FEL2015 - List of Authors (Huang, Z.)

Paper	Title	Page
MOB02	X-Ray FEL R&D: Brighter, Better and Cheaper	7
	Z. Huang SLAC, Menlo Park, California, USA
	The X-ray free-electron lasers (FELs), with nine to ten orders of magnitude improvement in peak brightness over the third-generation light sources, have demonstrated remarkable scientific capabilities. Despite the early success, X-ray FELs can still undergo dramatic transformations with accelerator and FEL R&D. In this talk, I will show examples of recent R&D efforts to increase X-ray coherence and brightness, to obtain better control of X-ray temporal and spectral properties, and to develop concepts for compact coherent sources.
	Slides MOB02 [18.004 MB]
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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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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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TUP070	Energy Jitter Minimization at LCLS	523
	L. Wang, A.L. Benwell, A. Brachmann, W.S. Colocho, F.-J. Decker, Z. Huang, T.J. Maxwell, T. Tao, J.L. Turner SLAC, Menlo Park, California, USA
	The energy jitters of the electron beam can affects the FEL in self-seeded modes if the jitter is large compared to the FEL parameter. We work in multiple ways to reduce the jitters, including hardware improvement, optimization linac set-up. This paper discusses the optimization of linac set-up. The solutions always suggest that we can largely reduce the energy jitter from a weak compression at BC1 and a stronger compression at BC2. Meanwhile a low beam energy at BC2 also reduce the energy jitter, which is confirmed by the experiment. The results can be explained by a simple model. Experimental results are also presented, demonstrating better than 20% and 40% relative energy jitter reduction for 13.6 and 4 GeV linac operation, respectively.
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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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WEP004	Energy Spread Constraints on Field Suppression in a Reverse Tapered Undulator	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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WEP005	Laser Heater Transverse Shaping to Improve Microbunching Suppresion for X-ray FELs	602
	S. Li Stanford University, Stanford, California, USA A.R. Fry, S. Gilevich, Z. Huang, A. Marinelli, D.F. Ratner, J. Robinson SLAC, Menlo Park, California, USA
	In X-ray free electron lasers (FELs), a small amount of initial density or energy modulation in the electron beam will be amplified through acceleration and bunch compression process. The undesired microbunching on the electron bunch will increase slice energy spread and degrade the FEL performance. The Linac Coherent Light Source (LCLS) laser heater (LH) system was installed to increase the uncorrelated energy spread in the electron beam in order to suppress the microbunching instability. The distribution of the induced energy spread depends strongly on the transverse profile of the heater laser and has a large effect on the microbunching suppression. In this paper we discuss strategies to shape the laser profile in order to obtain better suppression of microbunching. We present analysis to achieve the Gaussian-like energy spread using a Laguerre-Gaussian laser mode and study the efficiency and alignment tolerance for implementation.
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WEP023	Two Bunches with ns-Separation with LCLS	634
	F.-J. Decker, S. Gilevich, Z. Huang, H. Loos, A. Marinelli, C.A. Stan, J.L. Turner, Z. Van Hoover, S. Vetter SLAC, Menlo Park, California, USA
	Funding: Work supported by U.S. Department of Energy, Contract DE-AC02-76SF00515. The Linac Coherent Light Source (LCLS) delivers typically one bunch. Two bunches are interesting for pump / probe experiments. Two electron bunches with ps separation have been already produced using a split and delay in the laser which produces them on the gun cathode. Here we present the combination of two lasers with a combiner, this allows any time separation and is it limited to RF bucket spacing so far to about 40 ns limited by the setup of our beam containment system. Different beam energies were also provided and the most challenging part was a transverse separation of a few σs for the two beams. Although this setup was very jittery a successful user experiment was accomplished.
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WEP025	Effect of Microbunching on Seeding Schemes for LCLS-II	639
	G. Penn, J. Qiang LBNL, Berkeley, California, USA P. Emma, E. Hemsing, Z. Huang, G. Marcus, T.O. Raubenheimer, L. Wang SLAC, Menlo Park, California, USA
	Funding: This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. External seeding and self-seeding schemes are particularly sensitive to distortions and fluctuations in the electron beam profile. Wakefields and the microbunching instability are important sources of such imperfections. Even at modest levels, their influence can degrade the spectrum and decrease the output brightness. These effects are evaluated for seeded FELs at the soft X-ray beam line of LCLS-II. FEL simulations are performed in GENESIS based on various realistic electron distributions obtained using the IMPACT tracking code. The sensitivity depends on both the seeding scheme and the output wavelength.
	Poster WEP025 [0.962 MB]
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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

X-Ray FEL R&D: Brighter, Better and Cheaper

7

Z. Huang
SLAC, Menlo Park, California, USA

The X-ray free-electron lasers (FELs), with nine to ten orders of magnitude improvement in peak brightness over the third-generation light sources, have demonstrated remarkable scientific capabilities. Despite the early success, X-ray FELs can still undergo dramatic transformations with accelerator and FEL R&D. In this talk, I will show examples of recent R&D efforts to increase X-ray coherence and brightness, to obtain better control of X-ray temporal and spectral properties, and to develop concepts for compact coherent sources.

Slides MOB02 [18.004 MB]

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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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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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TUP070

Energy Jitter Minimization at LCLS

523

L. Wang, A.L. Benwell, A. Brachmann, W.S. Colocho, F.-J. Decker, Z. Huang, T.J. Maxwell, T. Tao, J.L. Turner
SLAC, Menlo Park, California, USA

The energy jitters of the electron beam can affects the FEL in self-seeded modes if the jitter is large compared to the FEL parameter. We work in multiple ways to reduce the jitters, including hardware improvement, optimization linac set-up. This paper discusses the optimization of linac set-up. The solutions always suggest that we can largely reduce the energy jitter from a weak compression at BC1 and a stronger compression at BC2. Meanwhile a low beam energy at BC2 also reduce the energy jitter, which is confirmed by the experiment. The results can be explained by a simple model. Experimental results are also presented, demonstrating better than 20% and 40% relative energy jitter reduction for 13.6 and 4 GeV linac operation, respectively.

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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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WEP004

Energy Spread Constraints on Field Suppression in a Reverse Tapered Undulator

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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WEP005

Laser Heater Transverse Shaping to Improve Microbunching Suppresion for X-ray FELs

602

S. Li
Stanford University, Stanford, California, USA
A.R. Fry, S. Gilevich, Z. Huang, A. Marinelli, D.F. Ratner, J. Robinson
SLAC, Menlo Park, California, USA

In X-ray free electron lasers (FELs), a small amount of initial density or energy modulation in the electron beam will be amplified through acceleration and bunch compression process. The undesired microbunching on the electron bunch will increase slice energy spread and degrade the FEL performance. The Linac Coherent Light Source (LCLS) laser heater (LH) system was installed to increase the uncorrelated energy spread in the electron beam in order to suppress the microbunching instability. The distribution of the induced energy spread depends strongly on the transverse profile of the heater laser and has a large effect on the microbunching suppression. In this paper we discuss strategies to shape the laser profile in order to obtain better suppression of microbunching. We present analysis to achieve the Gaussian-like energy spread using a Laguerre-Gaussian laser mode and study the efficiency and alignment tolerance for implementation.

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WEP023

Two Bunches with ns-Separation with LCLS

634

F.-J. Decker, S. Gilevich, Z. Huang, H. Loos, A. Marinelli, C.A. Stan, J.L. Turner, Z. Van Hoover, S. Vetter
SLAC, Menlo Park, California, USA

Funding: Work supported by U.S. Department of Energy, Contract DE-AC02-76SF00515.
The Linac Coherent Light Source (LCLS) delivers typically one bunch. Two bunches are interesting for pump / probe experiments. Two electron bunches with ps separation have been already produced using a split and delay in the laser which produces them on the gun cathode. Here we present the combination of two lasers with a combiner, this allows any time separation and is it limited to RF bucket spacing so far to about 40 ns limited by the setup of our beam containment system. Different beam energies were also provided and the most challenging part was a transverse separation of a few σs for the two beams. Although this setup was very jittery a successful user experiment was accomplished.

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WEP025

Effect of Microbunching on Seeding Schemes for LCLS-II

639

G. Penn, J. Qiang
LBNL, Berkeley, California, USA
P. Emma, E. Hemsing, Z. Huang, G. Marcus, T.O. Raubenheimer, L. Wang
SLAC, Menlo Park, California, USA

Funding: This work was supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.
External seeding and self-seeding schemes are particularly sensitive to distortions and fluctuations in the electron beam profile. Wakefields and the microbunching instability are important sources of such imperfections. Even at modest levels, their influence can degrade the spectrum and decrease the output brightness. These effects are evaluated for seeded FELs at the soft X-ray beam line of LCLS-II. FEL simulations are performed in GENESIS based on various realistic electron distributions obtained using the IMPACT tracking code. The sensitivity depends on both the seeding scheme and the output wavelength.

Poster WEP025 [0.962 MB]

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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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