FEL2015 - List of Keywords (bunching)

Paper	Title	Other Keywords	Page
MOC04	Operating of SXFEL in a Single Stage High Gain Harmonic Generation Scheme	laser, FEL, electron, linac	29
	G.L. Wang, D.X. Dai, G.R. Wu, X.M. Yang, W.Q. Zhang DICP, Dalian, People's Republic of China
	The beam energy spread at the entrance of undulator system is of paramount importance for efficient density modulation in high-gain seeded free-electron lasers (FELs). In this paper, the dependencies of high harmonic bunching efficiency in the high-gain harmonic generation (HGHG) schemes on the electron energy spread distribution are studied. Theoretical investigations and multi-dimensional numerical simulations are applied to the cases of uniform and saddle beam energy distributions and compared to a traditional Gaussian distribution. It shows that the uniform and saddle electron energy distributions significantly enhance the performance of HGHG-FELs. A numerical example demonstrates that, with the saddle distribution of sliced beam energy spread controlled by a laser heater, the 30th harmonic radiation can be directly generated by a single-stage seeding scheme for a soft x-ray FEL facility.
	Slides MOC04 [3.345 MB]
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MOP030	Study of Smith-Purcell Free Electron Laser Using Electron Bunch Produced By Micro-Pulse Electron Gun	electron, radiation, simulation, cavity	93
	J. Zhao, X.Y. Lu, W.W. Tan, D.Y. Yang, Y. Yang, Z.Q. Yang PKU, Beijing, People's Republic of China
	A Micro-Pulse electron Gun (MPG) with the frequency of 2856 MHz has been designed, constructed and tested. Some primary experimental studies have been carried out and electron beam with the average current of 6 mA has been detected which holds promise to use as an electron source of Smith-Purcell Free Electron Laser (SP-FEL) to produced Coherent Radiation. It is well known that Smith-Purcell radiation is one of the achievable ways to produce FEL. After many years study in theory and experiment, lots of new mechanisms and appearances have been discovered. Coherent Smith-Purcell Radiation was discovered in 1990s as well. Compared with incoherent Smith-Purcell Radiation, It can generate a more powerful and frequency locked coherent emission due to displaying all three of these enhancements, Ng (the number of grating periods), Ne (the number of electrons in the bunch), Nb (the number of electron bunch). Obviously, MPG is one of ideal electron sources of CSPR for that (1) S-band electron source can increase energy density at these frequencies, (2) picosecond or subpicosecond pulse can generate THz radiation, (3) low emittance makes the interactions between electron beam and granting more stable. All of the above will be displayed in the simulation of this article. The progress of the experiment with beam energy of 80 Kev, the average current of 6 mA is also introduced.
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MOP087	Multistage CSR Microbunching Gain Development in Transport or Recirculation Arcs	lattice, dipole, damping, optics	263
	C.-Y. Tsai Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA D. Douglas, R. Li, C. Tennant JLab, Newport News, Virginia, USA
	Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177. Coherent synchrotron radiation (CSR) induced microbunching instability has been one of the most challenging issues in the design of modern accelerators. A linear Vlasov solver has been developed [1] and applied to investigate the physical processes of microbunching gain amplification for several example lattices [2]. In this paper, by further extending the concept of stage gain as proposed by Huang and Kim [3], we develop a method to characterize the microbunching development in terms of stage orders that allow the quantitative comparison of optics impacts on microbunching gain for different lattices. We find that the microbunching instability in our demonstrated arcs has a distinguishing feature of multistage amplification (e.g, up to 6th stage amplification for our example transport arcs, in contrast to two-stage amplification for a typical 4-dipole bunch compressor chicane). We also try to connect lattice optics pattern with the obtained stage gain functions by a physical interpretation. This Vlasov analysis is validated by ELEGANT [4] tracking results with excellent agreement. [1] C. -Y. Tsai et al., MOP052, these proceedings [2] See, for example, C. -Y. Tsai et al., ERL2015 (TUICLH2034) [3] Z. Huang and K. -J. Kim, Phys. Rev. ST Accel. Beams 5, 074401 (2002) [4] M. Borland, APS Light Source Note LS-287 (2000)
	Poster MOP087 [1.962 MB]
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MOD02	Overview of Alternative Bunching and Current-shaping Techniques for Low-Energy Electron Beams	electron, laser, wakefield, radiation	274
	P. Piot Fermilab, Batavia, Illinois, USA P. Piot Northern Illinois University, DeKalb, Illinois, USA
	Funding: This work was supported by the U.S. Department of Energy contracts No. DE-SC0011831 to Northern Illinois University and No. DE-AC02-07CH11359 with the Fermi Research Alliance, LLC Techniques to bunch or shape an electron beam at low energies (E <15 MeV) have important implications toward the realization of table-top radiation sources [1] or to the design of compact multi-user free-electron lasers[2]. This paper provides an overview of alternative methods recently developed including techniques such as wakefield-based bunching, space-charge-driven microbunching via wave-breaking [3], ab-initio shaping of the electron-emission process [4], and phase space exchangers. Practical applications of some of these methods to foreseen free-electron-laser configurations are also briefly discussed [5]. [1] W. S. Graves, PRL 108, 263904 (2012) [2] A. Zholents, FEL14, 993 (2014) [3] P. Musumeci, PRL 106, 184801 (2011) [4] F. Lemery, PRSTAB 17, 112804 (2014) [5] G. Penco, PRL 112, 044801 (2014)
	Slides MOD02 [5.426 MB]
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TUC01	The Microbunching Instability and LCLS-II Lattice Design	simulation, laser, FEL, linac	308
	M. Venturini LBNL, Berkeley, California, USA
	The microbunching instability is a pervasive occurrence when high brightness electron beams are accelerated and transported through dispersive sections like bunch-compression chicanes or distributions beamlines. If uncontrolled the instability can severely compromise the performance of x-ray FELs, where beam high brightness is crucial. In this talk we discuss how consideration of the microbunching instability is informing the LCLS-II design and determining the specifications for the laser heater and transport lines. We also review some of the expected and not so-expected phenomena that we have encountered while carrying out high-resolution macroparticle simulations of the instability and the analytical models we have developed to interpret the numerical results.
	Slides TUC01 [4.206 MB]
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TUP071	A FAST Particle Tracking Code	wakefield, linac, collective-effects, space-charge	530
	L. Wang SLAC, Menlo Park, California, USA
	This paper presents a fast particle tracking (FPT) code for linac beam dynamics. It includes wake fields, coherent synchrotron radiation (CSR) and longitudinal space charge. We systematically benchmark the FPT with ELEGANT with different physics aspects: pure optics, wakefields, CSR and space-charge forces
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TUD01	COTR Resistant Profile Monitor	electron, laser, diagnostics, radiation	554
	H. Loos SLAC, Menlo Park, California, USA
	Funding: Work supported by DOE contract DE-AC02-76SF00515 Electron beam accelerators used as drivers for short wavelength FELs need ultra-high brightness beams with small emittances and highly compressed bunch lengths. The acceleration and beam transport process of such beams leads to micro-bunching instabilities which cause the emergence of coherent optical transition radiation (COTR). The effect of COTR on profile monitors based on OTR or fluorescent screens can be quite detrimental to their intended use to measure beam sizes and profiles. This presentation will review past observations of the beam diagnostics issues due to COTR and discuss various mitigation schemes for profile monitors as well as present experience with such implementations.
	Slides TUD01 [1.536 MB]
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WEP004	Energy Spread Constraints on Field Suppression in a Reverse Tapered Undulator	undulator, laser, FEL, simulation	597
	J.P. MacArthur, Z. Huang, A.A. Lutman, A. Marinelli, H.-D. Nuhn SLAC, Menlo Park, California, USA
	A 3.2 m variable polarization Delta undulator[1] has been installed at the end of the LCLS undulator line. The Delta undulator acts an an afterburner in this configuration, using bunching from upstream planar undulators to produce radiation with arbitrary polarization. To optimize the degree of polarization from this device, a reverse taper[2] has been proposed to suppress background radiation produced in upstream undulators while still microbunching the beam. Here we extend previous work on free electron lasers with a slowly varying undulator parameter[3] to show there is a strong energy spread dependence to the maximum allowable detune from resonance. At LCLS, this energy spread limitation keeps the reverse taper slope in the slowly varying regime and limits the achievable degree of circular polarization. [1] A. B. Temnykh, PRST-AB, 11, 120702, (2008). [2] E. A. Schneidmiller and M. V. Yurkov, PRST-AB, 16, 110702, (2013). [3] Z. Huang and G. Stupakov, PRST-AB, 8, 040702, (2005).
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WEP031	Measurements and Simulations of Seeded Electron Microbunches with Collective Effects	electron, simulation, laser, FEL	650
	K.E. Hacker, S. Khan, R. Molo DELTA, Dortmund, Germany S. Ackermann, J. Bödewadt, M. Dohlus, N. Ekanayake, T. Laarmann, H. Schlarb DESY, Hamburg, Germany L.L. Lazzarino, C. Lechner, Th. Maltezopoulos, T. Plath, J. Roßbach Uni HH, Hamburg, Germany
	Funding: The experiments were carried out at FLASH at DESY. BMBF contract No. 05K10PE1, 05K10PE3, 05K13GU4, and 05K13PE3, and the German Research Foundation program graduate school 1355. Measurements of the longitudinal phase-space distribution of electron bunches seeded with an external laser were done in order to study the impact of collective effects on seeded microbunches in free-electron lasers. Velocity bunching of a seeded microbunch appears to be a viable alternative to compression with a magnetic chicane under high-gain harmonic generation seeding conditions when the collective effects of Coulomb forces in a drift space and coherent synchrotron radiation in a chicane are considered. Measurements of these effects on seeded electron microbunches were performed with an RF deflecting structure and a dipole magnet which streak out the electron bunch for single-shot images of the longitudinal phase-space distribution. Particle tracking simulations in 3D predicted the compression dynamics of the seeded microbunches with collective effects.
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WEP045	Study on Beam Modulation Technique using a Masked Chicane at FAST (Fermilab Accelerator Science and Technology) Facility	simulation, dipole, space-charge, linac	665
	Y.-M. Shin, D.R. Broemmelsiek, D.J. Crawford, A.H. Lumpkin Fermilab, Batavia, Illinois, USA A.T. Green Northern Illinois Univerity, Dekalb, Illinois, USA
	Funding: This work was supported by the DOE contract No. DEAC02-07CH11359 to the Fermi Research Alliance LLC. Longitudinal density modulations on electron beams can improve machine performance of beam-driven accelerators and FELs with resonance beam-wave coupling. The sub-ps beam modulation has been studied with a masked chicane by the analytic model and simulations with the beam parameters of the Advanced Superconducting Test Accelerator (ASTA) in Fermilab. With the chicane design parameters (bending angle of 18 degree, bending radius of 0.95 m and R56 ~ - 0.19 m) and a nominal beam of 3-ps bunch length, the analytic model showed that a slit-mask with slit period 900 microns and aperture width 300 microns generates about 100 microns modulation periodicity with 2.4% correlated energy spread. With the designed slit mask and a 3- ps bunch, particle-in-cell simulations (CST-PS), including nonlinear energy distributions, space charge force, and coherent synchrotron radiation (CSR) effect, also result in ~ 100 microns of longitudinal modulation. The beam modulation has been extensively examined with three different beam conditions, 2.25 ps (0.25 nC), 3.25 ps (1 nC), and 4.75 ps (3.2 nC), by extended 3D tracking simulations (Elegant). The modulated bunch generation will be tested by a slit-mask installed at the chicane of the ASTA 50-MeV-injector beamline for beam-driven acceleration experiments.
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WEP067	Simulation of Cascaded Longitudinal-Space-Charge Amplifier at the Fermilab Accelerator Science & Technology (Fast) Facility	space-charge, simulation, radiation, impedance	707
	A. Halavanau, P. Piot Northern Illinois University, DeKalb, Illinois, USA P. Piot Fermilab, Batavia, Illinois, USA
	Funding: This work was supported by the US Department of Energy under contract DE-SC0011831 with Northern Illinois University. Cascaded longitudinal space-charge amplifier (LSCA) have been proposed as a mechanism to generate density modulation over broadband.[1] The scheme was recently demonstrated in the optical regime and confirmed the production of broadband optical radiation.[2] In this paper we investigate, via numerical simulations, the performances of a cascaded LSCA beamline at the Fermilab's Advanced Superconducting Test Accelerator (ASTA) to produce broadband ultraviolet radiation. Our studies are carried using a three-dimensional space charge algorithm coupled with ELEGANT [3] and based on a tree-based space-charge algorithm (see details in Ref. [4]) [1] M. Dohlus, PRSTAB, 14 090702 (2011). [2] A. Marinelli, PRL, 110 264802 (2013). [3] M. Borland, Advanced Photon Source, LS-287, 2000. [4] A. Halavanau, Proc. IPAC15, TUPMA007 (2015).
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WEP084	Microbunching-Instability-Induced Sidebands in a Seeded Free-Electron Laser	undulator, FEL, simulation, electron	741
	Z. Zhang TUB, Beijing, People's Republic of China Y. Ding, W.M. Fawley, Z. Huang, J. Krzywinski, A.A. Lutman, G. Marcus, A. Marinelli, D.F. Ratner SLAC, Menlo Park, California, USA
	The measured, self-seeded soft X-ray radiation spectrum corresponding to multiple effective undulator lengths of the LCLS exhibits a pedestal-like distribution around the seeded frequency. In the absence of a post-undulator monochromator, this contamination limits the spectral purity and may seriously degrade certain user applications. In general for either externally- or self-seeded FELs, such pedestals may originate with any time-varying property of the electron beam that can modulate the complex gain function. In this paper we specifically focus on the contributions of electron beam microbunching prior to the undulator. We show that both energy and density modulations can induce sidebands in a seeded FEL configuration. Analytic FEL theory and numerical simulations are used to analyze the sideband content relative to the amplified seeded signal, and to compare with experimental results.
	Poster WEP084 [1.263 MB]
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WED01	Commissioning of the Delta Polarizing Undulator at LCLS	undulator, polarization, radiation, electron	757
	H.-D. Nuhn, S.D. Anderson, R.N. Coffee, Y. Ding, Z. Huang, M. Ilchen, Yu.I. Levashov, A.A. Lutman, J.P. MacArthur, A. Marinelli, S.P. Moeller, F. Peters, Z.R. Wolf SLAC, Menlo Park, California, USA J. Buck XFEL. EU, Hamburg, Germany G. Hartmann, J. Viefhaus DESY, Hamburg, Germany A.O. Lindahl University of Gothenburg, Gothenburg, Sweden A.B. Temnykh Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA
	Funding: Work was supported by U.S. DOE, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515. A.B. Temnykh is supported U.S. National Science Foundation awards DMR-0807731 and DMR-DMR-0936384. The LCLS generates linearly polarized, intense, high brightness x-ray pulses from planar fixed-gap undulators, which provides only limited taper capability and lacks polarization control. The latter is of great importance for soft x-ray experiments. A new 3.2-m-long compact undulator (based on the Cornell University fixed-gap Delta design) has been developed and installed as the last LCLS undulator segment (U33) in October 2014. The Delta undulator provides full control of the polarization degree and K parameter through array position adjustments. Used on its own, it produces fully polarized spontaneous radiation in the selected state (linear, circular or elliptical). To increase the output power by orders of magnitude, the electron beam is micro-bunched by several (5-15) upstream LCLS undulator segments operated in the linear FEL regime. This micro-bunching process produces horizontally linear polarized (background) radiation. This unwanted radiation component has been greatly reduced by a reversed taper configuration, as suggested by Schneidmiller. Full elimination of the linear polarized component was achieved through spatial separation combined with transverse collimation. The paper will describe the methods tested during commissioning and will also present results of polarization measurements showing high degrees of circular polarization in the soft x-ray wavelength range.
	Slides WED01 [10.165 MB]
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Paper

Title

Other Keywords

Page

MOC04

Operating of SXFEL in a Single Stage High Gain Harmonic Generation Scheme

laser, FEL, electron, linac

29

G.L. Wang, D.X. Dai, G.R. Wu, X.M. Yang, W.Q. Zhang
DICP, Dalian, People's Republic of China

The beam energy spread at the entrance of undulator system is of paramount importance for efficient density modulation in high-gain seeded free-electron lasers (FELs). In this paper, the dependencies of high harmonic bunching efficiency in the high-gain harmonic generation (HGHG) schemes on the electron energy spread distribution are studied. Theoretical investigations and multi-dimensional numerical simulations are applied to the cases of uniform and saddle beam energy distributions and compared to a traditional Gaussian distribution. It shows that the uniform and saddle electron energy distributions significantly enhance the performance of HGHG-FELs. A numerical example demonstrates that, with the saddle distribution of sliced beam energy spread controlled by a laser heater, the 30th harmonic radiation can be directly generated by a single-stage seeding scheme for a soft x-ray FEL facility.

Slides MOC04 [3.345 MB]

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MOP030

Study of Smith-Purcell Free Electron Laser Using Electron Bunch Produced By Micro-Pulse Electron Gun

electron, radiation, simulation, cavity

93

J. Zhao, X.Y. Lu, W.W. Tan, D.Y. Yang, Y. Yang, Z.Q. Yang
PKU, Beijing, People's Republic of China

A Micro-Pulse electron Gun (MPG) with the frequency of 2856 MHz has been designed, constructed and tested. Some primary experimental studies have been carried out and electron beam with the average current of 6 mA has been detected which holds promise to use as an electron source of Smith-Purcell Free Electron Laser (SP-FEL) to produced Coherent Radiation. It is well known that Smith-Purcell radiation is one of the achievable ways to produce FEL. After many years study in theory and experiment, lots of new mechanisms and appearances have been discovered. Coherent Smith-Purcell Radiation was discovered in 1990s as well. Compared with incoherent Smith-Purcell Radiation, It can generate a more powerful and frequency locked coherent emission due to displaying all three of these enhancements, Ng (the number of grating periods), Ne (the number of electrons in the bunch), Nb (the number of electron bunch). Obviously, MPG is one of ideal electron sources of CSPR for that (1) S-band electron source can increase energy density at these frequencies, (2) picosecond or subpicosecond pulse can generate THz radiation, (3) low emittance makes the interactions between electron beam and granting more stable. All of the above will be displayed in the simulation of this article. The progress of the experiment with beam energy of 80 Kev, the average current of 6 mA is also introduced.

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MOP087

Multistage CSR Microbunching Gain Development in Transport or Recirculation Arcs

lattice, dipole, damping, optics

263

C.-Y. Tsai
Virginia Polytechnic Institute and State University, Blacksburg, Virginia, USA
D. Douglas, R. Li, C. Tennant
JLab, Newport News, Virginia, USA

Funding: Authored by Jefferson Science Associates, LLC under U.S. DOE Contract No. DE-AC05-06OR23177.
Coherent synchrotron radiation (CSR) induced microbunching instability has been one of the most challenging issues in the design of modern accelerators. A linear Vlasov solver has been developed [1] and applied to investigate the physical processes of microbunching gain amplification for several example lattices [2]. In this paper, by further extending the concept of stage gain as proposed by Huang and Kim [3], we develop a method to characterize the microbunching development in terms of stage orders that allow the quantitative comparison of optics impacts on microbunching gain for different lattices. We find that the microbunching instability in our demonstrated arcs has a distinguishing feature of multistage amplification (e.g, up to 6th stage amplification for our example transport arcs, in contrast to two-stage amplification for a typical 4-dipole bunch compressor chicane). We also try to connect lattice optics pattern with the obtained stage gain functions by a physical interpretation. This Vlasov analysis is validated by ELEGANT [4] tracking results with excellent agreement.
[1] C. -Y. Tsai et al., MOP052, these proceedings
[2] See, for example, C. -Y. Tsai et al., ERL2015 (TUICLH2034)
[3] Z. Huang and K. -J. Kim, Phys. Rev. ST Accel. Beams 5, 074401 (2002)
[4] M. Borland, APS Light Source Note LS-287 (2000)

Poster MOP087 [1.962 MB]

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MOD02

Overview of Alternative Bunching and Current-shaping Techniques for Low-Energy Electron Beams

electron, laser, wakefield, radiation

274

P. Piot
Fermilab, Batavia, Illinois, USA
P. Piot
Northern Illinois University, DeKalb, Illinois, USA

Funding: This work was supported by the U.S. Department of Energy contracts No. DE-SC0011831 to Northern Illinois University and No. DE-AC02-07CH11359 with the Fermi Research Alliance, LLC
Techniques to bunch or shape an electron beam at low energies (E <15 MeV) have important implications toward the realization of table-top radiation sources [1] or to the design of compact multi-user free-electron lasers[2]. This paper provides an overview of alternative methods recently developed including techniques such as wakefield-based bunching, space-charge-driven microbunching via wave-breaking [3], ab-initio shaping of the electron-emission process [4], and phase space exchangers. Practical applications of some of these methods to foreseen free-electron-laser configurations are also briefly discussed [5].
[1] W. S. Graves, PRL 108, 263904 (2012)
[2] A. Zholents, FEL14, 993 (2014)
[3] P. Musumeci, PRL 106, 184801 (2011)
[4] F. Lemery, PRSTAB 17, 112804 (2014)
[5] G. Penco, PRL 112, 044801 (2014)

Slides MOD02 [5.426 MB]

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TUC01

The Microbunching Instability and LCLS-II Lattice Design

simulation, laser, FEL, linac

308

M. Venturini
LBNL, Berkeley, California, USA

The microbunching instability is a pervasive occurrence when high brightness electron beams are accelerated and transported through dispersive sections like bunch-compression chicanes or distributions beamlines. If uncontrolled the instability can severely compromise the performance of x-ray FELs, where beam high brightness is crucial. In this talk we discuss how consideration of the microbunching instability is informing the LCLS-II design and determining the specifications for the laser heater and transport lines. We also review some of the expected and not so-expected phenomena that we have encountered while carrying out high-resolution macroparticle simulations of the instability and the analytical models we have developed to interpret the numerical results.

Slides TUC01 [4.206 MB]

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TUP071

A FAST Particle Tracking Code

wakefield, linac, collective-effects, space-charge

530

L. Wang
SLAC, Menlo Park, California, USA

This paper presents a fast particle tracking (FPT) code for linac beam dynamics. It includes wake fields, coherent synchrotron radiation (CSR) and longitudinal space charge. We systematically benchmark the FPT with ELEGANT with different physics aspects: pure optics, wakefields, CSR and space-charge forces

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TUD01

COTR Resistant Profile Monitor

electron, laser, diagnostics, radiation

554

H. Loos
SLAC, Menlo Park, California, USA

Funding: Work supported by DOE contract DE-AC02-76SF00515
Electron beam accelerators used as drivers for short wavelength FELs need ultra-high brightness beams with small emittances and highly compressed bunch lengths. The acceleration and beam transport process of such beams leads to micro-bunching instabilities which cause the emergence of coherent optical transition radiation (COTR). The effect of COTR on profile monitors based on OTR or fluorescent screens can be quite detrimental to their intended use to measure beam sizes and profiles. This presentation will review past observations of the beam diagnostics issues due to COTR and discuss various mitigation schemes for profile monitors as well as present experience with such implementations.

Slides TUD01 [1.536 MB]

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WEP004

Energy Spread Constraints on Field Suppression in a Reverse Tapered Undulator

undulator, laser, FEL, simulation

597

J.P. MacArthur, Z. Huang, A.A. Lutman, A. Marinelli, H.-D. Nuhn
SLAC, Menlo Park, California, USA

A 3.2 m variable polarization Delta undulator[1] has been installed at the end of the LCLS undulator line. The Delta undulator acts an an afterburner in this configuration, using bunching from upstream planar undulators to produce radiation with arbitrary polarization. To optimize the degree of polarization from this device, a reverse taper[2] has been proposed to suppress background radiation produced in upstream undulators while still microbunching the beam. Here we extend previous work on free electron lasers with a slowly varying undulator parameter[3] to show there is a strong energy spread dependence to the maximum allowable detune from resonance. At LCLS, this energy spread limitation keeps the reverse taper slope in the slowly varying regime and limits the achievable degree of circular polarization.
[1] A. B. Temnykh, PRST-AB, 11, 120702, (2008).
[2] E. A. Schneidmiller and M. V. Yurkov, PRST-AB, 16, 110702, (2013).
[3] Z. Huang and G. Stupakov, PRST-AB, 8, 040702, (2005).

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WEP031

Measurements and Simulations of Seeded Electron Microbunches with Collective Effects

electron, simulation, laser, FEL

650

K.E. Hacker, S. Khan, R. Molo
DELTA, Dortmund, Germany
S. Ackermann, J. Bödewadt, M. Dohlus, N. Ekanayake, T. Laarmann, H. Schlarb
DESY, Hamburg, Germany
L.L. Lazzarino, C. Lechner, Th. Maltezopoulos, T. Plath, J. Roßbach
Uni HH, Hamburg, Germany

Funding: The experiments were carried out at FLASH at DESY. BMBF contract No. 05K10PE1, 05K10PE3, 05K13GU4, and 05K13PE3, and the German Research Foundation program graduate school 1355.
Measurements of the longitudinal phase-space distribution of electron bunches seeded with an external laser were done in order to study the impact of collective effects on seeded microbunches in free-electron lasers. Velocity bunching of a seeded microbunch appears to be a viable alternative to compression with a magnetic chicane under high-gain harmonic generation seeding conditions when the collective effects of Coulomb forces in a drift space and coherent synchrotron radiation in a chicane are considered. Measurements of these effects on seeded electron microbunches were performed with an RF deflecting structure and a dipole magnet which streak out the electron bunch for single-shot images of the longitudinal phase-space distribution. Particle tracking simulations in 3D predicted the compression dynamics of the seeded microbunches with collective effects.

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WEP045

Study on Beam Modulation Technique using a Masked Chicane at FAST (Fermilab Accelerator Science and Technology) Facility

simulation, dipole, space-charge, linac

665

Y.-M. Shin, D.R. Broemmelsiek, D.J. Crawford, A.H. Lumpkin
Fermilab, Batavia, Illinois, USA
A.T. Green
Northern Illinois Univerity, Dekalb, Illinois, USA

Funding: This work was supported by the DOE contract No. DEAC02-07CH11359 to the Fermi Research Alliance LLC.
Longitudinal density modulations on electron beams can improve machine performance of beam-driven accelerators and FELs with resonance beam-wave coupling. The sub-ps beam modulation has been studied with a masked chicane by the analytic model and simulations with the beam parameters of the Advanced Superconducting Test Accelerator (ASTA) in Fermilab. With the chicane design parameters (bending angle of 18 degree, bending radius of 0.95 m and R56 ~ - 0.19 m) and a nominal beam of 3-ps bunch length, the analytic model showed that a slit-mask with slit period 900 microns and aperture width 300 microns generates about 100 microns modulation periodicity with 2.4% correlated energy spread. With the designed slit mask and a 3- ps bunch, particle-in-cell simulations (CST-PS), including nonlinear energy distributions, space charge force, and coherent synchrotron radiation (CSR) effect, also result in ~ 100 microns of longitudinal modulation. The beam modulation has been extensively examined with three different beam conditions, 2.25 ps (0.25 nC), 3.25 ps (1 nC), and 4.75 ps (3.2 nC), by extended 3D tracking simulations (Elegant). The modulated bunch generation will be tested by a slit-mask installed at the chicane of the ASTA 50-MeV-injector beamline for beam-driven acceleration experiments.

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WEP067

Simulation of Cascaded Longitudinal-Space-Charge Amplifier at the Fermilab Accelerator Science & Technology (Fast) Facility

space-charge, simulation, radiation, impedance

707

A. Halavanau, P. Piot
Northern Illinois University, DeKalb, Illinois, USA
P. Piot
Fermilab, Batavia, Illinois, USA

Funding: This work was supported by the US Department of Energy under contract DE-SC0011831 with Northern Illinois University.
Cascaded longitudinal space-charge amplifier (LSCA) have been proposed as a mechanism to generate density modulation over broadband.[1] The scheme was recently demonstrated in the optical regime and confirmed the production of broadband optical radiation.[2] In this paper we investigate, via numerical simulations, the performances of a cascaded LSCA beamline at the Fermilab's Advanced Superconducting Test Accelerator (ASTA) to produce broadband ultraviolet radiation. Our studies are carried using a three-dimensional space charge algorithm coupled with ELEGANT [3] and based on a tree-based space-charge algorithm (see details in Ref. [4])
[1] M. Dohlus, PRSTAB, 14 090702 (2011).
[2] A. Marinelli, PRL, 110 264802 (2013).
[3] M. Borland, Advanced Photon Source, LS-287, 2000.
[4] A. Halavanau, Proc. IPAC15, TUPMA007 (2015).

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WEP084

Microbunching-Instability-Induced Sidebands in a Seeded Free-Electron Laser

undulator, FEL, simulation, electron

741

Z. Zhang
TUB, Beijing, People's Republic of China
Y. Ding, W.M. Fawley, Z. Huang, J. Krzywinski, A.A. Lutman, G. Marcus, A. Marinelli, D.F. Ratner
SLAC, Menlo Park, California, USA

The measured, self-seeded soft X-ray radiation spectrum corresponding to multiple effective undulator lengths of the LCLS exhibits a pedestal-like distribution around the seeded frequency. In the absence of a post-undulator monochromator, this contamination limits the spectral purity and may seriously degrade certain user applications. In general for either externally- or self-seeded FELs, such pedestals may originate with any time-varying property of the electron beam that can modulate the complex gain function. In this paper we specifically focus on the contributions of electron beam microbunching prior to the undulator. We show that both energy and density modulations can induce sidebands in a seeded FEL configuration. Analytic FEL theory and numerical simulations are used to analyze the sideband content relative to the amplified seeded signal, and to compare with experimental results.

Poster WEP084 [1.263 MB]

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WED01

Commissioning of the Delta Polarizing Undulator at LCLS

undulator, polarization, radiation, electron

757

H.-D. Nuhn, S.D. Anderson, R.N. Coffee, Y. Ding, Z. Huang, M. Ilchen, Yu.I. Levashov, A.A. Lutman, J.P. MacArthur, A. Marinelli, S.P. Moeller, F. Peters, Z.R. Wolf
SLAC, Menlo Park, California, USA
J. Buck
XFEL. EU, Hamburg, Germany
G. Hartmann, J. Viefhaus
DESY, Hamburg, Germany
A.O. Lindahl
University of Gothenburg, Gothenburg, Sweden
A.B. Temnykh
Cornell University (CLASSE), Cornell Laboratory for Accelerator-Based Sciences and Education, Ithaca, New York, USA

Funding: Work was supported by U.S. DOE, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515. A.B. Temnykh is supported U.S. National Science Foundation awards DMR-0807731 and DMR-DMR-0936384.
The LCLS generates linearly polarized, intense, high brightness x-ray pulses from planar fixed-gap undulators, which provides only limited taper capability and lacks polarization control. The latter is of great importance for soft x-ray experiments. A new 3.2-m-long compact undulator (based on the Cornell University fixed-gap Delta design) has been developed and installed as the last LCLS undulator segment (U33) in October 2014. The Delta undulator provides full control of the polarization degree and K parameter through array position adjustments. Used on its own, it produces fully polarized spontaneous radiation in the selected state (linear, circular or elliptical). To increase the output power by orders of magnitude, the electron beam is micro-bunched by several (5-15) upstream LCLS undulator segments operated in the linear FEL regime. This micro-bunching process produces horizontally linear polarized (background) radiation. This unwanted radiation component has been greatly reduced by a reversed taper configuration, as suggested by Schneidmiller. Full elimination of the linear polarized component was achieved through spatial separation combined with transverse collimation. The paper will describe the methods tested during commissioning and will also present results of polarization measurements showing high degrees of circular polarization in the soft x-ray wavelength range.

Slides WED01 [10.165 MB]

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