|Conceptual Study of a Self-seeding Scheme at FLASH2
Funding: Supported by Federal Ministry of Education and Research of Germany under contract No. 05K1GU4 and 05K10PE1 and the German Research Foundation program graduate school 1355.
We present a conceptual study of a self-seeding installation at the new FEL beamline, FLASH2, at the free-electron laser at DESY, Hamburg. For self-seeding, light from a first set of undulators is filtered by a monochromator and thus acts as a seed for the gain process in the main undulator. This scheme has been tested at LCLS at SLAC with a diamond monochromator for hard X-rays and with a grating monochromator for soft X-rays covering energies between 700 and 1000 eV. For such a design to offer benefits at FLASH2, it must be modified to work with X-rays with wavelength of about 5 nm (248 eV) where the damage threshold of the monochromator in the setup and the divergence at longer wavelengths become an issue. An analysis of the potential performance and limitations of this setup is performed using GENESIS 1.3 and a method developed for the soft X-ray self-seeding experiment at the European XFEL. With a total of 9 undulators in the first stage and 8 undulators after the monochromator, a pulse energy contrast ratio of 4.5 was simulated with an initial peak current of 2.5 kA.
|Design Study for the PEHG Experiment at SDUV-FEL
Funding: This work was partially supported by National Natural Science Foundation of China (11475250, 11175240 and 11205234)
In this paper, design studies for the proof-of-principle experiment of the recently proposed phase-merging enhanced harmonic generation (PEHG) mechanism are presented. A dogleg and a new designed transverse gradient undulator should be added in the undulator system of SDUV-FEL to perform the phase-merging effect. With the help of 3D simulation codes, we show the possible performance of PEHG with the realistic parameters of SDUV-FEL.
* H. Deng, C. Feng, Phys. Rev. Lett. 111, 084801.
** C. Feng, H. Deng, D. Wang, Z. Zhao, New J. Phys.,16, 043021.
*** C. Feng, T. Zhang, H. Deng, Z. Zhao, Phys. Rev. ST Accel. Beams 17, 070701.
|Preparation for the CPA-CHG Experiment at SDUV-FEL
Chirped pulse amplification has been shown theoretically possible to generate ultra-short high intensity radiation pulses in a seeded FEL. To demonstrate the theoretical predictions, a CPA-CHG experiment is under preparation based on the SDUV-FEL recently. We will show the experiment layout, optical design studies and some preliminary measurement results of the optical systems in this paper.
* L. Yu, E. Johnson, D. Li, D. Umstader, Physical Review E 49 (1994) 4480
** C. Feng et al. Nuclear Instruments and Methods in Physics Research A 712 (2013) 113
|Chirped Pulse Amplification in a Seeded Free-electron Laser: Design of a Test Experiment at FERMI
|In solid-state lasers, frequency chirping is employed to stretch a short pulse prior to amplification, mitigating the problems related to high power in the active medium. After amplification, the chirp is compensated in order to recover short pulse duration and, hence, high peak power. Chirped pulse amplification (CPA) in seeded FEL’s relies on a similar principle: the seed pulse is stretched in time before interacting with the electron beam. This permits one to create bunching on a larger number of electrons, and to (approximately) linearly increase the output energy of the generated FEL pulse. In ideal conditions, the chirp carried by the phase of the seed pulse is transmitted to the output phase of the FEL pulse. Chirp compensation after the last undulator allows production of a short (ideally Fourier-transformed) pulse and, therefore, a larger peak power with respect to what obtained, for the same conditions, in standard (i.e., no-chirp-on-the-seed) operation mode. In this paper, we present the preparatory studies (i.e., numerical simulations and compressor design), which have been carried out at FERMI, in view of performing the first test experiment of CPA on a seeded FEL.
|Laser Seeding Schemes for Soft X-rays at LCLS-II
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 Nos. DE-AC02-05CH11231 and DE-AC02-76SF00515.
The initial design for LCLS-II incorporates both SASE and self-seeded configurations. Increased stability and/or coherence than is possible with either configuration may be provided by seeding with external lasers followed by one or more stages of harmonic generation, especially in the soft x-ray regime. External seeding also allows for increased flexibility, for example the ability to quickly vary the pulse duration. Studies of schemes based on high-gain harmonic generation and echo-enabled harmonic generation are presented, including realistic electron distributions based on tracking through the injector and linac.
|Measurements of the FEL-bandwidth Scaling with Harmonic Number in a HGHG FEL
|In this work we report recent measurements done at FERMI showing the dependence of the FEL bandwidth with respect to the seed laser harmonic at which the FEL is operated. Comparison of FEL spectra for different Fourier-limit seed and chirp pulses is also reported.
|Generation of Multiple Coherent Pulses in a Superradiant Free-Electron Laser
|We analyze the structure of the tail of a superradiant pulse, which is constituted by a train of sub-pulses with decaying amplitudes. We show how a trailing pulse, with pi phase advance from the leading pulse, is generated at the falling edge of the leading superradiant pulse, where the corresponding phase space is deeply saturated and the electrons become de-trapped by the reduced ponderomotive potential. Once the trailing pulse gains enough energy, it generates a second trailing pulse, and the process takes place again. By performing detailed simulations of the resulting electron phase space distribution and the FEL pulse spectral and temporal structure with PERSEO, we confirm that the deformation and re-bunching of the longitudinal phase space create a sequence of pulses. These results are compared to 3D simulations using the FEL code GENESIS 1.3 showing a good agreement.
|Two Color Experiments at SPARC: External and Intra-bunch Seeding
|The FEL-SPARC operation with two bunches at different energies has been characterized using different tools. The effect of the e-beam matching on the SASE operation has been explored by controlling the transport with an ad hoc developed control system, allowing either the transport in matching conditions and in different configurations. Since the two bunches have, at the entrance of the undulator, different phase space characteristics we used this effect as a knob to enhance or suppress one or the other FEL mode. We investigated the effect of seeding on both frequencies and observed beating waves, furthermore we observed effects of intra-bunch seeding when only one of the modes is seeded.
|Seeding Preparation at the FLASH2 Beamline
|At the FLASH Facility at DESY, Hamburg, a new undulator line, referred to as FLASH2, has been added. First electron beam in FLASH2 has reached the beam dump in May. Further commissioning, including first SASE, is foreseen for the remainder of the year. This FEL has been designed to include seeding in the future, such as a high-gain harmonic generation (HGHG) or an echo-enable harmonic generation (EEHG). The optimal choice for seeding at FLASH2 is under discussion. For seeding at FLASH2, an XUV spectrometer, a laser system based on optical parametric chirped-pulse amplification (OPCPA) as well as a system for a third harmonic generation (THG) including diagnostic tools have been prepared. We will report the current status.
|Perspectives for Imaging Single Protein Molecules with the Present Design of the European XFEL
|European XFEL aims to support imaging and structure determination of biological specimens between less than 0.1 microns and 1 micron size with working photon energies between 3 keV and 16 keV. This wide operation range is a cause for challenges to the focusing optics. A long propagation distance of about 900 m between x-ray source and sample leads to a large lateral photon beam size at the optics. Due to the large divergence of nominal X-ray pulses with durations shorter than 10 fs, one suffers diffraction from mirror apertures, leading to a 100-fold decrease in fluence at photon energies around 4 keV, which seem ideal for imaging of single biomolecules. Moreover, the nominal SASE1 is very far from the level required for single particle imaging. Here we show how it may be possible to optimize the SPB instrument for single biomolecule imaging with minimal additional costs and time, achieving diffraction without destruction at near-atomic resolution with 1013 photons in a 4 fs pulse at 4 keV photon energy and in a 100 nm focus, corresponding to a fluence of 1023 ph/cm2. This result is exemplified using the RNA Pol II molecule as a case study.
|Start-to-End Simulation for FLASH2 HGHG Option
|The Free-electron laser in Hamburg (FLASH) is the first FEL user facility to have produced extreme ultraviolet (XUV) and soft X-ray photons. In order to increase the beam time delivered to users, a major upgrade of FLASH named FLASH II is in progress. The electron beamline of FLASH2 consists of diagnostic and matching sections, a seeding undulator section and a SASE undulator section. In this paper, results from a start-to-end simulation for a FLASH2 High-Gain Harmonic Generation (HGHG) option are presented. For the beam dynamics simulation, space charge, coherent synchrotron radiation (CSR) and longitudinal cavity wake field effects are taken into account. In order to get electron beam bunches with small correlated and uncorrelated energy spread, RF parameters of the accelerating modules have been optimized as well as the parameters of the bunch compressors. Radiation simulations for the modulator and the radiator have been done with code Genesis 1.3 by using the particle distribution generated from the beam dynamics simulation. The results show that for a single stage HGHG, 33.6 nm wavelength FEL radiation can be seeded at FLASH2 with a 235 nm seeding laser.
|Enhancing Coherent Harmonic Generation using Tilted Laser Wavefronts
Funding: Work supported by BMBF (contract 05K13PE3)
Coherent Harmonic Generation (CHG) to produce ultrashort pulses of synchrotron radiation is based on the interaction of relativistic electrons in a storage ring with femtosecond laser pulses in an undulator. The resulting periodic energy modulation can be converted to a density modulation by a dispersive chicane, giving rise to coherent emission at harmonics of the laser wavelength in a second undulator. If the first undulator is in a section with non-zero dispersion, the density modulation can be enhanced using tilted laser wavefronts, thus delaying the phase-space distributions of electrons with different energy with respect to each other. The most simple way to realize the wavefront tilt would be to introduce a small crossing angle between the electron and laser beam. Details are discussed for the case of the CHG short-pulse facility at DELTA, a 1.5-GeV synchrotron light source at the TU Dortmund University, but HGHG and EEHG seeding of free-electron lasers could also be performed this way.
|Novel Seeding and Wavelength Shifting in Free Electron Lasers
|Free Electron Lasers provide the opportunity for continuous tunability of light from the mm-wave region to hard x-rays. For example, single-pass SASE operation allows very short wavelength operation without using optical cavity structure; unfortunately, the resulting photon beam is poor in terms of longitudinal coherence and shot-to-shot stability. Seeding can greatly improved the longitudinal coherence and shot-to-shot stability but this is dependent on the availability of a suitable seed pulse and then the challenge remains of switching the wavelength quickly. Here a new concept is investigated, one called wavelength shifting where a solid seed source is utilized and then the resultant wavelength shifted to the desired result. This new idea can improve the longitudinal coherence as well as stabilize the shot-to-shot output power while permitting continuous wavelength tunability.
|Broadly Tunable THz FEL Amplifier
Funding: MOST 102-2112-M-007 -002 -MY3, Taiwan
In this paper we present a broadly tunable sub-MW THz FEL amplifier driven by a photoinjector with a sub-kW seed THz source tunable between 0.7-2.0 THz. Specifically an S-band photoinjector at 2.856 GHz generate a 3.3-5.5 MeV electron bunch with 0.5 nC charge in a 4.25 ps rms bunch length, which is injected into a 2-m long undulator with a period of 18 mm and an rms undulator parameter of 0.98. The driver laser of the photoinjector is a frequency quadrupled amplified, mode-locked Nd:YVO4 laser at 1064 nm. We recycle the unconverted infrared laser at 1064 nm to pump a THz parametric amplifier using a lithium niobate crystal as its gain crystal. This THz parametric amplifier generates a transform-limited THz pulse with sub-kW power between 0.7 and 2.0 THz, which is seeded into the undulator to produce broadly tunable, transform-limited, sub-MW THz radiation through FEL amplification with a gain of about 3000. Since the pump laser of the THz OPA is derived from the driver laser of the photoinjector, the seed THz pulse is fully synchronized and overlapped with the electron bunch. Experimental progress of this work will be presented in the conference.
*Work supported by MoST under NSC 102-2112-M-007-002-MY3
|Poster MOP086 [1.269 MB]
|Upgrade Plans for the Short-pulse Facility at DELTA
Funding: Work supported by DFG, BMBF, FZ Jülich, and by the Land NRW.
DELTA is a 1.5-GeV synchrotron light source operated by the TU Dortmund University with a short-pulse facility based on Coherent Harmonic Generation (CHG) * to produce radiation with wavelengths in the VUV regime. Even shorter wavelengths can be generated by an upgrade based on the Echo-Enabled Harmonic Generation (EEHG) technique ** which requires additional magnetic chicanes and undulators. A new storage ring lattice provides enough free space for an EEHG setup and additionally for a femtoslicing undulator. Besides the new optics, first simulation results of EEHG will be presented.
* S. Khan et al., Sync. Rad. News 26, 3 (2013).
** G. Stupakov, Phys. Rev. Lett. 102, 074801 (2009).
|High Repetition Rate Energy Modulator System Utilizing a Laser Enhancement Cavity
|A high intensity laser field can be realized at a high repetition rate using an enhancement optical cavity scheme. We propose to apply the 100GW-level laser field inside the cavity for producing a micro-bunch structure in an electron bunch. Combining this system with an ERL scheme of accelerator, it can be used for a seeded FEL at a high repetition rate of ~100MHz continuous beam. The longitudinal electric field at the center area of a higher-order transverse mode of laser can be used to modulate beam energy at a period of the laser wavelength. A 250 MeV class two-loop ERL accelerator has been proposed in KEK as a future upgrade plan of existing 35 MeV ERL test accelerator. It will be able to provide a low emittance, small energy spread, short bunch electron beam at a high repetition rate of continuous operation. We propose to apply this beam to produce a seeded VUV coherent radiation. We will discuss the feasibility of the scheme and status of the laser modulator development.
|Overview of FEL Seeding Activities at FLASH
|The free-electron laser facility FLASH at DESY operates since several years in SASE mode, delivering high-intensity FEL pulses in the extreme ultra violet and soft x-ray wavelength range for users. In order to get more control of the characteristics of the FEL pulses external FEL seeding has proven to be a reliable method to do so. At FLASH, an experimental setup to test several different external seeding methods has been installed since 2010. After successful demonstration of direct seeding at 38 nm, the setup is now being commissioned to operate in HGHG and EEHG mode. Furthermore, other studies on laser induced effects on the electron beam dynamics will be performed. Beside the experimental activities, a design for the seeding option for the FLASH2 beamline is currently under investigation. The goal for that is to develop a concept which is compatible with the operation of FLASH1 and which satisfies the high demands of the future user community. In this contribution, we give an overview of the activities on FEL seeding at FLASH.
|Soft X-ray Self-seeding Simulation Methods and their Application for LCLS
|Self-seeding is a promising approach to significantly narrow the SASE bandwidth of XFELs to produce nearly transform-limited pulses. We study radiation propagation through the grating monochromator installed at LCLS. The monochromator design is based on a toroidal variable line spacing grating working at a fixed incidence angle mounting without an entrance slit. It covers the spectral range from 500eV to 1000eV. The optical system was studied using wave optics method to evaluate the performance of the self-seeding scheme. Our wave optics analysis takes into account the finite size of the coherent source, third-order aberrations and height error of the optical elements. Wave optics is the only method available, in combination with FEL simulations, to simulate performance of the monochromator without exit slit. Two approaches for time-dependent simulations are presented, compared and discussed. Also pulse-front tilt phenomenon effect is illustrated.
|Laser Heater and Seeded Free Electron Laser: Comments on Recent FERMI Experimental Results
|In this paper we consider the effect of laser heater on a seeded Free Electron Laser. We develop a model embedding the effect of the energy modulation induced by the heater with those due to the seeding. The present analysis is compatible with the experimental results obtained at FERMI displaying secondary maxima with increasing heater intensity. The problem has been considered ab-inito, by developing a numerical code based on the solution of the Liouville equation ruling the phase space evolution of the e-beam undergoing the various dynamical stages: Heating, Acceleration, Seeding. The treatment developed in the paper confirms and extends previous analyses and put in evidence further effects which can be tested in future experiments.
|X-ray Monochromators for Self-seeding XFELs in the Photon Energy Range Starting from 1.5 keV
Self-seeding of XFELs below 1 keV can be performed using grating monochromators . Forward-Bragg diffraction (wake) monochromators  were instrumental for achieving self-seeding in hard x-ray FELs in the photon energy range from 5 to 10 keV . Large photo-absorption makes extension into the lower photon range difficult. Here alternative schemes of x-ray monochromators are introduced and discussed for achieving self-seeding in a yet inaccessible spectral range starting from 1.5 keV.
 J. Feldhaus, et al., Opt. Commun. 140, 341 (1997).
. G. Geloni, V. Kocharyan, and E. Saldin, J. Mod. Opt. 58, 1391 (2011).
 J. Amann, et al., Nat. Photonics 6, 693 (2012).
|Indirect Measurements of NIR and UV Ultrashort Seed Laser Pulses using a Transverse Deflecting RF-Structure
Seeding of free-electron lasers (FELs) using external coherent optical pulses recently became an area of interest as users demand spectrally and temporally coherent FEL radiation which is not achievable in traditional self-amplified spontaneous emission operation mode. Since temporal and spectral properties of the seed laser pulses are directly imprinted on the electron bunch, a proper characterization of these seed pulses is needed. However, the lack of any measurement technique capable of characterizing ultrashort seed laser pulses at the laser-electron interaction region is a primary drawback. In this paper we report indirect measurements of seed laser pulses in an undulator section using a transverse deflecting RF-structure (TDS-LOLA) at the free-electron laser FLASH at DESY. Temporally chirped and unchirped seed pulse length measurements will be compared with second-harmonic generation frequency-resolved optical gating measurements and theoretical simulations. Using this technique we will demonstrate that pulse artifacts such as pre- and post-pulses in the seed pulse in the femtosecond and picosecond timescales can be identified without any temporal ambiguity.
Authors acknowledge the support received from FLASH team and many groups at DESY in preparation and commissioning of experiments. We thank our colleagues in the FLASH seeding team for their support.
|HGHG AND EEHG MICROBUNCHES WITH CSR AND LSC
Funding: Work supported by BMBF (contract 05K13PE3) and DESY
Longitudinal space charge (LSC) forces in a drift and coherent synchrotron radiation (CSR) in a chicane are relevant for high gain harmonic generation (HGHG) and echo enabled harmonic generation (EEHG) seeding designs. These factors determine whether or not the modulator can be located significantly upstream of the radiator. The benefits and dangers of having a drift in between the radiator and the modulator are investigated and a measurement of the LSC enabled reduction of the energy spread of a seeded beam is presented.
|Enhancing the Harmonic Content of an HGHG Microbunch
Funding: BMBF grant 05K10PE1 and DESY
High Gain Harmonic Generation (HGHG) seeding has been demonstrated in the visible and ultraviolet, but it is limited in performance at high harmonics of the seed by the initial uncorrelated energy spread of the electron beam. A recent proposal from SINAP using a chirped electron beam and a canted pole undulator has suggested a new mechanism for cooling the uncorrelated energy spread of the electron beam in order to improve the performance of HGHG seeding at high harmonics. This note reviews the mechanism, the limitation of the concept and extrapolates to some new concepts using analogous mechanisms derived from transverse gradients of the laser properties. The impact of CSR wakes on the vanishingly short microbunches produced by the methods are also investigated.
 H. Deng and C. Feng, Phys. Rev. Lett. 111, 084801 (2013)
|A Concept for Seeding 4-40 nm FEL Radiation at FLASH2
Funding: Work supported by BMBF (contract 05K13PE3)
This note describes a scheme to seed the FLASH2 FEL over a range of 4-40 nm without impacting SASE capabilities. This scheme combines multiple seeding techniques, builds on current infrastructure and offers a maximized range of performance with higher pulse energies than what are available at lower-peak current facilities. The concept relies on Echo Enabled Harmonic Generation (EEHG), cascaded seeding, and Second Harmonic Afterburners (SHAB) while maintaining the possibility to operate with High Gain Harmonic Generation (HGHG) seeding at >30 nm wavelengths.
|Hard X-ray Self-Seeding Setup and Results at SACLA
|In order to improve the spectral and temporal properties of XFEL, the self-seeding option based on the transmission crystal optics has been implemented in SACLA since 2012. The self-seeding setup composed of four dipole magnets that can generate up to 50 fs temporal delay and a diamond single crystal with the thickness of 180 micro-m has been installed at the position of the 9th undulator segment, which has been moved downstream. In 2013, the installation of all the components has been completed in August and the commissioning has been started in October. After a number of tuning processes such as the beam collimation and undulator K-value optimization, significant spectral narrowing has been confirmed at 10 keV with the C(400) Bragg reflection. The spectral bandwidth of seeded FEL is about 3 eV, which is nearly one order narrower than that of SASE measured without the diamond crystal. The peak spectral intensity of seeded FEL is about 5 times higher than that of SASE. Systematic optimization on beam properties is now in progress towards experimental use of seeded XFELs. This talk gives the overview of the plan, achieved results and ongoing R&D.
|Slides TUC01 [20.337 MB]
|Soft X-ray Self-seeding Setup and Results at LCLS
|The soft X-ray self seeding program was designed to provide near transform-limited pulses in the range of 500 eV to 1000 eV. The project was a three-way collaboration between SLAC, Lawrence Berkeley National Lab, and the Paul Scherrer Institute in Switzerland. Installation finished in the Fall of 2013, and after the early stages of commissioning we have measured up to 0.5mJ pulse energy and resolving powers of up to 5000 across the design wavelength range, representing a several-fold increase in the brightness compared to the normal LCLS operating mode. Future work will aim to increase the total pulse energy and establish self-seeding as a robust user operation mode.
|Slides TUC02 [10.464 MB]
|Generation of Optical Orbital Angular Momentum Using a Seeded Free Electron Laser
Funding: The research was in part funded by the TALENTS UP Programme (7th R&D Framework Programme, Specific Programme: PEOPLE - Marie Curie Actions - COFUND).
We propose an effective scheme for the generation of intense extreme-ultraviolet light beams carrying orbital angular momentum (OAM). The light is produced by a high-gain harmonic-generation free-electron laser (HGHG FEL), seeded using a laser pulse with a transverse staircase-like phase pattern. The transverse phase modulation in the seed laser is obtained by putting a phase-mask in front of the focusing lens, before the modulator. The staircase-like phase pattern is effectively transferred onto the electron beam in the modulator and the microbunching structure is preserved after frequency up-conversion in the radiator. During light amplification in the radiator, diffraction and mode selection drive the radiation profile towards a dominant OAM mode at saturation. With a seed laser at 260 nm, gigawatt power levels are obtained at wavelengths approaching those of soft x-rays. Compared to other proposed schemes to generate OAM with FELs, our approach is robust, easier to implement, and can be integrated into already existing FEL facilities without extensive modifications of the machine layout.
|Experimental Demonstration of Seeded Free-electron Laser Spectrum Control using Corrugated Structure
Funding: This work was partially supported by the Major State Basic Research Development Program of China (2011CB808300) and the National Natural Science Foundation of China (11175240, 11205234 and 11322550).
Corrugated structures [*] hold promising prospects as beam linearizer, beam de-chirper and beam stabilizer for a high brightness LINAC, and thus enhance the FEL performance significantly. So far, several electron beam test experiments of corrugated structure have been carried out on LINACs at PAL [**] and BNL [***]. More recently, a passive control experiment of FEL spectrum, by using a pair of corrugated planes has been successfully accmoplished at SDUV-FEL, which is the first real FEL experimental demonstration of such wakefield structure applications. The physical design, numerical simulations, the vacuum chamber manufacture and the experiment results are presented in this paper.
* K. L. F. Bane and G. Stupakov, NIMA 690 (2012) 106-110.
** P. Emma, M. Venturini, K. L. F. Bane, et al., PRL. 112 (2014) 034801.
*** M. Harrison, et al., NaPAc 2013, Pasadena, USA.
|Slides TUC04 [8.562 MB]