Krawczyk LANL, Los Alamos, New Mexico, USA A.H. Lumpkin, J. Ruan, R.M. Thurman-Keup Fermilab, Batavia, Illinois, USA
	Funding: Work supported by the subcontract (contract No: G2A62653) of LANL-LDRD program and DOE contract No. DEAC02-07CH11359 to the Fermi Research Alliance LLC. Long-range wakefields in superconducting RF (SCRF) cavities create complicated effects on beam dynamics in SCRF-based FEL beamlines. The driving bunch excites effectively an infinite number of structure modes (including HOMs) which oscillate within the SCRF cavity. Couplers with loads are used to damp the HOMs. However, these HOMs can persist for long periods of time in superconducting structures, which leads to long-range wakefields. Clear understanding of the long-range wakefield effects is a critical element for risk mitigation of future SCRF accelerators such as XFEL at DESY, LCLS-II XFEL, and MaRIE XFEL. We are currently developing numerical tools for simulating long-range wakefields in SCRF accelerators and plan to experimentally verify the tools by measuring these wakefields at the Fermilab Accelerator Science and Technology (FAST) facility. This paper previews the experimental conditions at the FAST 50 MeV beamline based on the simulation results.
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TUP022	Modeling and Optimization of the APS Photo-Injector Using OPAL for High Efficiency FEL Experiments	284
	C.C. Hall, D.L. Bruhwiler, S.D. Webb RadiaSoft LLC, Boulder, Colorado, USA A.Y. Murokh RadiaBeam, Santa Monica, California, USA P. Musumeci, Y. Park UCLA, Los Angeles, USA Y. Sun, A. Zholents ANL, Argonne, Illinois, USA
	Funding: This work was carried out with support for the United State Department of Energy, Office of Scientific Research, under SBIR contract number DE-SC0017161. The Linac Extension Area (LEA) is a new beamline planned as an extension of Argonne's APS linac. An S-band 1.6-cell copper photo-cathode (PC) RF gun has been installed and commissioned at the APS linac front end. The PC gun will provide a beam to the LEA for accelerator technology development and beam physics experiments, in interleaving with a thermionic RF gun which provides a beam for APS storage ring operations. Recently an experiment was proposed to demonstrate the TESSA high-efficiency concept at LEA. In support of this experiment, we have begun simulating the photo-injector using the code OPAL (Object-oriented Particle Accelerator Library). In this paper, we first benchmark OPAL simulations with the established APS photo-injector optimization using ASTRA and ELEGANT. Key beam parameters required for a successful high-efficiency TESSA demonstration are discussed.
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TUP023	Recent Developments and Plans for Two Bunch Operation with up to 1 μs Separation at LCLS	288
	F.-J. Decker, K.L.F. Bane, W.S. Colocho, A.A. Lutman, J.C. Sheppard SLAC, Menlo Park, California, USA
	Funding: Work supported by U.S. Department of Energy, Contract DE-AC02-76SF00515. To get two electron bunches with a separation of up to 1 microsecond at the Linac Coherent Light Source (LCLS) is important for LCLS-II developments. Two lasing bunches up to 220 ns have been demonstrated. Many issues have to be solved to get that separation increased by a factor of 5. The typical design and setup for one single bunch has to be questioned for many devices: RF pulse widths have to be widened, BPMs diagnostic can see only one bunch or a vector average, feedbacks have to be doubled up, the main Linac RF needs to run probably un-SLEDed, and special considerations have to be done for the Gun and L1X RF.
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TUP024	Stochastic Effects from Classical 3D Synchrotron Radiation	292
	B.W. Garcia, T.O. Raubenheimer SLAC, Menlo Park, California, USA R.D. Ryne LBNL, Berkeley, California, USA
	In most cases, the one-dimensional coherent synchrotron radiation wakefield gives an excellent approximation to the total coherent effect due to classical synchrotron radiation in bend magnets. However, full particle Liénard-Wiechert simulations have revealed that there is non-numerical, stochastic noise which generates fluctuations about the approximate 1D solution. We present a model for this stochastic term in which this noise is due to long-range interaction with a discrete number of synchrotron radiation cones. The nature of this noise and how it depends on the 3D dimensions of the beam are explored.
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TUP027	Cancellation of Coherent Synchrotron Radiation Kicks at LCLS	296
	D. Khan, T.O. Raubenheimer SLAC, Menlo Park, California, USA
	In this paper, we look at the cancellation of Coherent Synchrotron Radiation (CSR) induced emittance growth using a phase-advance manipulation technique pioneered by R. Hajima, and extended in the Courant-Snyder formalism by S. Di Mitri. Bending systems in a linear accelerator are essential for beam transport and bunch compression. With the ever-growing demands of high-energy, short wavelength free electron laser (FEL) drivers, the CSR effect has emerged to be a detrimental factor in emittance stability. Under linear approximation, it is showed that the CSR driven dispersive kicks in successive bending magnet systems can, with proper balancing of the linac optics, cancel each other to nullify transverse emittance growth. This technique of optics balancing in the constant bunch length regime is the focus of this paper. We will present our findings for the emittance measurements generated in Elegant simulations for the current LCLS-I dogleg system.
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TUP028	Approximated Expressions for the Coherent Synchrotron Radiation Effect in Bending Magnets	300
	D. Khan, T.O. Raubenheimer SLAC, Menlo Park, California, USA
	In this paper, we describe the development of simplified analytic expressions for the Coherent Synchrotron Radiation's (CSR) root-mean-square induced energy spread, typically found in the bending magnets of short bunch-length charged particle accelerators. The expressions are derived for a Gaussian longitudinal bunch distribution and compared with the full-rigor CSR wakefield integral expressions while entering, traversing and exiting a bending magnet. The validity of the expressions are then tested against ELEGANT with the simulation of an unchirped beam traveling across a bending magnet into a drift section, and the second stage bunch compressor (BC2) of the proposed LCLS-II beamline.
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TUP030	An Emittance-Preservation Study of a Five-Bend Chicane for the LCLS-II-HE Beamline	305
	D. Khan, T.O. Raubenheimer SLAC, Menlo Park, California, USA
	The Linac Coherent Light Source II (LCLS-II) is an upgrade intended to advance the great success of its predecessor, LCLS-I, to maintain its position at the forefront of X-ray science. The introduction of a niobium-metal superconducting linac for LCLS-II not only increases the repetition rate to the MHz level (from 120 Hz), but also boasts an average brightness many orders higher (~10,000) than that of LCLS-I. Though, these improvements do not come without a price: the peak brightness suffers by a factor of 10, owing its degradation to the impact of Coherent Synchrotron Radiation (CSR) diminishing the peak current of the beam in the second bunch compressor (BC2). In this paper, we discuss the impact of implementing a plug-compatible 5-bend chicane for BC2 on the beam's emittance dilution for a high-energy, low-emittance configuration of LCLS-II (LCLS-II-HE). The results are compared with that of a standard 4-bend chicane under various settings in Elegant and CSRTrack.
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TUP031	Design of a Dogleg Bunch Compressor with Tunable First-Order Longitudinal Dispersion	309
	W.K. Lau, M.C. Chou, N.Y. Huang, A.P. Lee NSRRC, Hsinchu, Taiwan J. Wu SLAC, Menlo Park, California, USA
	A nonlinear bunch compressor has been designed for the proposed NSRRC VUV FEL facility. It is a double dog-leg configuration that provides a first order longitudinal dispersion function (i.e. R56) with a sign opposite to that of a conventional four-dipole chicane. A large variation in the bunch length or the peak current for various operation conditions can be done by tuning R56. This can be realized by changing the longitudinal positions of the outside dipoles and adjusting the quadrupoles and sextupoles settings for desired bunch compression. Residual energy chirp left after bunch compression as revealed from ELEGANT simulation can be corrected by a capacitive dechirper structure when the bunch is slightly over-compressed.
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TUP034	Novel Aspects of Beam Dynamics in CeC SRF Gun and SRF Accelerator	313
	I. Petrushina SUNY SB, Stony Brook, New York, USA T. Hayes, Y.C. Jing, D. Kayran, V. Litvinenko, G. Narayan, I. Pinayev, F. Severino, K.S. Smith, G. Wang BNL, Upton, Long Island, New York, USA K. Mihara Stony Brook University, Stony Brook, USA K. Shih SBU, Stony Brook, New York, USA
	Funding: DoE NP office, grant DE-FOA-0000632, NSF grant PHY-1415252. A 15 MeV CW SRF accelerator had been commissioned at Brookhaven National Laboratory to test the coherent electron cooling concept. The accelerator consists of an SRF 113-MHz photoemission gun, two 500 MHz bunching cavities and a 704-MHz 5-cell SRF linac. In this paper we describe our experience with this system with focus on unusual phenomena, such as multipacting in the SRF gun. We also discuss issues of wakefields in the CeC accelerator.
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TUP035	CSR Wake Fields and Emittance Growth with a Discontinuous Galerkin Time Domain Method	317
	D. A. Bizzozero, H. De Gersem, E. Gjonaj TEMF, TU Darmstadt, Darmstadt, Germany
	Funding: Work supported by DESY. Coherent synchrotron radiation (CSR) is an essential consideration in modern accelerators and related electromagnetic structures. We present our current method to examine CSR in the time domain. The method uses a 2D Discontinous Galerkin (DG) discretization in the longitudinal and transverse coordinates (z,x) with a Fourier decomposition in the transverse coordinate y. After summation over modes, this treatment describes all electromagnetic field components at each space-time coordinate (z,x,y,t). Additionally, by alignment of mesh element interfaces along a source reference orbit, DG methods can handle discontinuous or thin sources in the transverse x direction. We present an overview of our method, illustrate it by calculating wake functions for a bunch compressor, and discuss a method for estimating emittance growth from the wake fields in future work.
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TUP072	Simulation of Phase Shifters Between FEL Amplifiers in Coherent Electron Cooling	386
	Y.C. Jing, V. Litvinenko, I. Pinayev BNL, Upton, Long Island, New York, USA V. Litvinenko Stony Brook University, Stony Brook, USA
	Coherent electron Cooling (CeC) is a proposed advanced beam cooling method that has the potential of reducing the ion beam emittance in significantly shorter amount of time compared to existing cooling methods. A high-gain FEL, composed of three permanent magnet helical wigglers, is acting as an amplifier of the ion's signals picked up by electron beam in CeC. A self-consistent simulation which takes the space and possible phase shifts between wigglers into account is crucial in determining the performance of the FEL. The authors developed an algorithm based on the well-used GENESIS code to properly treat the propagation of particles and radiations in between wigglers and predicted the FEL performance with different beamline layouts. The authors will present their simulation setup and results and provide hardware requirements for future operations and research at CeC.
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THB01	Beam Dynamics Optimization in High-Brightness Electron Injectors
	C.E. Mitchell, J. Qiang, F. Sannibale LBNL, Berkeley, California, USA
	Funding: This work was supported by the Office of Science of the U.S. Department of Energy under Contract Numbers DE-AC02-76SF00515, DE-AC02-05CH11231, and the LCLS-II project. The next generation of X-ray free electron lasers requires beams with increasingly high peak current and low emittances at ~MHz repetition rates, placing increased demands on the performance of high-brightness electron photoinjector sources. To explore the high-dimensional parameter space associated with photoinjector design, global multiobjective optimization methods based on genetic algorithms or similar tools are playing an increasingly critical role. We review our experience with applying these tools both to understand and to optimize simulated injector beam performance for projects such as LCLS-II (at SLAC) and the Advanced Photoinjector EXperiment (at LBNL), including both challenges and successes.
	Slides THB01 [11.471 MB]
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THB02	Non-Standard Use of Laser Heater for FEL Control and THz Generation	566
	E. Allaria, L. Badano, M.B. Danailov, A.A. Demidovich, S. Di Mitri, D. Gauthier, L. Giannessi, G. Penco, E. Roussel, P. Sigalotti, S. Spampinati, M. Trovò, M. Veronese Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy E. Roussel SOLEIL, Gif-sur-Yvette, France
	The laser heater system is currently used at various FEL facilities for an accurate control of the electron beam energy spread in order to suppress the micro-bunching instabilities that can develop in high brightness electron beams. More recently, studies and experiments have shown that laser-electron interaction developing in the laser heater can open new possibilities for tailoring the electron beam properties to meet special requirements. A suitable time-shaping of the laser heater pulse opened the door to the generation of (tens of) femtosecond-long FEL pulses. Using standard laser techniques it is also possible to imprint onto the electron bunch, energy and density modulations in the THz frequency range that, properly sustained through the accelerator, can be exploited for generation of coherent THz radiation at GeV beam energies. Such recent results at the FERMI FEL are here reported, together with near future plans.
	Slides THB02 [14.882 MB]
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THB03	High-Power, Narrow-Bandwidth THz Generation Using Laser-Electron Interaction in a Compact Accelerator
	Z. Huang, G. Marcus SLAC, Menlo Park, California, USA K. Kan ISIR, Osaka, Japan Z. Zhang TUB, Beijing, People's Republic of China
	We propose a method based on the slice energy-spread modulation to generate strong subpicosecond density bunching in high-intensity relativistic electron beams.* A laser pulse with periodic intensity envelope is used to modulate the slice energy spread of the electron beam, which can then be converted into density modulation after a dispersive section. In this paper, we study this method in a compact accelerator with electron energy on the order of 50 MeV. To interact with an infra-red laser, the modulation undulator is resonant with the laser at a harmonic frequency. We show the flexibility of this method to generate powerful, narrow-bandwidth radiation between 1-20 THz. The THz radiation can be generated at a very high-repetition rate that matches a high-repetition rate X-ray free-electron laser for pump-probe studies of novel materials. * Z. Zhang et al., Phys. Rev. Accel. Beams 20, 050701 (2017).
	Slides THB03 [3.539 MB]
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THB04	Two-Color Beam Generation via Wakefield Excitation
	S. Bettoni, E. Prat, S. Reiche PSI, Villigen PSI, Switzerland
	Several beam manipulation methods have been studied and experimentally tested to generate two-color photon beams in free electron laser facilities to accommodate the user requests. We propose to use the interaction of the beam with an oscillating longitudinal wakefield source to obtain a suitable electron beam structure. The bunch generates two sub-pulses with different energies and delays in time passing through a magnetic chicane after its longitudinal phase space has been modulated by the wakefield source. According to this approach, the power of the emitted radiation is not degraded compared to the monochromatic beam, and the set-up in the machine is quite simple because the bunch is manipulated only in the high energy section where the beam is more rigid. We present the design applied to SwissFEL. We identified the parameters and the corresponding range of tunability of the time and energy separation among the two sub-bunches. Reference: Phys. Rev. Accel. Beams 19, 050702 (2016)
	Slides THB04 [10.429 MB]
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THB05	Ion Effects in LCLS and LCLS-II
	T.O. Raubenheimer SLAC, Menlo Park, California, USA
	Funding: This work supported in part by the Department of Energy Contract DE-AC02-76-SF00515. The effect of ions are evaluated in LCLS and LCLS-II. X-ray FEL's require high-brightness beams, that generate very high internal beam fields which can potentially lead to tunneling ionization. The FEL performance is sensitive to betatron mismatches which can arise from the focusing due to ions. In LCLS, both collisional and tunneling ionization may have an impact on single electron bunches while LCLS-II will have the additional complication of trapped ions in the MHz-rate multi-bunch trains. Analytic and simulation results for the LCLS and LCLS-II are presented and scalings are described to estimate the effects in other X-ray FEL's.
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Paper

Title

Page

Numerical Studies on RF-Induced Trajectory Variations at the European XFEL

251

T. Hellert, B. Beutner, W. Decking, N. Walker
DESY, Hamburg, Germany

At the European X-Ray Free-Electron Laser, superconducting TESLA-type cavities are used for acceleration of the driving electron bunches. Due to the high achievable duty cycle, a long radio frequency (RF) pulse structure can be provided, which allows to operate the machine with long bunch trains. The designated pointing stability of the FEL radiation places stringent restrictions on the acceptable trajectory variations of individual electron bunches. Therefore a transverse intra-bunch-train feedback system (IBFB) is located upstream of the undulator section. However, intra-bunch-train variations of RF parameters and misalignment of RF structures induce significant trajectory variations that may exceed the capability of the IBFB. In this paper we give an estimate of the expected RF-induced intra-bunch-train trajectory variations for different machine realizations and investigate methods for their limitation.

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TUP003

First Beam Halo Measurements Using Wire Scanners at the European XFEL

255

S. Liu, V. Balandin, B. Beutner, W. Decking, L. Fröhlich, N. Golubeva, T. Lensch
DESY, Hamburg, Germany

Beam halo measurements and collimations are of great importance at the European XFEL, especially for the operation at high repetition rates (27000 pulses/s). First beam halo measurements have been performed during the commissioning using the wire scanners installed before and after the ~200 m long post-linac collimation section. We present the measurement results and the comparison of beam halo distributions before and after the collimation section.

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TUP004

Longitudinal Phase Space Optimization for the Hard X-ray Self-Seeding

259

S. Liu, W. Decking, G. Feng, V. Kocharyan, I. Zagorodnov
DESY, Hamburg, Germany
G. Geloni, S. Serkez
XFEL. EU, Hamburg, Germany

For the implementation of Hard X-ray Self-Seeding (HXRSS) at European XFEL, short electron-beam bunches (FWHM ≤ 50 fs) are preferred to mitigate spatio-temperal coupling effect and to fit to the seeding bump width. Therefore, operations with low charges (< 250 pC) are preferred. Longitudinal phase-space optimization has been performed for the 100 pC case by flattening the current distribution. Start-to-end simulations show that, with the optimized distribution, for the photon energy of 14.4 keV, the HXRSS output power, pulse energy and spectral intensity can be increased by a factor of approximately 2 as compared to the nominal working point.

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TUP005

Studies of the Transverse Beam Coupling in the European XFEL Injector

263

M. Scholz, B. Beutner
DESY, Hamburg, Germany

Coupling between the transverse plains leads to an increase of the horizontal and vertical electron beam emittances. The coupling can be measured with dedicated multi quadrupole scans while the correlations of the beam are observed on a screen. In this paper we show the results from first coupling studies in the European XFEL injector.

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TUP009

The Effect of Transverse Space Charge on Beam Evolution and Photon Coherence

266

Q.R. Marksteiner
LANL, Los Alamos, New Mexico, USA

An electron beam experiences a transverse electric field which tends to act like a defocusing force on the electron beam. This defocusing force will act with different strengths at different locations in the electron beam because the current varies along the beam. A simple, quasi-analytic method is presented to calculate the impact of this force on beam projected emittance. In addition, estimates are made regarding the degree to which this could degrade the transverse coherence of x-rays in an XFEL.

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TUP010

Double-Bunches for Two-Color Soft X-Ray Free-Electron Laser at the MAX IV Laboratory

269

J. Björklund Svensson, O. Lundh
Lund University, Lund, Sweden
J. Andersson, F. Curbis, M. Kotur, F. Lindau, E. Mansten, S. Thorin, S. Werin
MAX IV Laboratory, Lund University, Lund, Sweden

The ability to generate two-color free-electron laser (FEL) radiation enables a wider range of user experiments than just single-color FEL radiation. There are different schemes for generating the two colors, the original being to use a single bunch and two sets of undulators with different K-parameters. A development of the scheme has recently been shown, where two separate bunches in the same RF bucket are used for lasing at different wavelengths. We here investigate the feasibility of accelerating and compressing a double-bunch time structure generated in the photocathode electron gun for subsequent use in a soft X-ray FEL at the MAX IV Laboratory.

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TUP013

Experience and Initial Measurements of Magnetic Linearisation in the MAX IV Linac Bunch Compressors

273

S. Thorin, J. Andersson, M. Brandin, F. Curbis, L. Isaksson, M. Kotur, F. Lindau, E. Mansten, D. Olsson, R. Svärd, S. Werin
MAX IV Laboratory, Lund University, Lund, Sweden
J. Björklund Svensson
Lund University, Division of Atomic Physics, Lund, Sweden

The MAX IV Linac is now in routine operation for injection into two storage rings, and as a high-brightness driver for a Short Pulse Facility (SPF). In short-pulse mode the electron bunch is created in a photo cathode gun and compressed in two double achromat bunch compressors that also linearize longitudinal phase space with the second order transfer matrix element T566. T566 in the compressors can be tweaked with weak sextupoles located at high dispersion. In this paper we present the current experience from operating the bunch compressors at MAX IV and results from initial measurements of longitudinal phase space using the zero-crossing method.

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TUP015

Coherent Transition Radiation from Transversely Modulated Electron Beams

276

A. Halavanau, P. Piot
Northern Illinois University, DeKalb, Illinois, USA
S.P. Antipov, W. Liu, N.R. Neveu, J.G. Power, C. Whiteford, E.E. Wisniewski
ANL, Argonne, Illinois, USA
A.I. Benediktovitch
BSU, Minsk, Belarus, Belarus
S.N. Galyamin, A.V. Tyukhtin
Saint Petersburg State University, Saint Petersburg, Russia
D. Mihalcea, P. Piot
Fermilab, Batavia, Illinois, USA
N.R. Neveu
IIT, Chicago, Illinois, USA

A transverse laser-shaping optical setup using microlens arrays (MLAs), previously developed and employed at Argonne Wakefield Accelerator (AWA), allows the formation of both highly uniform and modulated (patterned) beams. In the latter case, transverse modulation is imposed in the sub-millimeter scale. In the present study, we report the simulations of backward coherent transition radiation (CTR) emitted from a transversely modulated beam. We compare the case of a uniform round beam against different transverse modulation wavelengths by generating CTR on a steel target and measuring the autocorrelation function of the resulting radiation with an interferometer. We particularly focus on the differences between round and patterned beam distributions and discuss possible future applications of this setup in THz radiation generation.

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TUP016

Beam-Dynamics Analysis of Long-Range Wakefield Effects on the SCRF Cavities at the Fast Facility

280

Y.-M. Shin
Northern Illinois University, DeKalb, Illinois, USA
K. Bishofberger, B.E. Carlsten, F.L. Krawczyk
LANL, Los Alamos, New Mexico, USA
A.H. Lumpkin, J. Ruan, R.M. Thurman-Keup
Fermilab, Batavia, Illinois, USA

Funding: Work supported by the subcontract (contract No: G2A62653) of LANL-LDRD program and DOE contract No. DEAC02-07CH11359 to the Fermi Research Alliance LLC.
Long-range wakefields in superconducting RF (SCRF) cavities create complicated effects on beam dynamics in SCRF-based FEL beamlines. The driving bunch excites effectively an infinite number of structure modes (including HOMs) which oscillate within the SCRF cavity. Couplers with loads are used to damp the HOMs. However, these HOMs can persist for long periods of time in superconducting structures, which leads to long-range wakefields. Clear understanding of the long-range wakefield effects is a critical element for risk mitigation of future SCRF accelerators such as XFEL at DESY, LCLS-II XFEL, and MaRIE XFEL. We are currently developing numerical tools for simulating long-range wakefields in SCRF accelerators and plan to experimentally verify the tools by measuring these wakefields at the Fermilab Accelerator Science and Technology (FAST) facility. This paper previews the experimental conditions at the FAST 50 MeV beamline based on the simulation results.

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TUP022

Modeling and Optimization of the APS Photo-Injector Using OPAL for High Efficiency FEL Experiments

284

C.C. Hall, D.L. Bruhwiler, S.D. Webb
RadiaSoft LLC, Boulder, Colorado, USA
A.Y. Murokh
RadiaBeam, Santa Monica, California, USA
P. Musumeci, Y. Park
UCLA, Los Angeles, USA
Y. Sun, A. Zholents
ANL, Argonne, Illinois, USA

Funding: This work was carried out with support for the United State Department of Energy, Office of Scientific Research, under SBIR contract number DE-SC0017161.
The Linac Extension Area (LEA) is a new beamline planned as an extension of Argonne's APS linac. An S-band 1.6-cell copper photo-cathode (PC) RF gun has been installed and commissioned at the APS linac front end. The PC gun will provide a beam to the LEA for accelerator technology development and beam physics experiments, in interleaving with a thermionic RF gun which provides a beam for APS storage ring operations. Recently an experiment was proposed to demonstrate the TESSA high-efficiency concept at LEA. In support of this experiment, we have begun simulating the photo-injector using the code OPAL (Object-oriented Particle Accelerator Library). In this paper, we first benchmark OPAL simulations with the established APS photo-injector optimization using ASTRA and ELEGANT. Key beam parameters required for a successful high-efficiency TESSA demonstration are discussed.

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TUP023

Recent Developments and Plans for Two Bunch Operation with up to 1 μs Separation at LCLS

288

F.-J. Decker, K.L.F. Bane, W.S. Colocho, A.A. Lutman, J.C. Sheppard
SLAC, Menlo Park, California, USA

Funding: Work supported by U.S. Department of Energy, Contract DE-AC02-76SF00515.
To get two electron bunches with a separation of up to 1 microsecond at the Linac Coherent Light Source (LCLS) is important for LCLS-II developments. Two lasing bunches up to 220 ns have been demonstrated. Many issues have to be solved to get that separation increased by a factor of 5. The typical design and setup for one single bunch has to be questioned for many devices: RF pulse widths have to be widened, BPMs diagnostic can see only one bunch or a vector average, feedbacks have to be doubled up, the main Linac RF needs to run probably un-SLEDed, and special considerations have to be done for the Gun and L1X RF.

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TUP024

Stochastic Effects from Classical 3D Synchrotron Radiation

292

B.W. Garcia, T.O. Raubenheimer
SLAC, Menlo Park, California, USA
R.D. Ryne
LBNL, Berkeley, California, USA

In most cases, the one-dimensional coherent synchrotron radiation wakefield gives an excellent approximation to the total coherent effect due to classical synchrotron radiation in bend magnets. However, full particle Liénard-Wiechert simulations have revealed that there is non-numerical, stochastic noise which generates fluctuations about the approximate 1D solution. We present a model for this stochastic term in which this noise is due to long-range interaction with a discrete number of synchrotron radiation cones. The nature of this noise and how it depends on the 3D dimensions of the beam are explored.

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TUP027

Cancellation of Coherent Synchrotron Radiation Kicks at LCLS

296

D. Khan, T.O. Raubenheimer
SLAC, Menlo Park, California, USA

In this paper, we look at the cancellation of Coherent Synchrotron Radiation (CSR) induced emittance growth using a phase-advance manipulation technique pioneered by R. Hajima, and extended in the Courant-Snyder formalism by S. Di Mitri. Bending systems in a linear accelerator are essential for beam transport and bunch compression. With the ever-growing demands of high-energy, short wavelength free electron laser (FEL) drivers, the CSR effect has emerged to be a detrimental factor in emittance stability. Under linear approximation, it is showed that the CSR driven dispersive kicks in successive bending magnet systems can, with proper balancing of the linac optics, cancel each other to nullify transverse emittance growth. This technique of optics balancing in the constant bunch length regime is the focus of this paper. We will present our findings for the emittance measurements generated in Elegant simulations for the current LCLS-I dogleg system.

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TUP028

Approximated Expressions for the Coherent Synchrotron Radiation Effect in Bending Magnets

300

D. Khan, T.O. Raubenheimer
SLAC, Menlo Park, California, USA

In this paper, we describe the development of simplified analytic expressions for the Coherent Synchrotron Radiation's (CSR) root-mean-square induced energy spread, typically found in the bending magnets of short bunch-length charged particle accelerators. The expressions are derived for a Gaussian longitudinal bunch distribution and compared with the full-rigor CSR wakefield integral expressions while entering, traversing and exiting a bending magnet. The validity of the expressions are then tested against ELEGANT with the simulation of an unchirped beam traveling across a bending magnet into a drift section, and the second stage bunch compressor (BC2) of the proposed LCLS-II beamline.

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TUP030

An Emittance-Preservation Study of a Five-Bend Chicane for the LCLS-II-HE Beamline

305

D. Khan, T.O. Raubenheimer
SLAC, Menlo Park, California, USA

The Linac Coherent Light Source II (LCLS-II) is an upgrade intended to advance the great success of its predecessor, LCLS-I, to maintain its position at the forefront of X-ray science. The introduction of a niobium-metal superconducting linac for LCLS-II not only increases the repetition rate to the MHz level (from 120 Hz), but also boasts an average brightness many orders higher (~10,000) than that of LCLS-I. Though, these improvements do not come without a price: the peak brightness suffers by a factor of 10, owing its degradation to the impact of Coherent Synchrotron Radiation (CSR) diminishing the peak current of the beam in the second bunch compressor (BC2). In this paper, we discuss the impact of implementing a plug-compatible 5-bend chicane for BC2 on the beam's emittance dilution for a high-energy, low-emittance configuration of LCLS-II (LCLS-II-HE). The results are compared with that of a standard 4-bend chicane under various settings in Elegant and CSRTrack.

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TUP031

Design of a Dogleg Bunch Compressor with Tunable First-Order Longitudinal Dispersion

309

W.K. Lau, M.C. Chou, N.Y. Huang, A.P. Lee
NSRRC, Hsinchu, Taiwan
J. Wu
SLAC, Menlo Park, California, USA

A nonlinear bunch compressor has been designed for the proposed NSRRC VUV FEL facility. It is a double dog-leg configuration that provides a first order longitudinal dispersion function (i.e. R56) with a sign opposite to that of a conventional four-dipole chicane. A large variation in the bunch length or the peak current for various operation conditions can be done by tuning R56. This can be realized by changing the longitudinal positions of the outside dipoles and adjusting the quadrupoles and sextupoles settings for desired bunch compression. Residual energy chirp left after bunch compression as revealed from ELEGANT simulation can be corrected by a capacitive dechirper structure when the bunch is slightly over-compressed.

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TUP034

Novel Aspects of Beam Dynamics in CeC SRF Gun and SRF Accelerator

313

I. Petrushina
SUNY SB, Stony Brook, New York, USA
T. Hayes, Y.C. Jing, D. Kayran, V. Litvinenko, G. Narayan, I. Pinayev, F. Severino, K.S. Smith, G. Wang
BNL, Upton, Long Island, New York, USA
K. Mihara
Stony Brook University, Stony Brook, USA
K. Shih
SBU, Stony Brook, New York, USA

Funding: DoE NP office, grant DE-FOA-0000632, NSF grant PHY-1415252.
A 15 MeV CW SRF accelerator had been commissioned at Brookhaven National Laboratory to test the coherent electron cooling concept. The accelerator consists of an SRF 113-MHz photoemission gun, two 500 MHz bunching cavities and a 704-MHz 5-cell SRF linac. In this paper we describe our experience with this system with focus on unusual phenomena, such as multipacting in the SRF gun. We also discuss issues of wakefields in the CeC accelerator.

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TUP035

CSR Wake Fields and Emittance Growth with a Discontinuous Galerkin Time Domain Method

317

D. A. Bizzozero, H. De Gersem, E. Gjonaj
TEMF, TU Darmstadt, Darmstadt, Germany

Funding: Work supported by DESY.
Coherent synchrotron radiation (CSR) is an essential consideration in modern accelerators and related electromagnetic structures. We present our current method to examine CSR in the time domain. The method uses a 2D Discontinous Galerkin (DG) discretization in the longitudinal and transverse coordinates (z,x) with a Fourier decomposition in the transverse coordinate y. After summation over modes, this treatment describes all electromagnetic field components at each space-time coordinate (z,x,y,t). Additionally, by alignment of mesh element interfaces along a source reference orbit, DG methods can handle discontinuous or thin sources in the transverse x direction. We present an overview of our method, illustrate it by calculating wake functions for a bunch compressor, and discuss a method for estimating emittance growth from the wake fields in future work.

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TUP072

Simulation of Phase Shifters Between FEL Amplifiers in Coherent Electron Cooling

386

Y.C. Jing, V. Litvinenko, I. Pinayev
BNL, Upton, Long Island, New York, USA
V. Litvinenko
Stony Brook University, Stony Brook, USA

Coherent electron Cooling (CeC) is a proposed advanced beam cooling method that has the potential of reducing the ion beam emittance in significantly shorter amount of time compared to existing cooling methods. A high-gain FEL, composed of three permanent magnet helical wigglers, is acting as an amplifier of the ion's signals picked up by electron beam in CeC. A self-consistent simulation which takes the space and possible phase shifts between wigglers into account is crucial in determining the performance of the FEL. The authors developed an algorithm based on the well-used GENESIS code to properly treat the propagation of particles and radiations in between wigglers and predicted the FEL performance with different beamline layouts. The authors will present their simulation setup and results and provide hardware requirements for future operations and research at CeC.

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THB01

Beam Dynamics Optimization in High-Brightness Electron Injectors

C.E. Mitchell, J. Qiang, F. Sannibale
LBNL, Berkeley, California, USA

Funding: This work was supported by the Office of Science of the U.S. Department of Energy under Contract Numbers DE-AC02-76SF00515, DE-AC02-05CH11231, and the LCLS-II project.
The next generation of X-ray free electron lasers requires beams with increasingly high peak current and low emittances at ~MHz repetition rates, placing increased demands on the performance of high-brightness electron photoinjector sources. To explore the high-dimensional parameter space associated with photoinjector design, global multiobjective optimization methods based on genetic algorithms or similar tools are playing an increasingly critical role. We review our experience with applying these tools both to understand and to optimize simulated injector beam performance for projects such as LCLS-II (at SLAC) and the Advanced Photoinjector EXperiment (at LBNL), including both challenges and successes.

Slides THB01 [11.471 MB]

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THB02

Non-Standard Use of Laser Heater for FEL Control and THz Generation

566

E. Allaria, L. Badano, M.B. Danailov, A.A. Demidovich, S. Di Mitri, D. Gauthier, L. Giannessi, G. Penco, E. Roussel, P. Sigalotti, S. Spampinati, M. Trovò, M. Veronese
Elettra-Sincrotrone Trieste S.C.p.A., Basovizza, Italy
E. Roussel
SOLEIL, Gif-sur-Yvette, France

The laser heater system is currently used at various FEL facilities for an accurate control of the electron beam energy spread in order to suppress the micro-bunching instabilities that can develop in high brightness electron beams. More recently, studies and experiments have shown that laser-electron interaction developing in the laser heater can open new possibilities for tailoring the electron beam properties to meet special requirements. A suitable time-shaping of the laser heater pulse opened the door to the generation of (tens of) femtosecond-long FEL pulses. Using standard laser techniques it is also possible to imprint onto the electron bunch, energy and density modulations in the THz frequency range that, properly sustained through the accelerator, can be exploited for generation of coherent THz radiation at GeV beam energies. Such recent results at the FERMI FEL are here reported, together with near future plans.

Slides THB02 [14.882 MB]

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THB03

High-Power, Narrow-Bandwidth THz Generation Using Laser-Electron Interaction in a Compact Accelerator

Z. Huang, G. Marcus
SLAC, Menlo Park, California, USA
K. Kan
ISIR, Osaka, Japan
Z. Zhang
TUB, Beijing, People's Republic of China

We propose a method based on the slice energy-spread modulation to generate strong subpicosecond density bunching in high-intensity relativistic electron beams.* A laser pulse with periodic intensity envelope is used to modulate the slice energy spread of the electron beam, which can then be converted into density modulation after a dispersive section. In this paper, we study this method in a compact accelerator with electron energy on the order of 50 MeV. To interact with an infra-red laser, the modulation undulator is resonant with the laser at a harmonic frequency. We show the flexibility of this method to generate powerful, narrow-bandwidth radiation between 1-20 THz. The THz radiation can be generated at a very high-repetition rate that matches a high-repetition rate X-ray free-electron laser for pump-probe studies of novel materials.
* Z. Zhang et al., Phys. Rev. Accel. Beams 20, 050701 (2017).

Slides THB03 [3.539 MB]

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THB04

Two-Color Beam Generation via Wakefield Excitation

S. Bettoni, E. Prat, S. Reiche
PSI, Villigen PSI, Switzerland

Several beam manipulation methods have been studied and experimentally tested to generate two-color photon beams in free electron laser facilities to accommodate the user requests. We propose to use the interaction of the beam with an oscillating longitudinal wakefield source to obtain a suitable electron beam structure. The bunch generates two sub-pulses with different energies and delays in time passing through a magnetic chicane after its longitudinal phase space has been modulated by the wakefield source. According to this approach, the power of the emitted radiation is not degraded compared to the monochromatic beam, and the set-up in the machine is quite simple because the bunch is manipulated only in the high energy section where the beam is more rigid. We present the design applied to SwissFEL. We identified the parameters and the corresponding range of tunability of the time and energy separation among the two sub-bunches.
Reference: Phys. Rev. Accel. Beams 19, 050702 (2016)

Slides THB04 [10.429 MB]

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THB05

Ion Effects in LCLS and LCLS-II

T.O. Raubenheimer
SLAC, Menlo Park, California, USA

Funding: This work supported in part by the Department of Energy Contract DE-AC02-76-SF00515.
The effect of ions are evaluated in LCLS and LCLS-II. X-ray FEL's require high-brightness beams, that generate very high internal beam fields which can potentially lead to tunneling ionization. The FEL performance is sensitive to betatron mismatches which can arise from the focusing due to ions. In LCLS, both collisional and tunneling ionization may have an impact on single electron bunches while LCLS-II will have the additional complication of trapped ions in the MHz-rate multi-bunch trains. Analytic and simulation results for the LCLS and LCLS-II are presented and scalings are described to estimate the effects in other X-ray FEL's.

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