FEL2011 - Table of Session: MOPB (FEL Theory and New Concepts)

Paper	Title	Page
MOPB05	Smith-Purcell Radiation with Negative-index Material	20
	D. Li, K. Imasaki ILT, Suita, Osaka, Japan M.R. Asakawa Kansai University, Osaka, Japan M. Hangyo, K. Takano ILE Osaka, Suita, Japan S. Miyamoto LASTI, Hyogo, Japan Y. Tsunawaki OSU, Daito, Osaka, Japan Z. Yang UESTC, Chengdu, Sichuan, People's Republic of China
	Smith-Purcell radiation from an electric line charge that moves, at constant speed, parallel to a grating made of metamaterial with negative index is analyzed. Through theoretical analysis and computations, we show that the Smith-Purcell radiation is stronger from a grating of negative-index material, than positive-index material and perfect conductor. Also, we found the radiation strongly depends on the values of permeability and permittivity.

MOPB06	Smith-Purcell Free Electron Laser with Bragg Reflector	24
	D. Li, K. Imasaki ILT, Suita, Osaka, Japan M.R. Asakawa Kansai University, Osaka, Japan M. Hangyo ILE Osaka, Suita, Japan S. Miyamoto LASTI, Hyogo, Japan Y. Tsunawaki OSU, Daito, Osaka, Japan Z. Yang UESTC, Chengdu, Sichuan, People's Republic of China
	Grating with Bragg reflectors for the Smith-Purcell free-electron laser is proposed to improve the reflection coefficient, resulting in enhancing the interaction of the surface wave with the electron beam and, consequently, relax the requirements to the electron beam. With the help of particle-in-cell simulations, it has been shown that, the usage of Bragg reflectors may improve the growth rate, shorten the time for the device to reach saturation and lower the start current for the operation of a Smith-Purcell free-electron laser.

MOPB07	Soft X-ray Free-electron Laser with a 10-time Reduced Size	28
	F.H. Chao, C.H. Chen, K.Y. Huang, Y.-C. Huang NTHU, Hsinchu, Taiwan
	We present a 30-m long soft x-ray FEL consisting of a 5-MeV photoinjector, a 150 MeV linac, a magnetic chicane compressor, and a 3-m long undulator. We propose to employ both the 3rd and the 4th harmonics of a Nd laser at 355 and 266 nm, respectively, to illuminate the cathode of the photoinjector. Owing to the beating of the two lasers, the emitted electron beam could be modulated at 282 THz. The electrons are further accelerated to 150 MeV and, after acceleration, compressed by 33 times in a magnetic chicane. The temporal compression of the electron macropulse increases the electron bunching frequency to 9.3 PHz, corresponding to a soft x-ray wavelength of 32.2 nm. We adopt a solenoid-derived staggered array undulator* with a 3 m length, 5 mm undulator period, and 1.2 mm gap. With a solenoid field of 10 kG, we estimate an undulator parameter of 0.4 and a corresponding radiation wavelength of 32.2 nm for a 150 MeV driving beam. With 3.3 kA peak current, 0.03% energy spread, 2 mm-mrad emittance, and 80-micron beam radius at the undulator entrance, the GENESIS code predicts 0.2 GW radiation power from the 3 m long undulator for an initial bunching factor of merely 10 ppm. * Y.C. Huang, H.C. Wang, R.H. Pantell, and J. Feinstein, "A staggered-array wiggler for far infrared, free-electron laser operation," IEEE J. Quantum Electronics 30 (1994) 1289.

MOPB08	Studies for Polarization Control at LCLS	31
	E. Allaria, Y.T. Ding, P. Emma, Z. Huang, H.-D. Nuhn, M. Rowen, J.J. Welch, J. Wu SLAC, Menlo Park, California, USA
	In order to improve the capabilities of LCLS to meet more of the user requirements it has been proposed to implement a method to produce circularly polarized coherent radiation in the LCLS free electron laser. In this work we will present the results of a new set of studies and simulations that have been done for adding polarization control to LCLS using circularly polarizing undulators. Attention has been focused mainly on the use of variable gap APPLE-II undulators to be used at the end of a long SASE radiator that is based on the standard planar LCLS undulators. Issues like polarization contamination from the planar polarized light, polarization fluctuation and the choice of undulator configuration have been studied.

MOPB15	Numerical Simulation of CAEP Compact FEL THz Source	35
	L.J. Chen, Q.K. Jia USTC/NSRL, Hefei, Anhui, People's Republic of China M. Li, X. Yang CAEP/IAE, Mianyang, Sichuan, People's Republic of China
	Free Election Laser Terahertz source is a good choice for THz source, whose wavelength is tunable. Using 1D FEL stimulation code FELO, we simulate the output characteristic of China Academy of Engineering Physics (CAEP) FEL THz, which is a waveguide FEL oscillator. The beam qualitys’ influence on the operation of FEL, such as energy dispersion, emittance and beam current, have been studied to designate a set of beam parameters. Besides, the output performance of FEL at different output coupling ratio is analyzed. The cavity detuning is discussed too. Meanwhile the influence of the position of the undulator in the cavity on the FEL performance is also studied.

MOPB16	New Tunable DUV Light Source for Seeding Free-electron Lasers	38
	N.Y. Joly, W. Chang, P. Hölzer, J. Nold, P.St.J. Russell, J.C. Travers Max Planck Institute for the Science of Light, Erlangen, Germany M.-E. Couprie, M. Labat SOLEIL, Gif-sur-Yvette, France
	Seeding of single-pass free-electron lasers is a promising approach for improving the temporal coherence compared to self-amplified spontaneous emission [1], at the same time reducing the saturation length and reinforcing the harmonic level. Convention lasers or harmonics generated in gas are usually used as coherent seeds [1]. However such sources require complicated set-up and have limited tuneability. Here, we suggest the use of a newly discovered and efficient source of UV light, continuously tunable from 120 nm to 320 nm. The extremely compact and simple set-up consists of 20 cm of hollow-core photonic crystal fibre filled with a noble gas at variable pressure up to a few tens of bar and pumped by ~1 μJ 30 fs pulses at 800 nm [2]. The process relies on a favourable sequence of linear and nonlinear effects: low pressure-tunable dispersion, pulse compression due to a combination of self-phase-modulation and anomalous dispersion, self-steepening and dispersive wave generation. Tunable diffraction-limited DUV pulses of ~50 nJ and fs duration are generated. Seeding of FEL is discussed. [1] G. Lambert et al., Nature Physics 4, 296-300 (2008) [2] N. Joly et al., accepted in PRL

MOPB17	Harmonic Generation for a Hard X-ray FEL	41
	Q.R. Marksteiner, K. Bishofberger, B.E. Carlsten, L.D. Duffy, N.A. Yampolsky LANL, Los Alamos, New Mexico, USA
	Funding: We gratefully acknowledge the support of the US Department of Energy through the LANL LDRD Propgram for this work. The proposed MaRIE XFEL at Los Alamos National Laboratory will generate ¼ Å, longitudinally coherent x-rays with a 20 GeV electron beam. A masked emittance exchanger can be used to generate coherent electron bunching at nm wavelengths. This masked emittance exchanger must be at 1 GeV in the accelerator, in order to mitigate debunching from incoherent synchrotron radiation (ISR). After this, the harmonic content must be stepped up by a factor of 200 in frequency and the electrons must be accelerated to 20 GeV. The nonlinear debunching effects in the accelerator from emittance must be mitigated by keeping the beam transversely large. There are several schemes to step the coherent bunching up to higher harmonics, all which require modulator and dispersive sections [1]. Echo-Enhanced Harmonic Generation, which requires large dispersion, must be incorporated at low energies, where ISR is reduced. Here we compare the usefulness of different harmonic generation schemes, and examine the possibility of placing successive harmonic generation sections at energies lower than 20 GeV in the accelerator line, with the accelerator sections in between used to introduce dispersion to the beam. [1] Phys. Rev. E 71, 046501 (2005), etc.

MOPB19	Using Laser Harmonics to Increase Bunching Factor in EEHG	45
	G.V. Stupakov SLAC, Menlo Park, California, USA M.S. Zolotorev LBNL, Berkeley, California, USA
	Funding: This work was supported by U.S. DOE Contracts No. DE-AC02-76SF00515 and DE-AC02-05CH11231 Echo-enabled harmonic generation (EEHG) is one of most promising approaches to seeding of soft x-ray FELs. It allows one to obtain beam bunching at high harmonics (of order of 100) of the laser frequency at a level of a few percent. In this paper we demonstrate that using the second and third harmonics of the laser radiation one can substantially increase the beam bunching: for a cold beam one can obtain values approaching 0.4 in the range of harmonic numbers 100~200. Such bunching factors are close to those achieved at saturation in the FEL process, which means that one can eliminate the lasing process and use coherent radiation of the pre-bunched beam in the undulator-radiator as a bright source of x-rays. We also discuss an option of using nonlinear dispersive elements to increase the bunching factor.

MOPB20	Effect of Coulomb Collisions on Echo-Enabled Harmonic Generation (EEHG)	49
	G.V. Stupakov SLAC, Menlo Park, California, USA
	Funding: Work supported by U.S. DOE Contract No. DE-AC02-76SF00515. Echo-enabled harmonic generation (EEHG) for FEL seeding uses two undulator-modulators and two chicanes to introduce a fine structure into the beam longitudinal phase space which, at the end of the system, transforms into high harmonic modulation of the beam current. As a result of this phase space manipulation, after the first chicane, the energy distribution function becomes a rapidly modulated function of energy, with the scale of the modulation of the order of the initial energy spread of the beam divided by the EEHG harmonic number. Small-angle Coulomb collisions between the particles of the beam (also known as intrabeam scattering) tend to smear out this modulation and hence to suppress the beam bunching. In this paper we calculate the EEHG bunching factor with account of the collisions and derive a simple scaling relation for the strength of the effect. Our estimates show that collisions become a limiting factor in EEGH seeding for harmonic numbers roughly exceeding 100.

MOPB21	Seeded Radiation Sources with Sawtooth Waveforms	53
	D.F. Ratner Stanford University, Stanford, California, USA A. Chao SLAC, Menlo Park, California, USA
	Despite the recent success of SASE-based FELs, there is still considerable interest in driving coherent radiation sources with external seeding. Seeding schemes, such as HGHG and EEHG, can increase longitudinal coherence, decrease saturation lengths, and improve performance of tapering, polarization control and other FEL features. Typically, seeding schemes start with a simple sinusoidal modulation, which is manipulated to provide bunching at a high harmonic of the original wavelength. In this paper, we consider variations starting with a sawtooth modulation. The sawtooth creates a clean phase space structure, providing a maximal bunching factor without the need for an FEL interaction. While a pure sawtooth modulation is a theoretical construct, it is possible to approach the waveform by combining two or more of the composite wavelengths. We give examples of sawtooth seeding for HGHG, EEHG and other schemes including compressed seeding, steady state microbunching, and reversible seeding. Finally, we note that the sawtooth modulation may aid in suppression of the microbunching instability.

MOPB23	Reversible Seeding in Storage Rings	57
	D.F. Ratner Stanford University, Stanford, California, USA A. Chao SLAC, Menlo Park, California, USA
	We propose to generate steady-state microbunching in a storage ring by implementing a reversible seeding scheme. High gain harmonic generation (HGHG) and echo-enhanced harmonic generation (EEHG) are two promising methods for microbunching linac electron beams. Because both schemes increase the energy spread of the seeded beam, they cannot drive a coherent radiator turn-by-turn in a storage ring. However, reversing the seeding process following the radiator minimizes the impact on the electron beam and may allow coherent radiation at or near the storage ring repetition rate. In this paper we describe the general idea and outline a proof-of-principle experiment.

MOPB25	Improvement of the Crossed Undulator Design for Effective Circular Polarization	61
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	The production of X-ray radiation with a high degree of circular polarization constitutes an important goal at XFEL facilities. A simple scheme to obtain circular polarization control with crossed undulators has been proposed so far. In its simplest configuration the crossed undulators consist of pair of short planar undulators in crossed position separated by an electromagnetic phase shifter. An advantage of this configuration is a fast helicity switching. A drawback is that a high degree of circular polarization (over 90%) can only be achieved for lengths of the insertion devices significantly shorter than the gain length, i.e. at output power significantly lower than the saturation power level. Here we propose to use a setup with two or more crossed undulators separated by phase shifters. This cascade crossed undulator scheme is distinguished, in performance, by a fast helicity switching, a high degree of circular polarization (over 95%) and a high output power level, comparable with the saturation power level in the baseline undulator at fundamental wavelength. We present feasibility study and exemplifications for the LCLS baseline in the soft X-ray regime.

MOPB26	Self-seeded Operation of the LCLS Hard X-ray FEL in the Long-bunch Mode of Operation	65
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	Self-seeding options for the LCLS baseline were recently investigated using a scheme which relies on a single-crystal monochromator in Bragg-transmission geometry. The LCLS low-charge (0.02 nC) mode of operation was considered in order to demonstrate the feasibility of the proposed scheme. The wakefield effects from the linac and from the undulator vacuum chamber are much reduced at such low charge, and can be ignored. In this paper we extend our previous investigations to the case of the LCLS mode of operation with nominal charge. Based on the LCLS start-to-end simulation for an electron beam charge of 0.25 nC, and accounting for the wakefields from the undulator vacuum chamber we demonstrate that the same simplest self-seeding system (two undulators with a single-crystal monochromator in between) is appropriate not only for short (few femtosecond) bunches, but for longer bunches too.

MOPB27	Circular Polarization Control for the LCLS Baseline in the Soft X-ray Regime	69
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	Several schemes have been discussed to obtain soft-polarization control in the context of the LCLS. We propose a novel method to generate 10 GW level power at the fundamental harmonic with 99% degree of circular polarization from the LCLS baseline. Its merits are low cost, simplicity and easy implementation. As in previously proposed methods, the microbunching of the planar undulator is used here as well. After the baseline undulator, the electron beam is sent through a 40 m long straight section, and subsequently through a short helical (APPLE II) radiator. The microbunching is easily preserved, and intense coherent radiation is emitted in the helical radiator. The background radiation from the baseline undulator can be suppressed by letting radiation and electrons through horizontal and vertical slits upstream the helical radiator, where the radiation spot size is about ten times larger than the electron bunch transverse size. Thin Be foils for the slits will preserve from electron losses. Other facilities e.g. LCLS II or the European XFEL may benefit from this work as well, due to availability of sufficiently long free space at the end of undulator tunnel.

MOPB28	Gas-filled Cell as a Narrow Bandwidth Bandpass Filter in the VUV Wavelength Range	73
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	We propose a method for spectrally filtering radiation in the VUV wavelength range by means of a monochromator constituted by a cell filled with a resonantly absorbing rare gas. Around particular wavelengths, the gas exhibits narrow-bandwidth absorbing resonances following the Fano profile. Within the photon energy range 60-65 eV, the correlation index of the Fano profiles for the photo-ionization spectra in He is equal to unity, meaning that the minimum of the cross-section is exactly zero. For sufficiently large column density in the gas cell, the spectrum of the incoming radiation will be attenuated by the background cross-section of many orders of magnitude, except for those wavelengths close to the point where the cross-section is zero. Remarkable advantages of a gas monochromator based on this principle are simplicity, efficiency and narrow-bandwidth. A gas monochromator installed in the experimental hall of a VUV SASE FEL facility would enable the delivery of a single-mode VUV laser beam. The design is identical to that of existing gas attenuator systems for VUV or X-ray FELs. We present feasibility study and exemplifications for the FLASH facility in the VUV regime.

MOPB29	Generation of Doublet Spectral Lines at Self-seeded X-ray FELs	77
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	We propose to extend our recently proposed single-crystal monochromatization setup to the case when two or more crystals are arranged in a series to spectrally filter the SASE radiation at two or more closely-spaced wavelengths within the FEL gain band. This allows for the production of doublet or multiplet spectral lines. We present simulation results for the LCLS baseline operating at two closely spaced wavelengths. We show that we can produce fully coherent radiation shared between two longitudinal modes. Mode spacing can be easily tuned within the FEL gain band. The proposed scheme allows for a modulation of the electron bunch at optical frequencies without a seed quantum laser. In fact, the XFEL output intensity contains an oscillating "mode-beat" component whose frequency is related to the frequency difference between the pair of longitudinal modes considered. At saturation one obtains FEL-induced optical modulations of energy loss and energy spread in the electron bunch, which can be converted into density modulation with a weak chicane behind the baseline undulator. Powerful coherent radiation, synchronized with the X-ray pulses, can then be generated with an OTR station.

MOPB30	The Effects of Betatron Motion on the Preservation of FEL Microbunching	81
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	In some options for circular polarization control at X-ray FELs, a helical radiator is placed a few ten meters distance behind the baseline undulator. If the microbunch structure induced in the baseline (planar) undulator can be preserved, intense coherent radiation is emitted in the helical radiator. The effects of betatron motion on the preservation ofμbunching in such in-line schemes should be accounting for. In this paper we present a comprehensive study of these effects. It is shown that one can work out an analytical expression for the debunching of an electron beam moving in a FODO lattice, strictly valid in the asymptote for a FODO cell much shorter than the betatron function. Further on, numerical studies can be used to demonstrate that the validity of such analytical expression goes beyond the above-mentioned asymptote, and can be used in much more a general context. Finally, a comparison with Genesis simulations is given.

MOPB31	Self-seeding Scheme with Gas Monochromator for Narrow-Bandwidth Soft X-Ray	85
	G. Geloni European XFEL GmbH, Hamburg, Germany V. Kocharyan, E. Saldin DESY, Hamburg, Germany
	We propose an extension of our recently-proposed single-crystal self-seeding scheme to the soft X-ray range using a cell filled with resonantly absorbing gas as monochromator. The transmittance spectrum in the gas exhibits an absorbing resonance with narrow bandwidth. Then, similarly to the hard X-ray case, the temporal waveform of the transmitted radiation pulse is characterized by a long monochromatic wake, whose power is much larger than the equivalent shot noise power in the electron bunch. The monochromatic wake of the radiation pulse is combined with the delayed electron bunch and amplified in the second undulator. The proposed setup is extremely simple, and composed of as few as two simple elements: a gas cell, to be filled with noble gas, and a short magnetic chicane. The installation of the magnetic chicane does not perturb the undulator focusing system and does not interfere with the baseline mode of operation.

MOPB32	System Trade Analysis for an FEL Facility	89
	M.W. Reinsch, B. Austin, J.N. Corlett, L.R. Doolittle, G. Penn, D. Prosnitz, J. Qiang, A. Sessler, M. Venturini, J.S. Wurtele LBNL, Berkeley, California, USA
	Designing an FEL from scratch requires the design team to balance multiple science needs, FEL and accelerator physics constraints and engineering limitations. STAFF (System Trade Analysis for an FEL Facility) enables the user to rapidly explore a large range of Linac and FEL design options. The model utilzes analytical models such as the Ming Xie formulas when appropriate and look-up tables when necessary to maintain speed, flexibility and extensiblity. STAFF allows for physics models for FEL harmonics, wake fields, cavity higher-order modes and aspects of linac particle dynamics. The code will permit the user to study error tolerances and multiple beamlines so as to explore the full capabilities of an entire user facility. This makes it possible to optimize the integrated system in terms of performance metrics such as photons/pulse, photons/sec and tunability range while ensuring that unrealistic requirements are not put on either the electron beam quality, undulator field/gap requirements or other system elements. This paper will describe preliminary results from STAFF as applied to a CW FEL soft X-ray facility.

Paper

Title

Page

Smith-Purcell Radiation with Negative-index Material

20

D. Li, K. Imasaki
ILT, Suita, Osaka, Japan
M.R. Asakawa
Kansai University, Osaka, Japan
M. Hangyo, K. Takano
ILE Osaka, Suita, Japan
S. Miyamoto
LASTI, Hyogo, Japan
Y. Tsunawaki
OSU, Daito, Osaka, Japan
Z. Yang
UESTC, Chengdu, Sichuan, People's Republic of China

Smith-Purcell radiation from an electric line charge that moves, at constant speed, parallel to a grating made of metamaterial with negative index is analyzed. Through theoretical analysis and computations, we show that the Smith-Purcell radiation is stronger from a grating of negative-index material, than positive-index material and perfect conductor. Also, we found the radiation strongly depends on the values of permeability and permittivity.

MOPB06

Smith-Purcell Free Electron Laser with Bragg Reflector

24

D. Li, K. Imasaki
ILT, Suita, Osaka, Japan
M.R. Asakawa
Kansai University, Osaka, Japan
M. Hangyo
ILE Osaka, Suita, Japan
S. Miyamoto
LASTI, Hyogo, Japan
Y. Tsunawaki
OSU, Daito, Osaka, Japan
Z. Yang
UESTC, Chengdu, Sichuan, People's Republic of China

Grating with Bragg reflectors for the Smith-Purcell free-electron laser is proposed to improve the reflection coefficient, resulting in enhancing the interaction of the surface wave with the electron beam and, consequently, relax the requirements to the electron beam. With the help of particle-in-cell simulations, it has been shown that, the usage of Bragg reflectors may improve the growth rate, shorten the time for the device to reach saturation and lower the start current for the operation of a Smith-Purcell free-electron laser.

MOPB07

Soft X-ray Free-electron Laser with a 10-time Reduced Size

28

F.H. Chao, C.H. Chen, K.Y. Huang, Y.-C. Huang
NTHU, Hsinchu, Taiwan

We present a 30-m long soft x-ray FEL consisting of a 5-MeV photoinjector, a 150 MeV linac, a magnetic chicane compressor, and a 3-m long undulator. We propose to employ both the 3rd and the 4th harmonics of a Nd laser at 355 and 266 nm, respectively, to illuminate the cathode of the photoinjector. Owing to the beating of the two lasers, the emitted electron beam could be modulated at 282 THz. The electrons are further accelerated to 150 MeV and, after acceleration, compressed by 33 times in a magnetic chicane. The temporal compression of the electron macropulse increases the electron bunching frequency to 9.3 PHz, corresponding to a soft x-ray wavelength of 32.2 nm. We adopt a solenoid-derived staggered array undulator* with a 3 m length, 5 mm undulator period, and 1.2 mm gap. With a solenoid field of 10 kG, we estimate an undulator parameter of 0.4 and a corresponding radiation wavelength of 32.2 nm for a 150 MeV driving beam. With 3.3 kA peak current, 0.03% energy spread, 2 mm-mrad emittance, and 80-micron beam radius at the undulator entrance, the GENESIS code predicts 0.2 GW radiation power from the 3 m long undulator for an initial bunching factor of merely 10 ppm.
* Y.C. Huang, H.C. Wang, R.H. Pantell, and J. Feinstein, "A staggered-array wiggler for far infrared, free-electron laser operation," IEEE J. Quantum Electronics 30 (1994) 1289.

MOPB08

Studies for Polarization Control at LCLS

31

E. Allaria, Y.T. Ding, P. Emma, Z. Huang, H.-D. Nuhn, M. Rowen, J.J. Welch, J. Wu
SLAC, Menlo Park, California, USA

In order to improve the capabilities of LCLS to meet more of the user requirements it has been proposed to implement a method to produce circularly polarized coherent radiation in the LCLS free electron laser. In this work we will present the results of a new set of studies and simulations that have been done for adding polarization control to LCLS using circularly polarizing undulators. Attention has been focused mainly on the use of variable gap APPLE-II undulators to be used at the end of a long SASE radiator that is based on the standard planar LCLS undulators. Issues like polarization contamination from the planar polarized light, polarization fluctuation and the choice of undulator configuration have been studied.

MOPB15

Numerical Simulation of CAEP Compact FEL THz Source

35

L.J. Chen, Q.K. Jia
USTC/NSRL, Hefei, Anhui, People's Republic of China
M. Li, X. Yang
CAEP/IAE, Mianyang, Sichuan, People's Republic of China

Free Election Laser Terahertz source is a good choice for THz source, whose wavelength is tunable. Using 1D FEL stimulation code FELO, we simulate the output characteristic of China Academy of Engineering Physics (CAEP) FEL THz, which is a waveguide FEL oscillator. The beam qualitys’ influence on the operation of FEL, such as energy dispersion, emittance and beam current, have been studied to designate a set of beam parameters. Besides, the output performance of FEL at different output coupling ratio is analyzed. The cavity detuning is discussed too. Meanwhile the influence of the position of the undulator in the cavity on the FEL performance is also studied.

MOPB16

New Tunable DUV Light Source for Seeding Free-electron Lasers

38

N.Y. Joly, W. Chang, P. Hölzer, J. Nold, P.St.J. Russell, J.C. Travers
Max Planck Institute for the Science of Light, Erlangen, Germany
M.-E. Couprie, M. Labat
SOLEIL, Gif-sur-Yvette, France

Seeding of single-pass free-electron lasers is a promising approach for improving the temporal coherence compared to self-amplified spontaneous emission [1], at the same time reducing the saturation length and reinforcing the harmonic level. Convention lasers or harmonics generated in gas are usually used as coherent seeds [1]. However such sources require complicated set-up and have limited tuneability. Here, we suggest the use of a newly discovered and efficient source of UV light, continuously tunable from 120 nm to 320 nm. The extremely compact and simple set-up consists of 20 cm of hollow-core photonic crystal fibre filled with a noble gas at variable pressure up to a few tens of bar and pumped by ~1 μJ 30 fs pulses at 800 nm [2]. The process relies on a favourable sequence of linear and nonlinear effects: low pressure-tunable dispersion, pulse compression due to a combination of self-phase-modulation and anomalous dispersion, self-steepening and dispersive wave generation. Tunable diffraction-limited DUV pulses of ~50 nJ and fs duration are generated. Seeding of FEL is discussed.
[1] G. Lambert et al., Nature Physics 4, 296-300 (2008)
[2] N. Joly et al., accepted in PRL

MOPB17

Harmonic Generation for a Hard X-ray FEL

41

Q.R. Marksteiner, K. Bishofberger, B.E. Carlsten, L.D. Duffy, N.A. Yampolsky
LANL, Los Alamos, New Mexico, USA

Funding: We gratefully acknowledge the support of the US Department of Energy through the LANL LDRD Propgram for this work.
The proposed MaRIE XFEL at Los Alamos National Laboratory will generate ¼ Å, longitudinally coherent x-rays with a 20 GeV electron beam. A masked emittance exchanger can be used to generate coherent electron bunching at nm wavelengths. This masked emittance exchanger must be at 1 GeV in the accelerator, in order to mitigate debunching from incoherent synchrotron radiation (ISR). After this, the harmonic content must be stepped up by a factor of 200 in frequency and the electrons must be accelerated to 20 GeV. The nonlinear debunching effects in the accelerator from emittance must be mitigated by keeping the beam transversely large. There are several schemes to step the coherent bunching up to higher harmonics, all which require modulator and dispersive sections [1]. Echo-Enhanced Harmonic Generation, which requires large dispersion, must be incorporated at low energies, where ISR is reduced. Here we compare the usefulness of different harmonic generation schemes, and examine the possibility of placing successive harmonic generation sections at energies lower than 20 GeV in the accelerator line, with the accelerator sections in between used to introduce dispersion to the beam.
[1] Phys. Rev. E 71, 046501 (2005), etc.

MOPB19

Using Laser Harmonics to Increase Bunching Factor in EEHG

45

G.V. Stupakov
SLAC, Menlo Park, California, USA
M.S. Zolotorev
LBNL, Berkeley, California, USA

Funding: This work was supported by U.S. DOE Contracts No. DE-AC02-76SF00515 and DE-AC02-05CH11231
Echo-enabled harmonic generation (EEHG) is one of most promising approaches to seeding of soft x-ray FELs. It allows one to obtain beam bunching at high harmonics (of order of 100) of the laser frequency at a level of a few percent. In this paper we demonstrate that using the second and third harmonics of the laser radiation one can substantially increase the beam bunching: for a cold beam one can obtain values approaching 0.4 in the range of harmonic numbers 100~200. Such bunching factors are close to those achieved at saturation in the FEL process, which means that one can eliminate the lasing process and use coherent radiation of the pre-bunched beam in the undulator-radiator as a bright source of x-rays. We also discuss an option of using nonlinear dispersive elements to increase the bunching factor.

MOPB20

Effect of Coulomb Collisions on Echo-Enabled Harmonic Generation (EEHG)

49

G.V. Stupakov
SLAC, Menlo Park, California, USA

Funding: Work supported by U.S. DOE Contract No. DE-AC02-76SF00515.
Echo-enabled harmonic generation (EEHG) for FEL seeding uses two undulator-modulators and two chicanes to introduce a fine structure into the beam longitudinal phase space which, at the end of the system, transforms into high harmonic modulation of the beam current. As a result of this phase space manipulation, after the first chicane, the energy distribution function becomes a rapidly modulated function of energy, with the scale of the modulation of the order of the initial energy spread of the beam divided by the EEHG harmonic number. Small-angle Coulomb collisions between the particles of the beam (also known as intrabeam scattering) tend to smear out this modulation and hence to suppress the beam bunching. In this paper we calculate the EEHG bunching factor with account of the collisions and derive a simple scaling relation for the strength of the effect. Our estimates show that collisions become a limiting factor in EEGH seeding for harmonic numbers roughly exceeding 100.

MOPB21

Seeded Radiation Sources with Sawtooth Waveforms

53

D.F. Ratner
Stanford University, Stanford, California, USA
A. Chao
SLAC, Menlo Park, California, USA

Despite the recent success of SASE-based FELs, there is still considerable interest in driving coherent radiation sources with external seeding. Seeding schemes, such as HGHG and EEHG, can increase longitudinal coherence, decrease saturation lengths, and improve performance of tapering, polarization control and other FEL features. Typically, seeding schemes start with a simple sinusoidal modulation, which is manipulated to provide bunching at a high harmonic of the original wavelength. In this paper, we consider variations starting with a sawtooth modulation. The sawtooth creates a clean phase space structure, providing a maximal bunching factor without the need for an FEL interaction. While a pure sawtooth modulation is a theoretical construct, it is possible to approach the waveform by combining two or more of the composite wavelengths. We give examples of sawtooth seeding for HGHG, EEHG and other schemes including compressed seeding, steady state microbunching, and reversible seeding. Finally, we note that the sawtooth modulation may aid in suppression of the microbunching instability.

MOPB23

Reversible Seeding in Storage Rings

57

D.F. Ratner
Stanford University, Stanford, California, USA
A. Chao
SLAC, Menlo Park, California, USA

We propose to generate steady-state microbunching in a storage ring by implementing a reversible seeding scheme. High gain harmonic generation (HGHG) and echo-enhanced harmonic generation (EEHG) are two promising methods for microbunching linac electron beams. Because both schemes increase the energy spread of the seeded beam, they cannot drive a coherent radiator turn-by-turn in a storage ring. However, reversing the seeding process following the radiator minimizes the impact on the electron beam and may allow coherent radiation at or near the storage ring repetition rate. In this paper we describe the general idea and outline a proof-of-principle experiment.

MOPB25

Improvement of the Crossed Undulator Design for Effective Circular Polarization

61

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

The production of X-ray radiation with a high degree of circular polarization constitutes an important goal at XFEL facilities. A simple scheme to obtain circular polarization control with crossed undulators has been proposed so far. In its simplest configuration the crossed undulators consist of pair of short planar undulators in crossed position separated by an electromagnetic phase shifter. An advantage of this configuration is a fast helicity switching. A drawback is that a high degree of circular polarization (over 90%) can only be achieved for lengths of the insertion devices significantly shorter than the gain length, i.e. at output power significantly lower than the saturation power level. Here we propose to use a setup with two or more crossed undulators separated by phase shifters. This cascade crossed undulator scheme is distinguished, in performance, by a fast helicity switching, a high degree of circular polarization (over 95%) and a high output power level, comparable with the saturation power level in the baseline undulator at fundamental wavelength. We present feasibility study and exemplifications for the LCLS baseline in the soft X-ray regime.

MOPB26

Self-seeded Operation of the LCLS Hard X-ray FEL in the Long-bunch Mode of Operation

65

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

Self-seeding options for the LCLS baseline were recently investigated using a scheme which relies on a single-crystal monochromator in Bragg-transmission geometry. The LCLS low-charge (0.02 nC) mode of operation was considered in order to demonstrate the feasibility of the proposed scheme. The wakefield effects from the linac and from the undulator vacuum chamber are much reduced at such low charge, and can be ignored. In this paper we extend our previous investigations to the case of the LCLS mode of operation with nominal charge. Based on the LCLS start-to-end simulation for an electron beam charge of 0.25 nC, and accounting for the wakefields from the undulator vacuum chamber we demonstrate that the same simplest self-seeding system (two undulators with a single-crystal monochromator in between) is appropriate not only for short (few femtosecond) bunches, but for longer bunches too.

MOPB27

Circular Polarization Control for the LCLS Baseline in the Soft X-ray Regime

69

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

Several schemes have been discussed to obtain soft-polarization control in the context of the LCLS. We propose a novel method to generate 10 GW level power at the fundamental harmonic with 99% degree of circular polarization from the LCLS baseline. Its merits are low cost, simplicity and easy implementation. As in previously proposed methods, the microbunching of the planar undulator is used here as well. After the baseline undulator, the electron beam is sent through a 40 m long straight section, and subsequently through a short helical (APPLE II) radiator. The microbunching is easily preserved, and intense coherent radiation is emitted in the helical radiator. The background radiation from the baseline undulator can be suppressed by letting radiation and electrons through horizontal and vertical slits upstream the helical radiator, where the radiation spot size is about ten times larger than the electron bunch transverse size. Thin Be foils for the slits will preserve from electron losses. Other facilities e.g. LCLS II or the European XFEL may benefit from this work as well, due to availability of sufficiently long free space at the end of undulator tunnel.

MOPB28

Gas-filled Cell as a Narrow Bandwidth Bandpass Filter in the VUV Wavelength Range

73

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

We propose a method for spectrally filtering radiation in the VUV wavelength range by means of a monochromator constituted by a cell filled with a resonantly absorbing rare gas. Around particular wavelengths, the gas exhibits narrow-bandwidth absorbing resonances following the Fano profile. Within the photon energy range 60-65 eV, the correlation index of the Fano profiles for the photo-ionization spectra in He is equal to unity, meaning that the minimum of the cross-section is exactly zero. For sufficiently large column density in the gas cell, the spectrum of the incoming radiation will be attenuated by the background cross-section of many orders of magnitude, except for those wavelengths close to the point where the cross-section is zero. Remarkable advantages of a gas monochromator based on this principle are simplicity, efficiency and narrow-bandwidth. A gas monochromator installed in the experimental hall of a VUV SASE FEL facility would enable the delivery of a single-mode VUV laser beam. The design is identical to that of existing gas attenuator systems for VUV or X-ray FELs. We present feasibility study and exemplifications for the FLASH facility in the VUV regime.

MOPB29

Generation of Doublet Spectral Lines at Self-seeded X-ray FELs

77

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

We propose to extend our recently proposed single-crystal monochromatization setup to the case when two or more crystals are arranged in a series to spectrally filter the SASE radiation at two or more closely-spaced wavelengths within the FEL gain band. This allows for the production of doublet or multiplet spectral lines. We present simulation results for the LCLS baseline operating at two closely spaced wavelengths. We show that we can produce fully coherent radiation shared between two longitudinal modes. Mode spacing can be easily tuned within the FEL gain band. The proposed scheme allows for a modulation of the electron bunch at optical frequencies without a seed quantum laser. In fact, the XFEL output intensity contains an oscillating "mode-beat" component whose frequency is related to the frequency difference between the pair of longitudinal modes considered. At saturation one obtains FEL-induced optical modulations of energy loss and energy spread in the electron bunch, which can be converted into density modulation with a weak chicane behind the baseline undulator. Powerful coherent radiation, synchronized with the X-ray pulses, can then be generated with an OTR station.

MOPB30

The Effects of Betatron Motion on the Preservation of FEL Microbunching

81

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

In some options for circular polarization control at X-ray FELs, a helical radiator is placed a few ten meters distance behind the baseline undulator. If the microbunch structure induced in the baseline (planar) undulator can be preserved, intense coherent radiation is emitted in the helical radiator. The effects of betatron motion on the preservation ofμbunching in such in-line schemes should be accounting for. In this paper we present a comprehensive study of these effects. It is shown that one can work out an analytical expression for the debunching of an electron beam moving in a FODO lattice, strictly valid in the asymptote for a FODO cell much shorter than the betatron function. Further on, numerical studies can be used to demonstrate that the validity of such analytical expression goes beyond the above-mentioned asymptote, and can be used in much more a general context. Finally, a comparison with Genesis simulations is given.

MOPB31

Self-seeding Scheme with Gas Monochromator for Narrow-Bandwidth Soft X-Ray

85

G. Geloni
European XFEL GmbH, Hamburg, Germany
V. Kocharyan, E. Saldin
DESY, Hamburg, Germany

We propose an extension of our recently-proposed single-crystal self-seeding scheme to the soft X-ray range using a cell filled with resonantly absorbing gas as monochromator. The transmittance spectrum in the gas exhibits an absorbing resonance with narrow bandwidth. Then, similarly to the hard X-ray case, the temporal waveform of the transmitted radiation pulse is characterized by a long monochromatic wake, whose power is much larger than the equivalent shot noise power in the electron bunch. The monochromatic wake of the radiation pulse is combined with the delayed electron bunch and amplified in the second undulator. The proposed setup is extremely simple, and composed of as few as two simple elements: a gas cell, to be filled with noble gas, and a short magnetic chicane. The installation of the magnetic chicane does not perturb the undulator focusing system and does not interfere with the baseline mode of operation.

MOPB32

System Trade Analysis for an FEL Facility

89

M.W. Reinsch, B. Austin, J.N. Corlett, L.R. Doolittle, G. Penn, D. Prosnitz, J. Qiang, A. Sessler, M. Venturini, J.S. Wurtele
LBNL, Berkeley, California, USA

Designing an FEL from scratch requires the design team to balance multiple science needs, FEL and accelerator physics constraints and engineering limitations. STAFF (System Trade Analysis for an FEL Facility) enables the user to rapidly explore a large range of Linac and FEL design options. The model utilzes analytical models such as the Ming Xie formulas when appropriate and look-up tables when necessary to maintain speed, flexibility and extensiblity. STAFF allows for physics models for FEL harmonics, wake fields, cavity higher-order modes and aspects of linac particle dynamics. The code will permit the user to study error tolerances and multiple beamlines so as to explore the full capabilities of an entire user facility. This makes it possible to optimize the integrated system in terms of performance metrics such as photons/pulse, photons/sec and tunability range while ensuring that unrealistic requirements are not put on either the electron beam quality, undulator field/gap requirements or other system elements. This paper will describe preliminary results from STAFF as applied to a CW FEL soft X-ray facility.