FEL2015 - List of Authors (Ding, Y.)

Paper	Title	Page
MOP076	Free-Electron Laser Driven by a 500 MeV Laser Plasma Accelerator Beam	217
	W. Qin, J.E. Chen, S. Huang, K.X. Liu, X.Q. Yan, L. Zeng PKU, Beijing, People's Republic of China Y. Ding, Z. Huang SLAC, Menlo Park, California, USA
	A laser plasma accelerator is under construction at Peking University and several hundred MeV electron beams are expected. In this paper we discuss applying a 500 MeV beam with 1% relative energy spread to FEL. Bunch decompression method is considered to deal with the large energy spread of the beam. Emittance growth induced by large divergence and energy spread in electron beam transport has been treated with the chromatic matching manipulation. Simulation shows that 100 MW level, 6.3 fs , 0.008 bandwidth output can be obtained for 30 nm FEL. TGU method with assumed matched beam is also discussed as a comparison.
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TUA01	Generating Femtosecond to Sub-Femtosecond X-Ray Pulses at Free-Electron Lasers
	Y. Ding SLAC, Menlo Park, California, USA
	Generation of high power, femtosecond to sub-femtosecond X-ray pulses is attracting much attention within the X-ray free-electron laser (FEL) user community. At the existing FEL facilities, such as the Linac Coherent Light Source at SLAC, low-charge operation mode and emittance-spoiling foil scheme have been developed to deliver sub-10 fs X-rays to user experiments. We recently performed extensive measurements on these two modes using the X-band transverse deflector diagnostic at LCLS, demonstrating a flexible pulse duration control with direct measurements. To achieve sub-fs X-ray pulses, a simple nonlinear compression mode has been recently developed with pulse duration about 200 as [1]. Experimental study of this sub-fs mode will be pursued in this year. We report the recent experimental studies for generating femtosecond to sub-fs x-ray pulses. [1] S. Huang, Y. Ding, Z. Huang, and J. Qiang, Phys. Rev. ST Accel. Beams 17, 120703 (2014)
	Slides TUA01 [2.922 MB]
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TUB03	Generating Femtosecond to Sub-Femtosecond X-Rays with a Modulated Chirped Beam in a Self-Seeded FEL
	S. Huang PKU, Beijing, People's Republic of China Y. Ding, Z. Huang, G. Marcus SLAC, Menlo Park, California, USA
	We propose a scheme to generate ultrashort soft X-ray pulses in a self-seeded FEL. In this scheme, a time-energy chirped electron beam is first modulated by an infrared laser with the wavelength of a few microns. It is then used to drive the self-seeded FEL. During the selfseeding section, besides the regular functions of the self-seeding chicane and the grating monochromator, the chicane is also used to shear the previously modulated electron beam, leading to current spikes in the temporal profile. Since the seeded pulse length from the chirped beam is much shorter than the electron bunch, we can choose to align the seed with one of the current spikes for generating a single short pulse. Simulations indicate that soft X-ray pulses with a fwhm of less than 1 fs and peak power at 10 GW level can be obtained.
	Slides TUB03 [1.585 MB]
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TUP007	High Fidelity Start-to-end Numerical Particle Simulations and Performance Studies for LCLS-II	342
	G. Marcus, Y. Ding, P. Emma, Z. Huang, T.O. Raubenheimer, L. Wang SLAC, Menlo Park, California, USA J. Qiang, M. Venturini LBNL, Berkeley, California, USA
	High fidelity numerical particle simulations that leverage a number of accelerator and FEL codes have been used to analyze the LCLS-II FEL performance. Together, the physics models that are included in these codes have been crucial in identifying, understanding, and mitigating a number of potential hazards that can adversely affect the FEL performance, some of which are discussed in papers submitted to this conference[, ]. Here, we present a broad overview of the LCLS-II FEL performance, based on these start-to-end simulations, for both the soft X-ray and hard X-ray undulators including both SASE and self-seeded operational modes. M. Venturini, et al., The microbunching instability and LCLS-II lattice design: lessons learned, FEL'15 ** Z. Zhang, et al., Microbunching-induced sidebands in a seeded free-electron laser, FEL'15
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TUP023	A Modified Self-Seeded X-ray FEL Scheme Towards Shorter Wavelengths	409
	L. Zeng, J.E. Chen, S. Huang, K.X. Liu, W. Qin PKU, Beijing, People's Republic of China Y. Ding, Z. Huang, G. Marcus SLAC, Menlo Park, California, USA
	We present a modified self-seeded FEL scheme for harmonic generation. Different from classical HGHG scheme whose seed laser is a conventional laser with longer wavelength, this scheme first uses a regular self-seeding monochromator to generate a seed laser, followed by a HGHG configuration to produce shorter-wavelength radiations. As an example, we perform start-to-end simulations to demonstrate the second and third harmonic FELs from a soft x-ray self-seeding case at the fundumental wavelength of 1.72 nm. The harmonic performance results will be discussed.
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WEB03	Progress Towards an X-ray FEL Oscillator
	K.-J. Kim, T. Kolodziej, R.R. Lindberg, D. Shu, Yu. Shvyd'ko, S. Stoupin ANL, Argonne, Ilinois, USA J. Arthur, Y. Ding, W.M. Fawley, J.C. Frisch, J.B. Hastings, Z. Huang, J. Krzywinski, G. Marcus, T.J. Maxwell SLAC, Menlo Park, California, USA
	Funding: Work at ANL supported under US Department of Energy contract DE-AC02-76SF00515 and at SLAC by the U.S. Department of Energy, Office of Science, under Contract No. DE-ACO2-O6CH11357 Issues and progress in R&D toward realizing an X-ray FEL oscillator will be discussed, including electron injector optimization, X-ray power density evolution on Bragg crystals throughout the lasing process, experimental efforts for testing radiation damage, evaluating the performance of compound refractive lenses (CRLs) as the focusing elements, and the basic considerations for mechanical layout. These will be discussed in the context of a concrete implementation scheme [] using the 4 GeV superconducting linac to be constructed at the LCLS-II. Time-dependent simulations of harmonic XFELO performance is discussed in another contribution to this conference by Zemella, et al. [] T. J. Maxwell et al., Feasibility study for an X-ray free electron laser oscillator, IPAC 2015, Richmond, Virginia, USA
	Slides WEB03 [6.452 MB]
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WEP003	Recent Understanding and Improvements of the LCLS Injector	592
	F. Zhou, D.K. Bohler, Y. Ding, S. Gilevich, Z. Huang, H. Loos, D.F. Ratner SLAC, Menlo Park, California, USA
	Funding: U.S. DOE contract No. DE-AC02-76SF00515. Ultraviolet drive laser and copper photocathode are the key systems for reliably delivering <0.4 micron of emittance and high brightness free electron laser (FEL) at the linac coherent light source (LCLS). Characterizing, optimizing and controlling laser distributions in both spatial and temporal directions are important for ultra-low emittance generation. Spatial truncated Gaussian laser profile has been demonstrated to produce better emittance than a spatial uniform beam. Sensitivity of the spatial laser distribution for the emittance is measured and analysed. Stacking two 2-ps Gaussian laser beams significantly improves emittance and eventually FEL performance at the LCLS in comparison to a single 2-ps Gaussian laser pulse. In addition, recent observations at the LCLS show that the micro-bunching effect depends strongly on the cathode spot locations. The dependence of the micro-bunching and FEL performance on the cathode spot location is mapped and discussed.
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WEP022	Photon Energies beyond the Selenium K-Edge at LCLS	630
	F.-J. Decker, W.S. Colocho, Y. Ding, R.H. Iverson, H. Loos, J. Sheppard, H. Smith, J.L. Turner SLAC, Menlo Park, California, USA
	Funding: Work supported by U.S. Department of Energy, Contract DE-AC02-76SF00515. The Linac Coherent Light Source (LCLS) was designed for a photon energies of 830 eV to 8.3 keV. This range was widened and up to 11.2 keV photons were already delivered for users. The Selenium K-edge at 12.6578 keV is very interesting since Selenium can replace Sulfur in biological structures and then that structure could be precisely measured. To reach this the electron energy would need to be raised by about 6% which initially didn't seem possible. The trick is to change the final compression scheme from a high correlated energy spread and moderate R56 in the compression chicane to moderate energy spread and high R56. The same bunch length can be achieved and RF energy is freed up, so the overall beam energy can be raised. Photons up to an energy of 12.82 keV (1.3% above the K-edge) with a pulse intensity of 0.93 mJ were achieved. The photon energy spread with this setup is wider at around 40-50 eV FWHM, since less correlated energy spread is left after the compression.
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WEP052	Studies of LCLS FEL Divergence	681
	J.L. Turner, P. Baxevanis, F.-J. Decker, Y. Ding, Z. Huang, J. Krzywinski, H. Loos, G. Marcus, N.P. Norvell SLAC, Menlo Park, California, USA
	Funding: This work was supported by U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515 Simulations show various impacts on x-ray divergence. With the motivation to maximize intensity at the focus, these beam studies were designed to study parameter space and beam qualities impacting divergence, and therefore aperture related clipping and diffraction. With multiple simultaneous users, beam constraints increase, requiring an improving knowledge of the mechanism of impact of changing parameters. These studies have that goal in order to improve beam control.
	Poster WEP052 [1.010 MB]
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WEP070	Start-to-End Simulation of the LCLS-II Beam Delivery System with Real Number of Electrons	714
	J. Qiang, C.E. Mitchell, C. F. Papadopoulos, M. Venturini LBNL, Berkeley, California, USA Y. Ding, P. Emma, Z. Huang, G. Marcus, Y. Nosochkov, T.O. Raubenheimer, L. Wang, M. Woodley SLAC, Menlo Park, California, USA
	The LCLS-II as a next generation high repetition rate FEL based X-ray light source will enable significant scientific discoveries. In this paper, we report on the progress in the design of the accelerator beam delivery system through start-to-end simulations. We will present simulation results for three cases, 20 pC, 100 pC and 300 pC that are transported through the hard X-ray line and the soft X-ray line for FEL radiation.
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WEP075	Femtosecond X-ray Pulse Generation with an Energy Chirped Electron Beam	722
	C. Emma, C. Pellegrini UCLA, Los Angeles, California, USA Y. Ding, Z. Huang, A.A. Lutman, G. Marcus, A. Marinelli, C. Pellegrini SLAC, Menlo Park, California, USA
	We study the generation of short (sub 10 fs) pulses in the X-ray spectral region using an energy chirped electron beam in a Self Amplified Spontaneous Emission Free Electron Laser (SASE FEL) and a self-seeding monochromator [1]-[2]. The monochromator filters a small bandwidth, short duration pulse from the frequency chirped SASE spectrum. This pulse is used to seed a small fraction of the long chirped beam, hence a short pulse with narrow bandwidth is amplified in the following undulators. We present start-to-end simulation results for LCLS operating in the soft X-ray self-seeded mode with an energy chirp of 1% over 30 fs and a bunch charge of 150pC. We demonstrate the potential to generate ~5 fs pulses with a bandwidth ~0.3eV. We also assess the possibility of further shortening the pulse by utilizing one more chicane after the self-seeding stage and shifting the radiation pulse to a 'fresh' part of the electron beam. Experimental study on this short pulse seeding mode has been planned at the LCLS.
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WEP084	Microbunching-Instability-Induced Sidebands in a Seeded Free-Electron Laser	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	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

Page

Free-Electron Laser Driven by a 500 MeV Laser Plasma Accelerator Beam

217

W. Qin, J.E. Chen, S. Huang, K.X. Liu, X.Q. Yan, L. Zeng
PKU, Beijing, People's Republic of China
Y. Ding, Z. Huang
SLAC, Menlo Park, California, USA

A laser plasma accelerator is under construction at Peking University and several hundred MeV electron beams are expected. In this paper we discuss applying a 500 MeV beam with 1% relative energy spread to FEL. Bunch decompression method is considered to deal with the large energy spread of the beam. Emittance growth induced by large divergence and energy spread in electron beam transport has been treated with the chromatic matching manipulation. Simulation shows that 100 MW level, 6.3 fs , 0.008 bandwidth output can be obtained for 30 nm FEL. TGU method with assumed matched beam is also discussed as a comparison.

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TUA01

Generating Femtosecond to Sub-Femtosecond X-Ray Pulses at Free-Electron Lasers

Y. Ding
SLAC, Menlo Park, California, USA

Generation of high power, femtosecond to sub-femtosecond X-ray pulses is attracting much attention within the X-ray free-electron laser (FEL) user community. At the existing FEL facilities, such as the Linac Coherent Light Source at SLAC, low-charge operation mode and emittance-spoiling foil scheme have been developed to deliver sub-10 fs X-rays to user experiments. We recently performed extensive measurements on these two modes using the X-band transverse deflector diagnostic at LCLS, demonstrating a flexible pulse duration control with direct measurements. To achieve sub-fs X-ray pulses, a simple nonlinear compression mode has been recently developed with pulse duration about 200 as [1]. Experimental study of this sub-fs mode will be pursued in this year. We report the recent experimental studies for generating femtosecond to sub-fs x-ray pulses.
[1] S. Huang, Y. Ding, Z. Huang, and J. Qiang, Phys. Rev. ST Accel. Beams 17, 120703 (2014)

Slides TUA01 [2.922 MB]

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TUB03

Generating Femtosecond to Sub-Femtosecond X-Rays with a Modulated Chirped Beam in a Self-Seeded FEL

S. Huang
PKU, Beijing, People's Republic of China
Y. Ding, Z. Huang, G. Marcus
SLAC, Menlo Park, California, USA

We propose a scheme to generate ultrashort soft X-ray pulses in a self-seeded FEL. In this scheme, a time-energy chirped electron beam is first modulated by an infrared laser with the wavelength of a few microns. It is then used to drive the self-seeded FEL. During the selfseeding section, besides the regular functions of the self-seeding chicane and the grating monochromator, the chicane is also used to shear the previously modulated electron beam, leading to current spikes in the temporal profile. Since the seeded pulse length from the chirped beam is much shorter than the electron bunch, we can choose to align the seed with one of the current spikes for generating a single short pulse. Simulations indicate that soft X-ray pulses with a fwhm of less than 1 fs and peak power at 10 GW level can be obtained.

Slides TUB03 [1.585 MB]

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TUP007

High Fidelity Start-to-end Numerical Particle Simulations and Performance Studies for LCLS-II

342

G. Marcus, Y. Ding, P. Emma, Z. Huang, T.O. Raubenheimer, L. Wang
SLAC, Menlo Park, California, USA
J. Qiang, M. Venturini
LBNL, Berkeley, California, USA

High fidelity numerical particle simulations that leverage a number of accelerator and FEL codes have been used to analyze the LCLS-II FEL performance. Together, the physics models that are included in these codes have been crucial in identifying, understanding, and mitigating a number of potential hazards that can adversely affect the FEL performance, some of which are discussed in papers submitted to this conference[*, **]. Here, we present a broad overview of the LCLS-II FEL performance, based on these start-to-end simulations, for both the soft X-ray and hard X-ray undulators including both SASE and self-seeded operational modes.
* M. Venturini, et al., The microbunching instability and LCLS-II lattice design: lessons learned, FEL'15
** Z. Zhang, et al., Microbunching-induced sidebands in a seeded free-electron laser, FEL'15

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TUP023

A Modified Self-Seeded X-ray FEL Scheme Towards Shorter Wavelengths

409

L. Zeng, J.E. Chen, S. Huang, K.X. Liu, W. Qin
PKU, Beijing, People's Republic of China
Y. Ding, Z. Huang, G. Marcus
SLAC, Menlo Park, California, USA

We present a modified self-seeded FEL scheme for harmonic generation. Different from classical HGHG scheme whose seed laser is a conventional laser with longer wavelength, this scheme first uses a regular self-seeding monochromator to generate a seed laser, followed by a HGHG configuration to produce shorter-wavelength radiations. As an example, we perform start-to-end simulations to demonstrate the second and third harmonic FELs from a soft x-ray self-seeding case at the fundumental wavelength of 1.72 nm. The harmonic performance results will be discussed.

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WEB03

Progress Towards an X-ray FEL Oscillator

K.-J. Kim, T. Kolodziej, R.R. Lindberg, D. Shu, Yu. Shvyd'ko, S. Stoupin
ANL, Argonne, Ilinois, USA
J. Arthur, Y. Ding, W.M. Fawley, J.C. Frisch, J.B. Hastings, Z. Huang, J. Krzywinski, G. Marcus, T.J. Maxwell
SLAC, Menlo Park, California, USA

Funding: Work at ANL supported under US Department of Energy contract DE-AC02-76SF00515 and at SLAC by the U.S. Department of Energy, Office of Science, under Contract No. DE-ACO2-O6CH11357
Issues and progress in R&D toward realizing an X-ray FEL oscillator will be discussed, including electron injector optimization, X-ray power density evolution on Bragg crystals throughout the lasing process, experimental efforts for testing radiation damage, evaluating the performance of compound refractive lenses (CRLs) as the focusing elements, and the basic considerations for mechanical layout. These will be discussed in the context of a concrete implementation scheme [*] using the 4 GeV superconducting linac to be constructed at the LCLS-II. Time-dependent simulations of harmonic XFELO performance is discussed in another contribution to this conference by Zemella, et al.
[*] T. J. Maxwell et al., Feasibility study for an X-ray free electron laser oscillator, IPAC 2015, Richmond, Virginia, USA

Slides WEB03 [6.452 MB]

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WEP003

Recent Understanding and Improvements of the LCLS Injector

592

F. Zhou, D.K. Bohler, Y. Ding, S. Gilevich, Z. Huang, H. Loos, D.F. Ratner
SLAC, Menlo Park, California, USA

Funding: U.S. DOE contract No. DE-AC02-76SF00515.
Ultraviolet drive laser and copper photocathode are the key systems for reliably delivering <0.4 micron of emittance and high brightness free electron laser (FEL) at the linac coherent light source (LCLS). Characterizing, optimizing and controlling laser distributions in both spatial and temporal directions are important for ultra-low emittance generation. Spatial truncated Gaussian laser profile has been demonstrated to produce better emittance than a spatial uniform beam. Sensitivity of the spatial laser distribution for the emittance is measured and analysed. Stacking two 2-ps Gaussian laser beams significantly improves emittance and eventually FEL performance at the LCLS in comparison to a single 2-ps Gaussian laser pulse. In addition, recent observations at the LCLS show that the micro-bunching effect depends strongly on the cathode spot locations. The dependence of the micro-bunching and FEL performance on the cathode spot location is mapped and discussed.

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WEP022

Photon Energies beyond the Selenium K-Edge at LCLS

630

F.-J. Decker, W.S. Colocho, Y. Ding, R.H. Iverson, H. Loos, J. Sheppard, H. Smith, J.L. Turner
SLAC, Menlo Park, California, USA

Funding: Work supported by U.S. Department of Energy, Contract DE-AC02-76SF00515.
The Linac Coherent Light Source (LCLS) was designed for a photon energies of 830 eV to 8.3 keV. This range was widened and up to 11.2 keV photons were already delivered for users. The Selenium K-edge at 12.6578 keV is very interesting since Selenium can replace Sulfur in biological structures and then that structure could be precisely measured. To reach this the electron energy would need to be raised by about 6% which initially didn't seem possible. The trick is to change the final compression scheme from a high correlated energy spread and moderate R56 in the compression chicane to moderate energy spread and high R56. The same bunch length can be achieved and RF energy is freed up, so the overall beam energy can be raised. Photons up to an energy of 12.82 keV (1.3% above the K-edge) with a pulse intensity of 0.93 mJ were achieved. The photon energy spread with this setup is wider at around 40-50 eV FWHM, since less correlated energy spread is left after the compression.

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WEP052

Studies of LCLS FEL Divergence

681

J.L. Turner, P. Baxevanis, F.-J. Decker, Y. Ding, Z. Huang, J. Krzywinski, H. Loos, G. Marcus, N.P. Norvell
SLAC, Menlo Park, California, USA

Funding: This work was supported by U.S. Department of Energy, Office of Basic Energy Sciences, under Contract DE-AC02-76SF00515
Simulations show various impacts on x-ray divergence. With the motivation to maximize intensity at the focus, these beam studies were designed to study parameter space and beam qualities impacting divergence, and therefore aperture related clipping and diffraction. With multiple simultaneous users, beam constraints increase, requiring an improving knowledge of the mechanism of impact of changing parameters. These studies have that goal in order to improve beam control.

Poster WEP052 [1.010 MB]

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WEP070

Start-to-End Simulation of the LCLS-II Beam Delivery System with Real Number of Electrons

714

J. Qiang, C.E. Mitchell, C. F. Papadopoulos, M. Venturini
LBNL, Berkeley, California, USA
Y. Ding, P. Emma, Z. Huang, G. Marcus, Y. Nosochkov, T.O. Raubenheimer, L. Wang, M. Woodley
SLAC, Menlo Park, California, USA

The LCLS-II as a next generation high repetition rate FEL based X-ray light source will enable significant scientific discoveries. In this paper, we report on the progress in the design of the accelerator beam delivery system through start-to-end simulations. We will present simulation results for three cases, 20 pC, 100 pC and 300 pC that are transported through the hard X-ray line and the soft X-ray line for FEL radiation.

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WEP075

Femtosecond X-ray Pulse Generation with an Energy Chirped Electron Beam

722

C. Emma, C. Pellegrini
UCLA, Los Angeles, California, USA
Y. Ding, Z. Huang, A.A. Lutman, G. Marcus, A. Marinelli, C. Pellegrini
SLAC, Menlo Park, California, USA

We study the generation of short (sub 10 fs) pulses in the X-ray spectral region using an energy chirped electron beam in a Self Amplified Spontaneous Emission Free Electron Laser (SASE FEL) and a self-seeding monochromator [1]-[2]. The monochromator filters a small bandwidth, short duration pulse from the frequency chirped SASE spectrum. This pulse is used to seed a small fraction of the long chirped beam, hence a short pulse with narrow bandwidth is amplified in the following undulators. We present start-to-end simulation results for LCLS operating in the soft X-ray self-seeded mode with an energy chirp of 1% over 30 fs and a bunch charge of 150pC. We demonstrate the potential to generate ~5 fs pulses with a bandwidth ~0.3eV. We also assess the possibility of further shortening the pulse by utilizing one more chicane after the self-seeding stage and shifting the radiation pulse to a 'fresh' part of the electron beam. Experimental study on this short pulse seeding mode has been planned at the LCLS.

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WEP084

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

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

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.

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