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Typically, in self-Amplified spontaneous emission free electron laser (SASE FEL) based short-pulse schemes, pulse duration is limited by FEL coherence time. For hard x-ray FELs, coherence time is in a few hundred attosecond range while for XUV and soft x-ray FELs it is in the femtosecond regime. In this paper the modification of so-called chirp-taper scheme is developed that allows to overcome the coherence time barrier. Numerical simulations for XUV and soft x-ray FEL user facility FLASH demonstrate that one can generate a few hundred attosecond long pulses in the wavelength range 2–10 nm with peak power reaching hundreds of megawatts. With several thousand pulses per second this can be a unique source for attosecond science.

Many high-gain free electron lasers worldwide are planning to incorporate seeding setups into their day-to-day operation. These techniques provide both longitudinal and transverse coherence and extended control of the output free electron laser radiation spectral properties. However, the output wavelength and repetition rate strongly depend on the properties of the seed laser system. With the laser peak power required for successful seeded operation, it is currently not possible to increase their repetition rate to an extent that it matches the electron bunch repetition rate of superconducting accelerators. Here, we investigate the advantages of a modification of standard seeding setups, by combining the seeding with the so-called optical klystron. With this new seeding setup, it is possible to decrease the seed laser power requirements and therefore, seed laser systems can increase their repetition rate at the same wavelength. We show simulation results in a high-gain harmonic generation (HGHG) setup for a range of harmonics (8th to 15th) and we verify the reduction of seed laser power required with an Optical Klystron HGHG scheme. Finally, we comment on the stability of the proposed setup to jitter sources and to shot-to-shot fluctuations and compare to the standard HGHG scheme.

The successful realization of high gain free-electron lasers has opened new possibilities to X-ray scientists for investigating matter in different states. The availability of unprecedented photon properties stimulated the development of new experimental techniques capable of taking full advantage of these options and has started a virtuous collaboration between machine experts and photon users to improve further and optimize the generated X-ray pulses. Over the recent years, this has led to the development of several advanced free-electron laser (FEL) schemes to tailor the photon properties to specific experimental demands. Presently, tunable wavelength X-ray pulses with extremely high brilliance and short pulse characteristics are a few of the many options available at FELs. Few facilities can offer options such as narrowband or extremely short pulses below one fs duration and simultaneous pulses of multiple colors enabling resonant X-ray pump—X-ray probe experiments with sub fs resolution. Fully coherent X-ray radiation (both spatial and temporal) can also be provided. This new option has stimulated the application of coherent control techniques to the X-ray world, allowing for experiments with few attoseconds resolution. FELs often operate at a relatively low repetition rate, typically on the order of tens of Hz. At FLASH and the European XFEL, however, the superconducting accelerators allow generating thousands of pulses per second. With the implementation of a new seeded FEL line and with an upgrade at FLASH linac, all the new features will become available in the soft X-ray spectral range down to the oxygen K edge with unprecedented average photon flux due to the high repetition rate of pulses.

High gain Free-electron lasers (FELs) are driven by linear accelerators that are currently capable of delivering electron bunches in high-repetition rates from kHz and up to several MHz. Short wavelength FELs are commonly operated in a self-amplified spontaneous emission (SASE) scheme which is, due to its stochastic nature, limited in temporal coherence. External seeding and harmonic generation techniques have been shown to be valid solutions for the generation of fully coherent and stable radiation up to the soft x-ray. However, the required seed lasers in the UV regime and shorter wavelengths cannot generate pulses with sufficient energy at the repetition-rates accessible with superconducting accelerators, hence the seed laser can become the limiting factor which determines the maximum repetition-rate for future seeded FEL based on superconducting linacs. In this paper, we address the need for generation of fully coherent radiation with higher average flux and we propose two different schemes which overcome the limitations and enable the generation of seeded radiation at high-repetitionrates. In one case, an optical cavity is used instead of the seed laser source. It retains a seed laser pulse and recirculates it to seed electron bunches in high-repetition-rates. In the other case, an optical klystron scheme is exploited in the seeding process: a low-intensity seed laser initiates the process in the first modulator and then, a dispersive section increases the density modulation and enables coherent emission in the second modulator which increases further the energy modulation. The resulting energy modulation is several times larger than the one possible with a standard seeding scheme and same laser intensity. Reducing the requirements in seed laser power, the proposed setup makes the use of seed lasers at higher repetition-rates or shorter wavelengths possible. Here, we show simulation results for the optical klystron scheme and we discuss experimental possibilities.

Harmonic lasing provides an opportunity to extend the photon energy range of existing and planned x-ray free electron laser (FEL) user facilities. Contrary to nonlinear harmonic generation, harmonic lasing can generate a much more intense, stable, and narrow-band FEL beam. Another interesting application is harmonic lasing self-seeding that allows to improve the longitudinal coherence and spectral power of a self-amplified spontaneous emission FEL. This concept was tested at the soft x-ray FEL user facility FLASH in the range of 4.5–15 nm and at Pohang accelerator laboratory X-ray Free Electron Laser (XFEL) at 1 nm. In this paper we present recent results from the European XFEL where we successfully demonstrated harmonic lasing at 5.9 Angstrom and 2.8 Angstrom. In the latter case we obtained both third and fifth harmonic lasing and, for the first time, operated a harmonic lasing cascade (fifth-third-first harmonics of the undulator). These results pave the way for reaching very high photon energies, up to 100 keV.

This paper describes a detailed simulation of the influence of optical imperfections in a multilayer Bragg mirror-based split-and-delay unit on the focused and unfocused beam profiles with relative time delays within the coherence time for hard X-ray pump/X-ray probe experiments at the European XFEL., For the High-Energy-Density (HED) beamline at the SASE2 undulator of the European XFEL, a hard X-ray split-and-delay unit (SDU) has been built enabling time-resolved pump/probe experiments with photon energies between 5 keV and 24 keV. The optical layout of the SDU is based on geometrical wavefront splitting and multilayer Bragg mirrors. Maximum delays between Δτ = ±1 ps at 24 keV and Δτ = ±23 ps at 5 keV will be possible. Time-dependent wavefront propagation simulations were performed by means of the Synchrotron Radiation Workshop (SRW) software in order to investigate the impact of the optical layout, including diffraction on the beam splitter and recombiner edges and the three-dimensional topography of all eight mirrors, on the spatio-temporal properties of the XFEL pulses. The radiation is generated from noise by the code FAST which simulates the self-amplified spontaneous emission (SASE) process. A fast Fourier transformation evaluation of the disturbed interference pattern yields for ideal mirror surfaces a coherence time of τc = 0.23 fs and deduces one of τc = 0.21 fs for the real mirrors, thus with an error of Δτ = 0.02 fs which is smaller than the deviation resulting from shot-to-shot fluctuations of SASE2 pulses. The wavefronts are focused by means of compound refractive lenses in order to achieve fluences of a few hundred mJ mm−2 within a spot width of 20 µm (FWHM) diameter. Coherence effects and optics imperfections increase the peak intensity between 200 and 400% for pulse delays within the coherence time. Additionally, the influence of two off-set mirrors in the HED beamline are discussed. Further, we show the fluence distribution for Δz = ±3 mm around the focal spot along the optical axis. The simulations show that the topographies of the mirrors of the SDU are good enough to support X-ray pump/X-ray probe experiments.

Applied Sciences 10(8), 2728 (2020). doi:10.3390/app10082728, X-ray pump/X-ray probe applications are made possible at X-ray Free Electron Laser (XFEL) facilities by generating two X-ray pulses with different wavelengths and controllable temporal delay. In order to enable this capability at the European XFEL, an upgrade project to equip the soft X-ray SASE3 beamline with a magnetic chicane is underway. In the present paper we describe the status of the project, its scientific focus and expected performance, including start-to-end simulations of the photon beam transport up to the sample, as well as recent experimental results demonstrating two-color lasing at photon energies of 805 eV + 835 eV and 910 eV + 950 eV. Additionally, we discuss methods to analyze the spectral properties and the intensity of the generated radiation to provide on-line diagnostics for future user experiments., Published by MDPI, Basel

13th International Conference on Synchrotron Radiation Instrumentation, SRI2018, Taipei, Taiwan, 10 Jun 2018 - 15 Jun 2018; AIP conference proceedings 2054(1), 030019 (2019). doi:10.1063/1.5084582, Knowledge of different properties of the radiation from X-ray FEL is very important for planning experiments. An understanding of such properties can come from start-to-end simulations of experiments: modern FEL simulation allows to reliably predict the output radiation pulses from X-ray FEL. We present a web a ccessible XFEL photon pulses simulation database showing data calculated with the FAST simulation framework., Published by AIP, Melville, NY

Determining fluctuations in focus properties is essential for many experiments at Self-Amplified-Spontaneous-Emission (SASE) based Free-Electron-Lasers (FELs), in particular for imaging single non-crystalline biological particles. We report on a diffractive imaging technique to fully characterize highly focused, single-shot pulses using an iterative phase retrieval algorithm, and benchmark it against an existing Hartmann wavefront sensor. The results, both theoretical and experimental, demonstrate the effectiveness of this technique to provide a comprehensive and convenient shot-to-shot measurement of focused-pulse wave fields and source-point positional variations without the need for manipulative optics between the focus and the detector.

Proc. SPIE, 2017. - ISBN - doi:10.1117/12.2265567 Advances in X-ray Free-Electron Lasers Instrumentation IV, Prague, Czech Republic, 24 Apr 2017 - 27 Apr 2017; Proceedings of SPIE 10237, 102370I (2017). doi:10.1117/12.2265567, The radiation from SASE FEL has always limited value of the degree of transverse coherence. When transverse size of the electron beam significantly exceeds diffraction limit, the mode competition effect does not provide the selection of the ground mode, and spatial coherence degrades due to contribution of the higher order transverse modes. It is important that the most strong higher modes are azimythally non-symmetric which leads to fluctuations of the spot size and of the pointing stability of the photon beam. These fluctuations are fundamental and originate from the shot noise in the electron beam. The effect of the pointing instability becomes more pronouncing for shorter wavelengths. We analyze in detail the case of optimized SASE FEL and derive universal dependencies applicable to all operating and planned x-ray FELs. It is shown that x-ray FELs driven by low energy electron beams will exhibit poor spatial coherence and bad pointing stability., Published by SPIE, Bellingham, Wash.

Radiation from the SASE FEL operating in the linear regime holds properties of completely chaotic polarized light. Measurements of the SASE FEL gain curve allows to determine saturation length which is strictly connected with coherence time. Statistical analysis of the fluctuations of the radiation energies measured with different spatial apertures allows one to determine the number of the longitudinal and transverse modes. Thus, with these simple measurements it becomes possible to determine the degree of transverse coherence, coherence time, and photon pulse duration. In this report we present theoretical background and experimental results obtained at free electron laser FLASH.

8th International Particle Accelerator Conference, IPAC17, Copenhagen, Denmark, 15 May 2017 - 19 May 2017 ; Geneva : JACoW, 2635-2638(2017). doi:10.18429/JACoW-IPAC2017-WEPAB027, We report on the results of the first operation of a frequency doubler at free electron laser FLASH2. The scheme uses the feature of the variable gap undulator. Undulator is divided in two parts. The second part of the undulator is tuned to the double frequency of the first part. Amplification process in the first undulator part is stopped at the onset of the nonlinear regime, such that nonlinear higher harmonic bunching in the electron beam density becomes pronouncing, but the radiation level is still small to disturb the electron beam significantly. Modulated electron beam enters the second part of the undulator and generates radiation at the 2nd harmonic. Frequency doubler allows operation in a two-color mode and operation at shorter wavelengths with respect to standard SASE scheme. Tuning of the electron beam trajectory, phase shifters and compression allows to tune intensities of the first and the second harmonic. The shortest wavelength of 3.1 nm (photon energy 400 eV) has been achieved with frequency doubler scheme, which is significantly below the design value for the standard SASE option., Published by JACoW, Geneva

Proc. SPIE, 2017. - ISBN - doi:10.1117/12.2268140 Advances in X-ray Free-Electron Lasers Instrumentation IV, Prague, Czech Republic, 24 Apr 2017 - 27 Apr 2017; Proceedings of SPIE 10237, 102370G (2017). doi:10.1117/12.2268140, We discuss theoretical background and experimental verification of advanced schemes for X-ray FELs using variable gap undulators (harmonic lasing self-seeded FEL, reverse taper etc.) Harmonic lasing in XFELs is an opportunity to extend operating range of existing and planned X-ray FEL user facilities. Contrary to nonlinear harmonic generation, harmonic lasing can provide much more intense, stable, and narrow-band FEL beam which is easier to handle due to the suppressed fundamental. Another interesting application of harmonic lasing is Harmonic Lasing Self-Seeded (HLSS) FEL that allows to improve longitudinal coherence and spectral power of a SASE FEL. Recently this concept was successfully tested at the soft X-ray FEL user facility FLASH in the wavelength range between 4.5 nm and 15 nm. That was also the first experimental demonstration of harmonic lasing in a high-gain FEL and at a short wavelength (before it worked only in infrared FEL oscillators). Another innovative scheme that was tested at FLASH2 is the reverse tapering that can be used to produce circularly polarized radiation from a dedicated afterburner with strongly suppressed linearly polarized radiation from the main undulator. This scheme can also be used for an efficient background-free production of harmonics in an afterburner. Experiments on the frequency doubling that allowed to reach the shortest wavelength at FLASH as well as on post-saturation tapering to produce a record intencity in XUV regime are also discussed., Published by SPIE, Bellingham, Wash.

SPIE Optics + Optoelectronics, Prague, Czech Republic; Proceedings of SPIE 10237, 102370S (2017). doi:10.1117/12.2270552, Simulations of experiments at modern light sources, such as optical laser laboratories, synchrotrons, and free electron lasers, become increasingly important for the successful preparation, execution, and analysis of these experiments investigating ever more complex physical systems, e.g. biomolecules, complex materials, and ultra–short lived states of matter at extreme conditions. We have implemented a platform for complete start–to–end simulations of various types of photon science experiments, tracking the radiation from the source through the beam transport optics to the sample or target under investigation, its interaction with and scattering from the sample, and registration in a photon detector. This tool allows researchers and facility operators to simulate their experiments and instruments under real life conditions, identify promising and unattainable regions of the parameter space and ultimately make better use of valuable beamtime. In this paper, we present an overview about status and future development of the simulation platform and discuss three applications: 1.) Single–particle imaging of biomolecules using x–ray free electron lasers and optimization of x–ray pulse properties, 2.) x–ray scattering diagnostics of hot dense plasmas in high power laser–matter interaction and identification of plasma instabilities, and 3.) x–ray absorption spectroscopy in warm dense matter created by high energy laser–matter interaction and pulse shape optimization for low–isentrope dynamic compression., Published by SPIE, Bellingham, Wash.

The optimal XFEL pulse duration for single-particle imaging of small proteins is narrowed down to the 3–9 fs range, using start-to-end simulations of a single-particle imaging experiment at the European XFEL., Single-particle imaging with X-ray free-electron lasers (XFELs) has the potential to provide structural information at atomic resolution for non-crystalline biomolecules. This potential exists because ultra-short intense pulses can produce interpretable diffraction data notwithstanding radiation damage. This paper explores the impact of pulse duration on the interpretability of diffraction data using comprehensive and realistic simulations of an imaging experiment at the European X-ray Free-Electron Laser. It is found that the optimal pulse duration for molecules with a few thousand atoms at 5 keV lies between 3 and 9 fs.

Physical review accelerators and beams 20(2), 020705(2017). doi:10.1103/PhysRevAccelBeams.20.020705, Harmonic lasing is a possible mode of operation of X-ray FEL user facilities that allows us to provide brilliant beams of higher energy photons for user experiments. Another useful application of harmonic lasing is the so-called harmonic lasing self-seeded free electron laser (HLSS FEL) that allows the improvement of spectral brightness of these facilities. In the past, harmonic lasing has been demonstrated in the FEL oscillators in infrared and visible wavelength ranges, but not in high-gain FELs and not at short wavelengths. In this paper we report on the first evidence of the harmonic lasing and the first operation of the HLSS FEL at the soft X-ray FEL user facility FLASH in the wavelength range between 4.5 nm and 15 nm. Spectral brightness was improved in comparison with self-amplified spontaneous emission (SASE) FEL by a factor of six in the exponential gain regime. A better performance of HLSS FEL with respect to SASE FEL in the post-saturation regime with a tapered undulator was observed as well. The first demonstration of harmonic lasing in a high-gain FEL and at short wavelengths paves the way for a variety of applications of this new operation mode in X-ray FELs., Published by American Physical Society, College Park, MD

Scientific reports 6, 24791(2016). doi:10.1038/srep24791, The advent of newer, brighter, and more coherent X-ray sources, such as X-ray Free-Electron Lasers (XFELs), represents a tremendous growth in the potential to apply coherent X-rays to determine the structure of materials from the micron-scale down to the Angstrom-scale. There is a significant need for a multi-physics simulation framework to perform source-to-detector simulations for a single particle imaging experiment, including (i) the multidimensional simulation of the X-ray source; (ii) simulation of the wave-optics propagation of the coherent XFEL beams; (iii) atomistic modelling of photon-material interactions; (iv) simulation of the time-dependent diffraction process, including incoherent scattering; (v) assembling noisy and incomplete diffraction intensities into a three-dimensional data set using the Expansion-Maximisation-Compression (EMC) algorithm and (vi) phase retrieval to obtain structural information. We demonstrate the framework by simulating a single-particle experiment for a nitrogenase iron protein using parameters of the SPB/SFX instrument of the European XFEL. This exercise demonstrably yields interpretable consequences for structure determination that are crucial yet currently unavailable for experiment design., Published by Nature Publishing Group, London

We discuss the progress in the R&D program for a future upgrade of the European XFEL facility, namely for an operation in the continuous wave (cw) and long pulse (lp) modes, which will allow for significantly more flexibility in the electron and photon beam time structure. Results of cw/lp runs with preseries XFEL cryomodules and status of components needed for the new operation modes are presented here.

FLASH is the first free electron laser user facility operating in the vacuum ultraviolet and soft x-ray wavelength range. Many user experiments require knowledge of the spatial and temporal coherence properties of the radiation. In this paper we present an analysis of the coherence properties of the radiation for the fundamental and for the higher odd frequency harmonics. We show that temporal and spatial coherence reach maximum close to the FEL saturation but may degrade significantly in the post-saturation regime. We also find that the pointing stability of short FEL pulses is limited due to the fact that non-azimuthal FEL eigenmodes are not sufficiently suppressed. We discuss possible ways for improving the degree of transverse coherence and the pointing stability., 17 pages, 19 figures

We describe statistical and coherence properties of the radiation from x-ray free electron lasers (XFEL) operating in the post-saturation regime. We consider practical case of the SASE3 FEL at the European XFEL. We perform comparison of the main characteristics of the X-ray FEL operating in the post-saturation regime with and without undulator tapering: efficiency, coherence time and degree of transverse coherence., 6 pages, 5 figures

Present bunch compression scheme at the VUV FEL at DESY is essentially nonlinear and naturally results in the formation of a short, high-current leading peak (spike) in the density distribution that produces FEL radiation. The main feature of the considered mode of operation is the production of short, down to 20 fs radiation pulses with GW-level peak power and contrast of 80%.

One of the most challenging tasks for extreme ultraviolet, soft and hard x ray free electron laser photon diagnostics is the precise determination of the photon pulse duration, which is typically in the sub 100 fs range. Nine different methods, able to determine such ultrashort photon pulse durations, were compared experimentally at FLASH, the self amplified spontaneous emission free electron laser at DESY in Hamburg, in order to identify advantages and disadvantages of different methods. Radiation pulses at a wavelength of 13.5 and 24.0 nm together with the corresponding electron bunch duration were measured by indirect methods like analyzing spectral correlations, statistical fluctuations, and energy modulations of the electron bunch and also by direct methods like autocorrelation techniques, terahertz streaking, or reflectivity changes of solid state samples. In this paper, we present a comprehensive overview of the various techniques and a comparison of the individual experimental results. The information gained is of utmost importance for the future development of reliable pulse duration monitors indispensable for successful experiments with ultrashort extreme ultraviolet pulses

Phase 1 of the vacuum ultra-violet (VUV) free-electron laser (FEL) at the TESLA Test Facility (TTF) recently concluded operation. It successfully demonstrated the saturation of a SASE FEL in the in the wavelength range of 80-120 nm. We present a posteriori start-to-end numerical simulations of this FEL. These simulations are based on the programs Astra and elegant for the generation and transport of the electron distribution. An independent simulation of the intricate beam dynamics in the magnetic bunch compressor is performed with the program CSRtrack. The SASE FEL process is simulated with the code FAST. From our detailed simulations and the resulting phase space distribution at the undulator entrance, we found that the FEL was driven only by a small fraction (slice) of the electron bunch. This "lasing slice" is located in the head of the bunch, and has a peak current of approximately 3 kA. A strong energy chirp (due to the space charge field after compression) within this slice had a significant influence on the FEL operation. Our study shows that the radiation pulse duration is about 40 fs (FWHM) with a corresponding peak power of 1.5 GW. The simulated FEL properties are compared with various experimental data and found to be in excellent agreement., Comment: 25 pages, 19 figures

The aim of the present work is to investigate the influence of a tapered undulator on the main parameters of an free electron laser (FEL). The experiments were carried out at the Darmstadt FEL at an electron beam energy of 31 MeV and a wavelength of 7 μm . The effect of tapering depth on various important free electron laser characteristics such as the wavelength corresponding to the peak of the spectral distribution, the spectral width, the small signal gain and the electron energy extraction efficiency has been experimentally observed and quantitatively understood. It is shown for example that the wavelength shift is well described taking into account short pulse effects. Furthermore, for the first time, it has been found that for some values of the tapering depth the FEL efficiency exceeds that of the uniform undulator by more than 35% and 18% for negative and positive tapering, respectively. The experimental data of the spectral distributions for different tapering values are compared with predictions based on simulations for the interaction processes in the tapered undulator and with the experimental results recently obtained at the IR DEMO FEL at the Thomas Jefferson National Accelerator Facility.

Re-examination of dogmatic “truths” can sometimes yield surprises. For years we were led to believe that famous Schwinger's formulas are directly applicable to describe synchrotron radiation from dipole magnet and even now no attention is usually paid to the region of applicability of these expressions. While such formulas are valid in order to describe radiation from a dipole in the X-ray range, their long-wavelength asymptote are not valid, in general. In the long-wavelength region, Schwinger's formulas must be analyzed from a critical viewpoint, and corrections must be discussed when one is looking for an application to CSR-based diagnostics. In this paper, we perform such a task by means of a consistent use of similarity techniques, discussing the limits of validity of Schwinger's formulas which arise from a finite magnet length, from a finite distance of the detector to the sources and from diffraction effects (due to the presence of vacuum pipe and aperture limitations).

VUV SASE FEL at the TESLA Test Facility (Phase 1) was successfully running and reached saturation in the wavelength range 80– 120 nm . We present a posteriori start-to-end simulations of this machine. The codes Astra and elegant are used to track particle distribution from the cathode to the undulator entrance. An independent simulation of the beam dynamics in the bunch compressor is performed with the code CSRtrack. SASE FEL process is simulated with the code FAST. Simulation results are in a good agreement with the measured properties of SASE FEL radiation.

In the limit for a large distance between bunch and detector and under the assumption that the entire process, i.e. radiation and detection, happens in vacuum, one can use the well-known Schwinger formulas in order to describe the single-particle radiation in the case of circular motion. Nevertheless, these formulas cannot be applied for particles moving in an arc of a circle. In this paper, we present a characterization of coherent synchrotron radiation (CSR) pulses in the time-domain as they are emitted by an electron bunch moving in an arc of a circle. This can be used in order to give a quantitative estimation of the effects of a finite bending magnet extension on the characteristics of the CSR pulse.

In this paper, we discuss a method for non-destructive measurements of the longitudinal profile of sub-picosecond electron bunches for X-ray free electron lasers. The method is based on the detection of the coherent synchrotron radiation (CSR) produced by a bunch passing through an undulator. Coherent radiation energy within a central cone turns out to be proportional, per pulse, to the square modulus of the bunch form-factor at the resonant frequency of the fundamental harmonic. An attractive feature of the proposed technique is the absence of any apparent limitation which would distort measurements. Indeed, the radiation process takes place in vacuum and is described by analytical formulae. CSR propagates to the detector placed in vacuum. Since CSR energy is in the range up to a fraction of mJ, a simple bolometer is used to measure the energy with a high accuracy. The proposed technique is very sensitive and it is capable of probing the electron bunches with a resolution down to a few microns.

An effective and practical technique based on the detection of the coherent synchrotron radiation (CSR) spectrum can be used to characterize the profile function of ultra-short bunches. The CSR spectrum measurement has an important limitation: no spectral phase information is available, and the complete profile function cannot be obtained in general. In this paper we propose to use constrained deconvolution method for bunch profile reconstruction based on a priori-known information about formation of the electron bunch. Application of the method is illustrated with practically important example of a bunch formed in a single bunch-compressor. Downstream of the bunch compressor the bunch charge distribution is strongly non-Gaussian with a narrow leading peak and a long tail. The longitudinal bunch distribution is derived by measuring the bunch tail constant with a streak camera and by using a priory available information about profile function.

A nondestructive radiation detector unit is under construction at the TESLA Test Facility at DESY providing monitoring of the radiation (pulse energy, transverse intensity distribution, and statistical properties). The concept of the detector design is based on experience obtained during operation of TTF FEL, Phase 1. Key element of the detector is a wide dynamic range micro-channel plate (MCP) which detects scattered radiation from a thin gold wire crossing photon beam. Operating wavelength range of the detector is from 6 to 100 nm . An important feature of the MCP-based detector is that it is capable to cover all dynamic range of the radiation intensity, from the level of spontaneous emission up to the saturation level of SASE FEL.

In this paper, we study a possible microbunching instability in the TESLA Test Facility linac. A longitudinal space charge is found to be the main effect driving the instability. Analytical estimates show that initial perturbations of beam current in the range 0.5– 1 mm are amplified by a factor of a few hundred when the beam passes two bunch compressors. A method to suppress the instability is discussed.

It is shown that although the soft X-ray SASE FEL photon beam from undulator is in principle a single user tool, just like an optical laser, the optical distribution system based on multifacet reflectors provide efficient ways to generate a multi-user facility—very similar to present-day synchrotron radiation facilities. Multifacet reflectors involve multiple reflections from a series of plane mirrors. These make use of the principle of total external reflectance. The candidate mirror materials include C, Mo, and Si which should yield a reflectance of 40–80% for an arc angle of 90° in the wavelength range 6– 100 nm . Multifacet reflector arcs can be repeated a number of times to form a complete ring. In order to make efficient use of the new SASE FEL source, it is proposed to install this photon ring distribution system in the experimental hall of soft X-ray FEL laboratory. A few tens beam lines with different experiments can be served by a single FEL source. Using movable mirrors in each photon ring cell, it is possible to quickly switch the FEL photon beam from one experiment to the other, thus providing simultaneous multi-user capability.

A closer inspection of the statistical properties of the third-harmonic radiation from the SASE FEL reveals that it is possible to select single, temporary coherent radiation spikes by using a simple intensity trigger. A carefully designed optical system for splitting, delaying, filtering, and recombining the radiation would then allow time-resolved measurements with resolution down to the coherence time of the FEL, i.e. a few femtoseconds in the case of the TTF FEL.

Abstrac In this paper we describe the regeneration of electron bunches from the RF gun by back-reflected radiation from VUV SASE FEL at the TESLA Test Facility (TTF) at DESY. The SASE FEL was running at the wavelength 96 nm with 30-100 fs pulses, SASE pulse energy was up to 20 µJ. “Nominal” electron bunches for lasing were produced by RF gun with Cs2Te photocathode driven by UV quantum laser system. “Parasitic” bunches with a charge up to 1–1.5 nC were extracted from the RF gun due to VUV FEL radiation reflected from the mirror (placed downstream of the undulator) to the cathode. Nontrivial dependence of a charge on a SASE pulse energy was found. © 2003 Elsevier Science B.V. All rights reserved. PACS: 41.60. Cr; 52.75. M; 42.62. Cf

We analyze statistical properties of the radiation from a SASE FEL driven by short electron bunches. In the linear regime the radiation from a SASE FEL is a Gaussian random process. When approaching saturation point, statistical properties of the radiation change drastically on a scale of one field gain length. Particular attention is devoted to the analysis of fluctuations of total energy in the radiation pulse and after a narrow-band monochromator. It was found that fluctuations at saturation are significantly suppressed when electron pulse length becomes comparable with cooperation length.

We present a comprehensive analysis of coherence properties of radiation from a SASE FEL. The effective transverse correlation function is calculated by means of numerical simulations with the code FAST. This allows us to calculate the area and degree of transverse coherence. Evolution of these parameters is traced from the beginning of the undulator up to the deep nonlinear regime. It is shown that the degree of transverse coherence reaches maximum value at the saturation point. Then, it drops drastically because of poor transverse coherence of the radiation produced in the nonlinear regime.

In this paper, we consider a klystron-like mechanism of amplification of parasitic density modulations in an electron bunch passing a magnetic bunch compressor. Analytical expressions are derived for the small-signal gain. The effect of wakefields in front of the bunch compressor is analyzed by using a model of linear compression which assumes linear correlated energy chirp and linear dependence of a path length on energy deviation. Analysis of the density modulation growth due to coherent synchrotron radiation inside bends of the magnetic bunch compressor is done for the simplified case of no correlated energy chirp (no compression). Analytical results of this paper can be used for benchmarking numerical simulation codes.

New proposal of a fs X-ray facility, which is described in this paper, is based on the use of X-ray SASE FEL combined with a fs quantum laser. An ultrashort laser pulse is used for modulation of the energy and density of the electrons within a slice of the electron bunch at a frequency ωopt. The density modulation exiting the modulator (energy-modulation undulator and dispersion section) is about 10%. Following the modulator the beam enters an X-ray SASE FEL undulator, and is bunched at a frequency ω0. This leads to an amplitude modulation of the beam density at the sidebands ω0±ωopt. The sideband density modulation takes place at the part of the electron pulse defined by the duration of the seed laser pulse that is much shorter than the electron pulse. Following the SASE FEL undulator the beam and SASE radiation enter undulator section (radiator) which is resonant at the frequency ω0−ωopt. Because the beam has a large component of bunching at the sideband, coherent emission is copiously produced within fs slice of the electron bunch. Separation of the sideband frequency from the central frequency by a monochromator is used to distinguish the fs pulses from the sub-ps intense SASE pulses.

In this paper we describe the extension of the soft X-ray SASE FEL at the TESLA Test Facility (TTF) at DESY for generation of femtosecond pulses. The proposed scheme operates as follows. The first stage is a conventional FEL amplifier seeded by 523 nm external laser. A zero area optical pulse (i.e. the pulse with zero value of optical field in the central area of the pulse) is timed to overlap with the electron bunch. Radiation power is amplified up to the saturation level. Following the first stage the electron beam enters the main 6 nm SASE undulator. Large energy spread is induced in the significant fraction of the electron beam due to the FEL interaction process, and only a small part of the electron bunch (near the center of zero area light pulse) is able to produce radiation in the 6 nm SASE FEL. The SASE FEL described in this paper will provide soft X-ray pulses with 30 fs (FWHM) duration. On the basis of the TTF parameters it should be possible to achieve an average brilliance of 10 22 photons / s 1 / mrad 2 / mm 2 / (0.1% BW). The average number of photons can exceed 1012 photon/pulse.